an overcurrent trip

What Is Overcurrent? (Causes, Effects, and Protection)

Overcurrent is a destructive fault that can damage small, as well as, large motors, electric devices, and home appliances.

In this article, I will discuss the current increase (overcurrent) and answer the most important questions about it. let’s get started.

Table of Contents

what is overcurrent?

Overcurrent refers to an electrical condition in a circuit where the current flowing through the conductors exceeds the rated or designed current-carrying capacity of the components, such as wires, fuses, circuit breakers, and other protective devices.

Overcurrent can occur due to various factors and can have different consequences, including overheating, component damage, fires, or circuit malfunction.

Overcurrent example

A clear overcurrent example is, when a motor with a rated current of 35A, you can find rated current on the motor nameplate, draws 55A for any reason, it can be a mechanical loading as I discussed in the article Motor Overcurrent , this motor is an overcurrent situation.

Another example is if a current of 167A passes through a cable with a current ampacity of 135A, this cable is over the current situation.

In general, any electrical equipment has a rated current, whenever the current exceeds this value, it’s an overcurrent situation.

Overcurrent effects

Uncontrolled electric overcurrent leads to excessive heat generation and can damage or burn equipment.

A circuit wiring overheats when an overcurrent occurs. There is a possibility that the insulation could melt, and fire can break out as a result.

Circuit overload causes the breaker to trip, opening up and shutting off the power supply. The overload could cause the wiring to overheat and melt, causing a short circuit and leading to a house fire.

In addition to the heat loss from increasing current,  the rising current can also:

• Damage the circuit
• Burn resistors and electronic components
• Damage to electric equipment and home appliances
• And even cause fire to break out around the circuit.

What causes overcurrent in a circuit?

Overcurrent in electrical circuits can occur for various reasons, and it is important to identify and address the underlying causes to maintain the safety and proper functioning of electrical systems. Here are some common causes of overcurrent:

Excessive Load : One of the most common causes of overcurrent is simply overloading a circuit. This happens when the total electrical load connected to a circuit exceeds its designed capacity. Examples include plugging too many appliances into a single outlet or running too many devices on a circuit.

Short Circuits : A short circuit occurs when there is a low-resistance path between two conductors, causing a rapid and excessive flow of current. This can result from damaged insulation, exposed wires, or loose connections. Short circuits are particularly dangerous and can lead to electrical fires.

Ground Faults : Ground faults occur when an unintended electrical connection is established between a live conductor and the ground (earth). This can happen due to damaged insulation or faulty equipment. Ground faults can lead to overcurrent situations and pose safety hazards.

Equipment Malfunctions : Malfunctions within electrical equipment, such as motors, transformers, and appliances, can cause overcurrent. This can be due to internal faults, mechanical issues, or electrical problems within the equipment itself.

Power Surges : Sudden and temporary increases in voltage, known as power surges or voltage spikes, can cause overcurrent in circuits. These surges can result from lightning strikes, utility grid fluctuations, or switching events.

Inrush Current : Some devices, particularly motors and transformers, experience high inrush currents when they are initially energized. Inrush current can temporarily exceed the normal operating current and may trip protective devices if not accounted for in the design.

Circuit Imbalances : In three-phase electrical systems, imbalances in current between the phases can lead to overcurrent in one or more phases. This can occur due to unequal loads or issues with the supply system.

Faulty Wiring and Connections : Poorly installed or deteriorated wiring and connections can increase electrical resistance, leading to overheating and overcurrent. This can be a result of corrosion, loose connections, or inadequate wire size.

Environmental Factors : Environmental factors, such as extreme temperatures, humidity, or exposure to corrosive substances, can degrade wiring and insulation over time, potentially causing overcurrent issues.

Circuit Design Flaws : Errors or flaws in the initial design of electrical circuits, including the selection of protective devices and wire sizes, can lead to overcurrent problems if the circuit is not properly matched to its intended use.

To prevent overcurrent and its associated risks, electrical systems are designed with protective measures, including circuit breakers, fuses, and ground fault circuit interrupters (GFCIs).

These devices are selected based on the expected load and potential risks associated with the circuit.

Regular maintenance and inspections are also essential to identify and address any issues that may lead to overcurrent.

How can you prevent over-current?

Electrical circuits have circuit breakers , surge protectors, and electrical fuses to prevent potential disasters.

The concept of preventing a power outage due to an overcurrent is quite straightforward: you should avoid overloading a circuit in the first place.

There are typically separate circuits for each room of the house, with heavy-duty appliances like an electric dryer or oven requiring a separate dedicated line.

In order to prevent your circuits from overcurrent, let us take a look at how you can do it.

Your circuit load should be calculated

For the workplace, never run any power tool or electrical equipment before you check its power rating, and verify the power source is suitable for it.

Only authorized persons are allowed to add new loads to any power source.

All authorized departments should participate when choosing new equipment. For example, when choosing a new pump, electrical and mechanical engineers should cooperate to make sure the pump and the motor ratings are the same, to prevent motor overloading.

For home circuits, the majority of circuits are rated for 15 to 20 amps, and if you understand how much current your lights and appliances use, you can roughly estimate how much current is safe to put in.

It may be necessary to move the light strand to another room if your load is approaching 80% before plugging in the next strand to avoid overcurrent.

• Never use extension cords without asking an authorized electrician.
• Never overload the outlet with a lot of appliances.
• Ask for advice before connecting any new appliance.
• If you buy a new house, hire an authorized electrician to check its circuits and wiring.

Use large appliances with caution

When searching for a location to plug your lights in, it’s a good idea to stay away from spaces that already include a lot of equipment, such as the kitchen.

To make more current available for those priceless decorations, you may disconnect any equipment or devices you don’t intend to use to avoid overcurrent.

Make LED lighting a priority

Investing in LED lights is one method to make your current setup considerably more festive.

LED lights consume significantly less power than conventional bulbs, saving you money on your electricity bill and easing the pressure on your wiring to avoid overcurrent.

Use Protection devices

The built-in defense against overcurrent in your home is the circuit breaker. They are housed in your electrical panels and turn off electricity to particular areas of the structure if they detect erratic electrical currents.

You may protect yourself against short circuits, when necessary, by doing routine circuit breaker maintenance to make sure it is operating correctly.

Why does the current increase when the load increases?

Each load requires its own current, the more connected loads, the higher the current they draw. To explain it to you, suppose the electrical circuit is a pipeline, the loads are faucets, and the more faucets you open the more water you draw.

In the case of electrical Motors

In case we have a motor (the load), this motor requires electric current depending on its power rating.

If the motor starts with no load, it draws a no-load current, it’s smaller than the full-load current.

By increasing the mechanical load, the motor needs to provide more torque to overcome this load, and of course, it will draw more current .

In case of home loads

In case of connecting more loads on home or a building outlet. Electrical loads are typically connected in parallel.

If you have a load raised, it means the added load is connected in parallel with the existing load.

Parallel connections always decrease the equivalent impedance. Therefore, the current increases.

When loads are connected in parallel, each load draws its own currency from the circuit according to the requirements of that load. Therefore, the current increases when the load is increased.

How can I tell if my electrical panel is overloaded?

If your panel main circuit breaker trips , and when you turn it on again it works for a while and then trips again, it’s a clear sign of a possible overloaded panel.

But, keep in mind that there are other reasons for circuit breaker tripping, you can hire an electrician to measure the currents of the panel, check the man circuit breaker rating, and make sure it’s overloaded.

If you know how to use a current clamp, you can measure the current by it, check the circuit breaker rating, and make sure the CB rating is greater than the measured one.

Make sure to switch on all loads during the measuring process, this is the only way to have a clear idea about the panel load.

One more thing, an electrical engineer can calculate the panel loads, check the circuit breaker’s ratings, and make sure it’s not overloaded.

It’s crucial to watch out for clues that your electrical panel could be generating an excessively high level of electricity when you start putting in more and more appliances for any reason.

To assist you in recognizing when this could be happening, we’ve listed a few telling indications.

Panel Overheating, the overheating that might result from excessive current flowing through your breakers or cables is also a sign of an overloaded panel.

In my workplace, we use thermal imaging to check that panels are not overloaded.

What happens when an electric current increases?

When the electric current increases, the following points can happen:

In the case of electric motors

The higher the current, the higher it temperature rise. This temperature increase can cause the motor insulation to fail, and lead to an internal short circuit between windings and complete damage if the protection device fails to trip the circuit.

In the case of home wiring

Excessive current trips the circuit breaker of your home panel. Of course, it will increase the temperature of the wiring, the panel, and the circuit breaker.

If the increased current is caused by an appliance, the appliance will get damaged if the CB fails to trip.

Ready to dive in? Check out my comprehensive article Avoiding Overcurrent Situations: Tips to Keep Your Circuits and Devices Safe now

In the case of electric cables

Cable temperature rise can cause the insulation to get damaged. When the insulation gets damaged, the live phase and the neutral or another phase will get in touch and a short circuit happens.

In the case of an electric generator

The generator will heat up, its winding insulation will melt and a short circuit occur between its phases. Yep, it’s complete damage.

We can say, that an overcurrent could cause the circuit wiring, as well as the electrical equipment to overheat. This could lead to, equipment damage or melting of the insulation and a possible fire.

The heat loss from increasing current can cause damage to the circuit, burn resistors, damage electrical equipment and home appliances, or even cause fire to the surroundings.

Circuits are designed to work with a particular voltage and resistance . If the excessive current flows in a conductor , the results will be very bad if the protection device fails to trip the circuit.

Why does the current increase in temperature?

From a physics point of view.

Electric current is the flow of electrons in a conductor. As the current increases, the electron flow will increase.

Due to the increased flow of electrons, the collision of electrons will intensify generating heat energy and power losses. This is why the electric current increases the temperature.

From an electricity point of view,

any conductor has a resistance, when current flows through the conductor it faces its resistance.

From the power equation we have, Power = I 2 *R, which means that increasing ‘I’ and the ‘R’ increases the power loss in the shape of heat and raises the temperature.

The temperature rise beyond the allowed limits on any device is directly linked to the fact that you have exceeded power consumption higher than the rated value.

Any equipment is designed to meet a specific power consumption, if the limit is exceeded its power, gets heated.

ِAny electric current value causes produce temperature, the more current the higher the temperature rise.

Starting from the equipment-rated current to the overcurrent and ending at the short circuit current. All these currents produce heat, and the value of the temperature rise differs from one currency to another. 

Read also my article about Electric transformer temperature rise .  and the other one , Motor temperature rise causes and limits.

Why does an increase in current increase power losses?

Power loss is a result of the conductor resistance that the current faces while passing through the conductor.

As you can see, from the below power loss equation, the power is proportional to the square of the current. So, if the current increases, the power losses will increase much faster.

As we know, Power = I 2 × R, This means, the higher the current the higher energy loss in the conductor in the form of heat . As I mentioned above.

The increase of power loss in the conductor increases the conductor’s temperature. We don’t have an ideal conductor with zero resistance, if we do its power loss will be zero.

What is Overcurrent Protection?

Overcurrent protection is a fundamental aspect of electrical safety that involves the use of devices and measures to detect and limit excessive electrical current in a circuit.

The primary purpose of overcurrent protection is to prevent electrical circuits and equipment from being subjected to current levels that exceed their designed capacity, which can lead to overheating, damage, fires, and safety hazards.

Overcurrent protection is achieved through various protective devices and strategies, including:

Fuses : Fuses are passive overcurrent protection devices that consist of a wire or element that melts or breaks when subjected to excessive current. When a circuit experiences an overcurrent condition, the fuse element melts, opening the circuit and disconnecting power. Fuses are available in various ratings to match the current-carrying capacity of the protected circuit.

Circuit Breakers : Circuit breakers are automatic overcurrent protection devices that can be reset after they trip. They consist of a switch-like mechanism that opens the circuit when an overcurrent condition is detected. Circuit breakers are available in different types, including thermal-magnetic and electronic, and they can be designed to provide protection against short circuits, overloads, or both.

Ground Fault Circuit Interrupters (GFCIs) : GFCIs are specialized overcurrent protection devices used to prevent electrical shock hazards. They detect imbalances in current flow between the hot and neutral conductors and trip the circuit if a ground fault is detected. GFCIs are commonly used in locations where electrical devices may come into contact with water, such as bathrooms and kitchens.

Arc Fault Circuit Interrupters (AFCIs) : AFCIs are designed to detect and interrupt arcing faults, which can lead to electrical fires. They monitor the circuit for abnormal arcing conditions and trip the circuit if such conditions are detected, reducing the risk of fire.

Motor Overload Relays : These are used to protect electric motors from overcurrent conditions that can occur during motor startup or due to mechanical problems. Overload relays monitor the motor’s current and trip the circuit if the current exceeds a specified threshold for an extended period.

Current Limiting Devices : These devices are designed to limit the magnitude of fault currents in a circuit, reducing the potential damage caused by short circuits and overcurrent events.

Selective Coordination : In complex electrical systems, selective coordination is a strategy that involves coordinating the settings of protective devices (e.g., circuit breakers and fuses) to ensure that only the device closest to the fault opens, minimizing downtime and disruptions in the event of an overcurrent condition.

Proper Circuit Design : Ensuring that circuits are appropriately designed with proper wire sizes, protection devices, and load considerations is essential for overcurrent protection. Designing circuits to match the expected loads and operating conditions helps prevent overcurrent situations.

Overcurrent protection devices and strategies are critical for electrical safety, as they help prevent electrical fires, equipment damage, and electrical hazards.

The selection and installation of the appropriate protective devices depend on the specific requirements and characteristics of the electrical circuits and equipment being protected.

Over-current protection importance

In order to keep yourself and others safe from many risky situations, overcurrent protection is important.

Every electric circuit must have circuit overcurrent protection. If the current levels of an electric circuit exceed the safe limits for which they were built, the circuit may be harmed or even destroyed.

Circuit wires may become too hot if there is an overcurrent. In turn, fire and insulation melting might result due to this situation.

There may be serious consequences if a circuit is not equipped with overcurrent protection. An electrical shock, fire, or electrocution can result from overcurrent, which can destroy unprotected electronic devices and cause human injury and fatality.

The purpose of an overcurrent protection device is to protect against dangerously high temperatures in conductors or their insulation . It is crucial to match the conductor size and current rating of the protection device.

We can say, that overcurrent protection is essential because it protects humans and equipment as well against destructive and fatal accidents.

Is current protection and surge protection the same?

No, it’s different; overcurrent protection protects the excessive current flow in the circuit, and surge protection protects against excessive voltage or spikes of voltage to the circuit.

Overcurrent protection is the protection against excessive currents beyond the acceptable current rating of the equipment. It generally operates instantly.

Magnetic circuit breakers, fuses , and overcurrent relays commonly provide overcurrent protection.

High and low-voltage power distribution systems, as well as control systems, frequently incorporate overcurrent protectors.

On the other hand, Surge protection protects equipment against power surges and voltage spikes while blocking voltage over a safe threshold.

When a threshold is an overrated voltage, a surge protector shorts to ground voltage or blocks the voltage.

Low-voltage power distribution systems frequently utilize SPD, sometimes referred to as surge protector, lightning protector, and lightning arrester, as a lightning protection device.

It is often linked to the line in parallel or series to discharge the wave.

Difference Between Overcurrent and Overload Protection

Overloading equipment causes it to draw an overcurrent. We, electrical engineers, usually use the word overload in case the equipment gets more load, electrical or mechanical, than it’s designed to handle.

For example, a motor is overloaded means its mechanical load is greater than the motor-rated power so, it draws an overcurrent.

Another example is when a cable is overloaded, it means the cable load, electrical load, draws current greater than the cable current carrying capacity, so the cable has over current passing through it.

Overcurrent protection is the protection against excessive currents beyond the acceptable designed current rating of equipment or an electric device . Short circuits, arc faults,s, and earth faults are overcurrent types.

On the other hand, overload protection is protection against overloading equipment that causes it to face an overcurrent situation, and would cause the equipment to overheat.

A well-designed circuit and periodical inspection of loads can help prevent overload

Hence, an overload is also some kind of overcurrent. Slow-acting fuses and overload relays are commonly used for overload protection devices.

A thermal-magnetic CB as well as the dual element fuse has both thermal and magnetic elements which means that it could provide both overcurrent and overload protection.

How does overcurrent protection work?

According to the circuit current rating of equipment or circuit, overcurrent protection devices have a current rating.

In case of any fault, when the circuit current exceeds the rating current of protection devices, it cuts the circuit’s supply, by the thermal or magnetic effect of the overcurrent. As you know, current has both thermal effects, and if the current passes through a coil it will produce a magnetic effect.

The thermal effect can melt a fuse to protect the circuit and can trigger the mechanical mechanism of a circuit breaker to trip the circuit.

The magnetic effect also can trigger the circuit breaker faster than the thermal effect. By using this working principle, overcurrent protection devices protect equipment or circuits.

The thermal effect is used for overload protection, lower values of overcurrent, while the magnetic effect, the faster, is used for protection against the short circuit over currents.

Fuse and circuit breakers are overcurrent protection devices that contain time/current characteristics (TCC) that specify how long it takes to clear the fault for a specific value of fault current. The higher the overcurrent value, the faster the tripping time of the circuit breaker.

The fuse’s metal strip or wire melts when too much current flows across it, cutting off the current flow. Fuses are sacrificial components, which means an overcurrent destroys them.

On the other hand, circuit breakers turn off when they face any fault or a short circuit, unlike fuses, which melt to break the circuit. Circuit breakers can therefore be reused.

Can overcurrent damage an overcurrent protection device?

Yes, overcurrent can potentially damage an overcurrent protection device, such as a fuse or a circuit breaker, if the device is subjected to current levels exceeding its rated capacity for an extended period.

While these protection devices are designed to withstand short-term overcurrent conditions and perform their protective function, they are not invulnerable and can be damaged under certain circumstances:

• Sustained Overloads : If a circuit experiences a sustained overload, where the current exceeds the device’s rated capacity for an extended period, it can cause overheating of the protection device. Prolonged overheating may damage the device, affecting its ability to function properly in the future. For example, a fuse may blow or a circuit breaker may trip due to overheating.
• Fault Currents : In the case of a short circuit or a severe fault current event, the magnitude of the current can be extremely high. While protection devices are designed to handle these situations and quickly interrupt the fault current, the stress from such high current levels can potentially damage the internal components of the device.
• Inadequate Device Rating : Using a protection device with an inadequate current rating for the circuit it is intended to protect can result in the device being damaged during an overcurrent event. For instance, if a lower-rated fuse or circuit breaker is used in a circuit with a consistently high current load, it may trip or blow repeatedly, leading to damage.
• Device Wear and Tear : Like any mechanical or electrical component, overcurrent protection devices can experience wear and tear over time. Frequent tripping, exposure to adverse environmental conditions, or improper handling can contribute to device degradation and reduce their effectiveness.

It’s essential to select protection devices with appropriate current ratings for the circuits they are intended to protect.

Regular maintenance and inspection of protection devices are also important to ensure their continued reliability and performance.

Damaged or malfunctioning protection devices should be replaced promptly to maintain electrical safety and prevent potential hazards.

Can over-current damage a circuit breaker?

Yes, overcurrent can potentially damage a circuit breaker, but circuit breakers are designed to withstand and interrupt overcurrent conditions within their specified ratings.

However, there are limits to their capacity, and extreme overcurrent events or sustained overloads can lead to damage or failure.

Here are some considerations regarding overcurrent and circuit breaker damage:

To prevent damage to circuit breakers and ensure their reliable operation, it’s important to adhere to the following practices:

• Select circuit breakers with appropriate current ratings for the circuits they protect.
• Avoid overloading circuits beyond their designed capacity.
• Properly maintain and inspect circuit breakers to identify signs of wear or damage.
• Replace damaged or malfunctioning circuit breakers promptly.
• Ensure that circuit breakers are operated within their specified voltage and current ranges.

While circuit breakers are designed to provide overcurrent protection and are more robust than fuses, they are not immune to damage under extreme or improper operating conditions.

Regular maintenance and adherence to electrical safety standards are essential to maintain the integrity and reliability of circuit breakers in electrical systems.

For more information about Why circuit breakers go bad, read my article.

What happens if an over-current protection device is oversized?

If an over-current protection device (such as a fuse or circuit breaker) is oversized for the circuit it is meant to protect , it can lead to several potential issues and compromises in electrical safety and equipment protection. Here are some consequences of using an over-sized protection device:

• Reduced Protection : The primary purpose of an over-current protection device is to safeguard the circuit and connected equipment from excessive current levels, which can cause overheating, fires, and equipment damage. When an oversized device is used, it may not trip or open the circuit as intended when overcurrent conditions occur. This means that the circuit and equipment may not be adequately protected from short circuits, overloads, or faults.
• Increased Fault Energy : An oversized protection device can allow excessive fault currents to flow without tripping or interrupting the circuit. This can result in higher fault energy levels in the event of a short circuit, increasing the potential for electrical arc flash incidents and equipment damage.
• Equipment Damage : Oversizing protection devices can lead to damage to downstream equipment. For example, if a circuit breaker with a much higher current rating than the circuit’s load capacity is used, the equipment connected to the circuit may not be adequately protected from overcurrents. This can lead to overheating and damage to the equipment, including motors, transformers, and wires.
• Inefficiency : Oversized protection devices can be less efficient in terms of energy consumption. They may allow unnecessarily high currents to flow for longer periods before tripping, leading to increased energy losses and potentially higher operating costs.
• Violation of Electrical Codes and Standards : Oversizing protection devices can lead to non-compliance with electrical codes and standards. Electrical codes specify the correct sizing and selection of protection devices based on the characteristics of the circuit and equipment being protected. Using an over-sized device may result in non-compliance and regulatory violations.
• Misleading Fault Analysis : Oversized protection devices can make it challenging to identify and analyze faults or overcurrent events within a circuit. Since the device does not operate as expected under fault conditions, troubleshooting and diagnosing electrical issues can become more complex.

To ensure proper electrical safety and equipment protection, it is crucial to select and install protection devices with current ratings that match the circuit’s load and characteristics.

The selection of protection devices should be in accordance with electrical codes and standards. Additionally, regular maintenance and testing of protection devices are essential to verify their proper operation and compliance with safety requirements.

Oversized protection devices should be replaced with appropriately sized ones to maintain electrical safety and equipment protection.

What if an overcurrent protective device opens slower than expected?

If an overcurrent protective device, such as a circuit breaker or a fuse, opens slower than expected, it can have several implications and potential consequences, depending on the specific circumstances and the nature of the overcurrent condition:

• Equipment Damage : Slower tripping of the protective device can allow excessive current to flow through the circuit for a longer duration. This prolonged overcurrent can result in overheating of wiring, equipment, and components, potentially causing damage or degradation.
• Fire Hazard : Overcurrents can generate heat, and if the protective device does not trip promptly, the prolonged overcurrent can increase the risk of electrical fires. Electrical components and insulation may become damaged due to excessive heat.
• Safety Hazards : Slower tripping of protective devices can pose safety hazards to individuals in the vicinity. For example, if a fault condition occurs and the circuit breaker does not trip quickly, it may expose people to electrical shock hazards.
• Compromised Protection : The primary purpose of overcurrent protection is to prevent electrical circuits and equipment from being subjected to current levels that exceed their designed capacity. When a protective device opens slower than expected, it compromises the protection provided by that device.

To address the situation when an overcurrent protective device is not operating as expected, here are some steps to consider:

• Investigation : Identify the cause of the slow operation. It could be due to a malfunctioning protective device, loose connections, or other issues within the electrical circuit.
• Immediate Shutdown : If there is an ongoing overcurrent condition and the protective device is not responding as expected, it is essential to de-energize the circuit immediately to prevent further damage or hazards. Disconnect power at the main switch or circuit breaker panel if necessary.
• Professional Inspection : Seek the assistance of a qualified electrician or technician to inspect the protective device, the circuit, and any associated equipment. They can diagnose the issue and recommend appropriate corrective actions, such as repairing or replacing the protective device.
• Maintenance and Testing : Regularly maintain and test protective devices to ensure they operate within their specified response times. Periodic testing and maintenance can help identify potential issues before they become critical.
• Replacement : If the protective device is found to be faulty or not operating correctly, it should be replaced promptly with a properly rated and functioning device to restore adequate protection.

Ensuring that overcurrent protective devices operate correctly and promptly is crucial for electrical safety and equipment protection.

Any signs of malfunction or unexpected behavior should be addressed promptly to prevent potential hazards and equipment damage.

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Terms and Definitions

3 Overload and Overcurrent Protection

Click play on the following audio player to listen along as you read this section.

Inrush Current

When a motor is first started, before the shaft has a chance to pick up speed and begin to rotate, the characteristics of the stator coil are that of a short circuit. As such the motor starts to draw very high values of current . This current creates a magnetic field that causes the motor shaft to spin, and that spinning action creates a counter-EMF (CEMF), which limits the current to its normal running value.

The initial high value of current is called inrush and can cause severe line disturbances and nuisance tripping if fuses and circuit breakers are not sized accordingly.

The term “ overload ” describes a moderate and gradual rise in the value of current over a relatively long period of time. It is caused by excessive amounts of current drawn by a motor, which may be as high as six times the rated current. This is caused by too much load on a motor. Systems are protected by overload protection relays . While overloads are allowed for a short time (usually minutes), prolonged overloads will use thermal action to cause a protective device to trip.

Overcurrent

The term “ overcurrent ” (sometimes called a short circuit or a ground fault) describes a sharp and fast rise in current over a short period of time (fractions of a second). Circuits and equipment are protected from overcurrent situations by fuses or circuit breakers.

In these cases, the value of current is far greater than the nominal line current and can indeed be anywhere from six times to many hundreds of times greater the normal rated current value of current.

There are several causes of overcurrent situations. For example, when a bolted fault occurs—either a line to ground or a line to line fault. This causes a very large value of current to be drawn because of the inversely proportional relationship between the resistance of a circuit and the current drawn.

Another less intuitive cause of short circuits is when an induction motor starts. When a three-phase induction motor is first energized, the stator windings consist of a very low resistance path. This draws a very large inrush current which is indistinguishable from a standard short circuit, except that it quickly drops down to the rated value of current drawn by the motor. This is due to the CEMF (counter-electromotive force) developed by the rotating shaft of the motor. When the motor is spinning, a CEMF limits the current to safe values. When the motor is not spinning, a very large value of current is drawn from the source. This current is sometimes called locked-rotor current , and motor starters and overcurrent devices must be rated to safely handle this value of current.

Effects of short circuits

Two of the main negative outputs of overcurrents are:

• Thermal energy : High values of current will create lots of heat, which can damage equipment and wires. Thermal energy can be expressed by I 2 t (current squared times time)—the longer the fault persists, the greater the potential thermal damage.
• Mechanical forces : High-fault currents can create powerful magnetic fields and exert huge magnetic stress on busbars and equipment, sometimes warping them out of shape and creating other problems.

Large values of fault current can cause damage very quickly, so overcurrent protective devices must act very quickly to clear the fault. There are two main categories of overcurrent protective devices: fuses and circuit breakers.

A fuse is a simple device that protects the conductors and equipment of a circuit from damage due to higher than normal fault values. It is designed to be the weakest link in a circuit.

A fuse is an insulated tube containing a strip of conductive metal (fuse-link) that has a lower melting point than either copper or aluminum. The fuse link has narrow, resistive segments that concentrate the current and cause the temperature at those points to rise.

In a short circuit, the fuse elements burn open in just a fraction of a second. The higher the values of fault current, the faster the fuse will react.

In an overload situation, the fuse elements can take many seconds or even minutes before thermal actions cause the fuse link to melt open.

Fuses come in two categories: Fast-acting fuse (Type P) and time-delay fuse (Type D).

Fuses used in motor circuits have to withstand the intense inrush current when the motor is started, and so we use time-delay fuses, also known as “dual-element fuses.”

Common ratings

All overcurrent devices must be operated within their rated values. Three of the most important ratings are voltage, current, and interrupting capacity.

Voltage rating

Fuses and circuit breakers must be rated for at least the value of the voltage of the circuit they are designed to protect.

When a fuse or circuit breaker interrupts a fault current, it must safely extinguish the arc and prevent it from reestablishing itself. Therefore, the voltage rating of a fuse or circuit breaker must be equal to or greater than the system voltage.

For example, a fuse rated at 240V RMS will be acceptable for use in a 120V circuit. However, it would exceed the fuse’s voltage rating to use it in a 600V circuit.

Continuous-duty rating

Continuous-duty rating describes the maximum rated value of RMS current that the overcurrent device is designed to handle on a continuous basis without tripping. Generally speaking, the ampere rating of the fuse or breaker should not exceed the current carrying capacity of the circuit, but there are exceptions, such as certain motor circuits.

Interrupting capacity

When a short circuit or ground fault occurs, the circuit resistance drops to effectively zero ohms , causing very large values of current to flow. This extremely fast rise in fault current can cause damage to wires and equipment through overheating and must be extinguished as quickly as possible.

The interrupting-capacity (IC) rating of an overcurrent device is the maximum fault current that the device can interrupt without damage to itself. Most circuit breakers and fuses have an IC rating of 10,000 amps.

For systems capable of larger fault currents, high-rupture capacity (HRC) fuses can interrupt currents up to 200,000 amps by using an arc-quenching filler such as silica sand to help interrupt the fault.

The rate of flow of an electric charge, measured in amperes (or amps). When one coulomb of charge moves past one point in once second, current is said to flow at a rate of one ampere. Current flows from negative potential to a positive potential through a load.

The initial high value of current produced when an inductive load is first energized.

An insulated tube containing a strip of conductive metal that has a lower melting point than either copper or aluminum. It protects a circuit from damage because it will melt in overload or overcurrent situations and break the connection with the rest of the circuit.

An automatic device that is designed to safely disconnect circuits under fault conditions. Most circuit breakers provide Overload and Overcurrent protection, and are rated in Volts, Amps and Horsepower.

A moderate and gradual rise in the value of current over a relatively long period of time that is caused by excessive amounts of current drawn by a motor due to too much load being put on the motor.

A heater element paired with normally-closed contacts that open once the heater gets too hot. Two types of relays are the bimetallic strip and the melting solder pot.

A sharp and fast rise in current over a short period of time (fractions of a second) where the value of current is far greater than the nominal line current.

The opposition to the flow of current in an electric circuit, measured in ohms (Ω).

The current drawn by a motor when the motor is not spinning.

A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide overload protection .

The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts (V).

The maximum amount of voltage that a fuse, circuit breaker, switch-gear or motor starter can handle. The voltage rating of a fuse or circuit breaker must be equal to or greater than the system voltage.

The maximum rated value of RMS current that the overcurrent device is designed to handle on a continuous basis without tripping.

The unit used to measure electrical current. It is equal to a flow of one coulomb per second. It may also be called "amp."

The unit used to measure electrical resistance (Ω). It takes one volt to push one amp through one ohm of resistance.

The maximum fault current that an overcurrent device can interrupt without damage to itself. Most circuit breakers and fuses have an IC rating of 10,000 amps.

Basic Motor Control Copyright © 2020 by Aaron Lee and Chad Flinn is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Overcurrent Protection of ship electrical system

The general term “overcurrent” applies to a relatively small increase over the full load current ( FLC ) rating (e.g. due to mechanical overloading of a motor) rather than the massive current increase caused by a short-circuit fault.

Generally, an overcurrent , supplied from a CT, is detected by a relay with an appropriate time-delay to match the protected circuit.

Short-circuit faults in LV distribution circuits are mainly detected and cleared almost instantaneously bf fuses, MCCBs or MCBs.

Main supply feeders are usually protected against short-circuits by circuit breakers with instantaneous magnetic trip action.

Overcurrent relay types on ship:

Magnetic Thermal Electronic

All relay types have an inverse current- time characteristic called OCIT (over- current inverse time), i.e. the bigger the current the faster it will operate.

A magnetic relay , directly converts the current into an electromagnetic force to operate a trip switch.

An electronic overcurrent relay usually converts the measured current into a proportional voltage. This is then compared with a set voltage level within the unit which may be digital or analogue.

In analogue unit the time delay is obtained by the time taken to charge up a capacitor.

This type of relay has separate adjustments for overcurrent and time settings together with an instantaneous trip.

Both the magnetic and electronic relays can be designed to give an almost instantaneous trip (typically less than 0.05 seconds or 50 ms) to clear a short- circuit fault.

Thermal relays are commonly fitted in moulded case circuit breakers (MCCBs) and in miniature circuit-breakers (MCBs) to give a “long time” thermal overcurrent trip in addition to a magnetic action for an instantaneous trip with a short-circuit fault.

Overcurrent protection relays in large power circuits are generally driven by current transformers(CTs).

All overcurrent relays can be tested by injecting calibrated test currents into them to check their current trip levels and time delay settings.

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What Happens When an Electrical Circuit Overloads

Timothy Thiele has an associate degree in electronics and is an IBEW Local #176 Union Electrician with over 30 years of experience in residential, commercial, and industrial wiring.

Southern Stock / Getty Images

If you’ve ever plugged in one too many holiday lights, switched on a vacuum, or cranked up a space heater only to have the lights or appliance suddenly shut off, you’ve created an electrical circuit overload. The shutdown was triggered by the circuit’s breaker (or fuses ) in your home’s service panel. And while circuit breakers are reliable and do a good job preventing house fires due to overloads, the safest strategy is to manage your electricity usage to prevent overloads in the first place.

What Is an Electrical Circuit Overload?

An electrical circuit overload occurs when you draw more electricity than a circuit can safely handle.

How Do Electrical Circuit Overloads Work?

Electrical circuits are designed to handle a limited amount of electricity. Circuits are made up of wiring, a breaker (or a fuse, in old wiring systems), and devices (such as light fixtures, appliances, and anything plugged into an outlet). The electricity usage of each device (when running) adds to the total LOAD on the circuit. Exceeding the rated load for the circuit wiring causes the circuit breaker to trip, shutting off the power to the entire circuit.

If there were no breaker in the circuit, an overload would cause the circuit wiring to overheat, which could melt the wire insulation and lead to a fire. Different circuits have different load ratings so that some circuits can provide more electricity than others. Home electrical systems are designed around typical household usage, but there’s nothing to prevent us from plugging in too many devices on the same circuit. However, the more you know about the layout of your home’s circuits the more easily you can prevent overloads.

Signs of Overloaded Circuits

The most obvious sign of an electrical circuit overload is a breaker tripping and shutting off all the power. Other signs can be less noticeable:

• Dimming lights, especially if lights dim when you turn on appliances or more lights.
• Buzzing outlets or switches.
• Outlet or switch covers that are warm to the touch.
• Burning odors from outlets or switches.
• Scorched plugs or outlets.
• Power tools, appliances, or electronics that seem to lack sufficient power.

Buzzing sounds , burning smells, and unusually warm devices also can indicate other wiring problems, such as loose connections or short circuits . If any of these problems persist after you’ve taken steps to prevent circuit overloads, contact an electrician.

Mapping Your Home’s Circuits

The first step to preventing electrical circuit overload is to learn which circuits power which devices. When you’ve mapped the basic circuit layout, you can calculate the safe load rating of each circuit to get a sense of how many things you can operate on that circuit. For example, if your kitchen lights dim when you turn on your toaster oven (a power-hungry appliance), that tells you that the toaster and lights are on the same circuit (even though they shouldn’t be) and that you’re close to maxing out the circuit capacity. Mapping the circuits also can tell you if there’s a need for new circuits to meet the normal demands of the household.

Mapping circuits are simple (if repetitive): Get a notepad and a pencil. Open the door to your home’s service panel (breaker box) and turn off one of the breakers with the number 15 or 20 stamped on the end of the breaker switch. (Don’t bother with the breakers stamped with 30, 40, 50, or higher numbers; these are high-voltage circuits for appliances like electric ranges, water heaters, and clothes dryers, and you’re not plugging ordinary appliances into these circuits.) Note on the pad where the circuit lies in the panel so you can identify it later.

Next, walk through the house and try all the lights, ceiling fans, and plug-in appliances. Write down everything that doesn’t have power, and note the room it is in. Also, test each outlet with a voltage tester or receptacle tester, or even a plug-in light or lamp, recording all that don’t work. You don’t necessarily have to go through the entire house for each circuit. And if your electrician was thorough, there may be helpful labels next to the breakers, indicating the circuit areas (“Southeast bedroom,” “Garage lights,” etc.). But for accurate mapping, you should test each area broadly because circuits can have oddball members—a microwave on a hallway lighting circuit, for example.

After you’ve tested the circuit area, go back to the panel, turn on the first breaker, then turn off the next one in the row, and repeat the test. Repeat the process for all of the “15” and “20” circuits.

Calculating Circuit Loads

Your circuit map tells you which devices are powered by each circuit. Now you have to calculate how much power those devices are using. To do that, you need a quick lesson in electrical energy. Electricity is measured in watts; a 100-watt light bulb uses 100 watts of electricity. A watt is the product of voltage (volts) and amperage (amps):

1 volt x 1 amp = 1 watt

To calculate the total load on each circuit, add up the wattage of all the devices on that circuit. Light bulbs and many small appliances have labels noting their wattage. If a device gives you only amps, multiply the amp value by 120 (the voltage of standard circuits) to find the wattage. Include all devices that are permanently wired to the circuit as well as plug-in appliances that you don’t move very often (like a toaster oven, or a heater in a particularly cold room).

Compare the total wattage of each circuit to the load rating of that circuit. The circuits with “15” breakers are rated for 15 amps. The maximum load rating of one of these circuits is 1,800 watts:

120 volts x 15 amps = 1,800 watts

If you try using more than 1,800 watts on that circuit, you will overload it, and the breaker will trip.

The circuits with “20” breakers are rated for 20 amps and have a maximum load rating of 2,400 watts:

120 volts x 20 amps = 2,400 watts

Compare the wattage total (how much electricity you’re using) and the load rating for each circuit. For example, a 15-amp circuit serving lights and outlets in a living area might be providing power for 500 watts for lighting, 500 watts for the TV and cable box, and 200 watts for the sound system, for a total of 1,200 watts. If you plug in a 700-watt vacuum while the TV, stereo, and lights are on, you’ll exceed the 1,500-watt rating on the circuit breaker, causing it to trip and shut off the power.

The maximum load on each circuit isn’t the ideal target. For a margin of safety, it’s best if the normal load on a circuit does not exceed 80 percent of the maximum (rated) load. For a 15-amp circuit, the safe load target is 1,440 watts; for a 20-amp circuit, the safe load is 1,920 watts.

If your circuit calculations indicate that you’re drawing more wattage from a circuit than the safe load number—or you’re exceeding the rated load and frequently overloading the circuit—there are a few ways to reduce the load on the circuit to prevent overload:

• Move plug-in appliances to a circuit that is less-used (use your mapping and circuit calculations to identify circuits that have plenty of available wattages).
• Remember not to turn on too many things at once. For example, turn off the TV and sound system while you vacuum (you can’t hear them anyway).
• Reduce lighting loads by replacing incandescent or halogen light bulbs with energy-efficient LED (preferably) or CFL (fluorescent) bulbs.
• Install new circuits for high-demand devices. For example, if you run a lot of power tools in your garage workshop, but your garage is wired with all of the outlets and lights on the same 15-amp circuit, install a new 20-amp circuit supplying a few new outlets for your tools.

Electrical Safety in the Home . National Fire Protection Agency.

More from The Spruce

• Codes and Standards

Overcurrent Protection: Fuses or Breakers?

When designing non-residential electrical systems, engineers must consider electrical protection from many perspectives. although the entire electrical distribution system of a facility is important—from switchyard to light bulbs—“the overcurrent protective system is the very heart of the electrical distribution system,” wrote george farrell and frank valvoda, pe, in a 1....

What has changed since the first publication of the popular article series, “The Art of Protecting Electrical Systems,” is the introduction of more electrically intuitive devices, which have changed electrical design practices significantly.

Overcurrent

Overcurrent is current that exceeds the ampere rating of conductors, equipment or devices under conditions of use, and includes both short circuits and overloads. During a short circuit, current flows outside its normal path. Insulation breakdown or faulty equipment connections can cause short circuits. The load determines circuit current during normal fault-free conditions. However, during a short circuit, electrical current bypasses the load, taking the path of least resistance. System impedance—or AC resistance—determines short circuit or fault current magnitude, which ranges from fractions of an amp to 200 kA or more.

An overload is an overcurrent condition within normal current paths—there is no insulation breakdown. However, if an overload is allowed to persist, it causes equipment or wiring damage. Temporary overloads can be harmless; sustained overloads can cause damage.

Temporary overloads may be caused by momentarily pushing equipment past its limit, such as cutting too deeply with a milling machine or from starting large motors or other inductive loads. Temporary overloads occur frequently, are typically harmless and should be allowed to subside. An overcurrent protective device (OCPD) should not open the circuit, allowing motors to start and loads to stabilize.

Sustained overloads can be caused by continually overloading electrically-driven mechanical equipment, failed bearings or other equipment malfunctions. They also are caused by installing loads such as equipment or additional lighting circuits that increase power demand beyond planned capacity. If sustained overloads are not disconnected within appropriate time limits, they eventually will overheat circuit components and cause thermal damage to insulation and equipment.

When starting up a new facility, after system modification or equipment installation, it’s possible to encounter crossed phase wiring, or perhaps even bolted faults. “Bolted faults are characterized by a solidly connected fault path causing high levels of current to flow through this solid connection,” said Todd Lottman, product manager of services at St. Louis-based Cooper Bussmann. “This type of fault is commonly used when testing electrical equipment for short-circuit current ratings and overcurrent protective devices for interrupting ratings.”

Protecting with fuses and breakers

Overcurrent scenarios dictate the type of overcurrent protection that should be used. The National Electrical Code (NEC) has established basic power system overcurrent protection requirements and recognizes fuses and circuit breakers as the two basic types of OCPDs. According to the NEC, a fuse is an overcurrent protective device with a circuit-opening fusible element that is heated and severed by the passage of overcurrent through it. A circuit breaker is a device designed to open and close a circuit by non-automatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.

Fuses and circuit breakers are available in a variety of sizes and ratings. Their similar yet different features and characteristics allow electrical system designers to choose devices appropriate for a facility’s electrical system.

Fuses are single-pole devices—an individual fuse can open only one phase of a multi-phase circuit. However, multiple individual fuses can be applied together in a disconnect to protect a multi-phase system. Low-voltage fuses are available in sizes from fractions of an amp to thousands of amps at voltage ratings up to 600 volts. They are available with short-circuit interrupting ratings of 200 kA or more.

Some fuse types are classified as current limiting. According to the NEC, current-limiting fuses “…reduce the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having comparable impedance.” This means that a current-limiting fuse will open quickly—within one-half cycle—when subjected to a high-level fault.

Fuses cannot be given an external command to trip. By nature, fuses offer very reliable current limiting features. Also, they can operate independently—they do not require an overload relay with instrument transformers to tell them when to blow.

When using fuses, a separate disconnect must be used in many situations because they are designed to open under overcurrent conditions only. However, when using circuit breakers, a separate disconnect is not required because breakers are designed to be opened and closed manually, as well as when subjected to an overcurrent condition.

Circuit breakers

Low voltage circuit breakers differ in construction, operation and maintenance requirements depending on how and where they are used. Circuit breakers are available as 1-, 2-, 3- or 4-pole devices, and rated from 10 amps to thousands of amps. Short-circuit interrupting ratings of circuit breakers are available up to 200 kA.

Low voltage circuit breaker types include molded-case circuit breaker (MCCB) and low-voltage power circuit breaker (LVPCB). The internal parts of an MCCB are enclosed in a molded case of insulating material. This type of breaker is not designed to be opened, which means that it is not field maintainable. MCCBs are used in panelboards, switchboards, motor control centers (MCCs), equipment control panels and as stand-alone disconnects inside separate enclosures.

LVPCBs are used in low-voltage drawout switchgear. They are typically larger and more rugged than MCCBs, and are usually field maintainable. One characteristic that most power circuit breakers have in common is they are rated for continuous operation at 100% of their current ratings in their enclosures, which is not the case with all types of low voltage circuit breakers. LVPCBs have short time and interrupting ratings, allowing them to be used for selectivity and coordination with downstream devices.

Low-voltage circuit breakers can have a toggle mechanism or a two-step stored energy mechanism. The MCCB has a toggle mechanism with a distinct tripped position, which is typically midway between on and off. The LVPCB has a two-step stored energy mechanism, which uses an energy storage device, such as a spring, that is charged and then released, or discharged to close the circuit breaker.

Current limiting OCPDs

Many fuses and some breakers are categorized as current limiting. Within its current-limiting range, a current-limiting device is designed to interrupt all currents; limit the peak current (compared to a solid conductor with the same impedance); and open the circuit in less than one-half cycle (at 60 Hz) after the occurrence of a fault.

A common fuse myth is that it will blow as soon as the current flowing through it exceeds its rated value. Truth is, a typical fuse has an inverse time-current characteristic: the higher the current, the faster the fuse will blow. As the amount of overcurrent increases, the opening time of the fuse decreases exponentially. However, a single fuse class has only a single time-current characteristic, which cannot be adjusted.

Whether fuses or circuit breakers, the current-limiting range of an individual protective device falls between its threshold current and its interrupting rating. Threshold current of an overcurrent device is the specific amount of current that causes it to open the circuit in less than

SCCR and interrupting ratings

It should be noted that there are different ratings involved in overcurrent protection. Short circuit current rating (SCCR) is the same as withstand rating. The rating represents how much short circuit current a device can withstand without self-destructing. “A withstand rating is the maximum RMS symmetrical short-circuit current at which the equipment has been tested under specified conditions,” explained Farrell and Valvoda. “At the end of the test the equipment must be in ‘substantially’ the same condition as prior to the test.”

SCCR is applicable to non-interrupting equipment including switches; busway or bus duct; switchgear and switchboards; motor starters; contactors; MCCs; and similar equipment. If a short circuit occurs, every component in the system through which the fault current flows must safely withstand the effects of the current, which include heating and magnetic stresses. The protective device must break the current path reliably and safely.

Article 100 of the NEC defines interrupting rating as “the highest current at rated voltage that a device is intended to interrupt under standard test conditions.” The Fine Print Note (FPN) to Article 100 states “Equipment intended to break current at other than fault levels may have its interrupting rating implied in other ratings, such as horsepower or locked rotor current.”

Section 110-9 of the NEC states: “Equipment intended to break current at fault levels shall have an interrupting rating sufficient for the system voltage and the current which is available at the line terminals of the equipment… Equipment intended to break current at other than fault levels shall have an interrupting rating at system voltage sufficient for the current that must be interrupted.”

Arc flash considerations

Existing facilities are investing intense efforts in complying with the NEC and NFPA 70E. It’s more difficult to change the status quo than to engineer in compliance at the beginning. However, opportunities for consulting engineers exist for both existing and new projects. New facilities can be designed correctly before construction begins. Existing facilities must be analyzed, and modified if necessary.

Regardless of whether an electrical system is being designed for a new facility or an existing one, an arc flash hazard analysis must be done to ensure workers are protected from this potentially lethal threat. It is necessary to know the bolted-fault-current value when doing arc flash analysis calculations. It is also necessary to know the available fault current, which should be available from the utility.

Bolted fault current is not the same as arcing current. “Arcing faults differ in the fact that the current actually flows through ionized air causing an arc,” Lottman said. “The major difference between these two types of faults is that the energy in a bolted fault condition is dissipated in the faulted equipment while an arcing fault releases energy out into the surrounding environment.”

Coordination issues

Selective coordination minimizes downtime caused by nuisance tripping. Joe Schomaker, senior product manager at Cooper Bussmann, said that selective coordination involves “isolating an overloaded or faulted circuit from the remainder of the electrical system by having only the nearest upstream overcurrent protective device open. Without selective coordination, a single faulted circuit can shut down an entire facility.”

Prior to NFPA 70E and the work done to develop IEEE 1584, most facilities were designed to protect the electrical system and its loads from damage while avoiding nuisance interruptions. However, protecting workers from arcing faults is now a necessary part of the equation. Safety should never be an afterthought.

On the surface it appears that selective coordination and safety from arc flash hazards are opposites. However, some believe that coordination and safety can be achieved in the same system. “Protecting people, while protecting the system and providing continuity of service are not mutually exclusive goals,” said Joe Weigel, product manager, Square D Services, Schneider Electric, in Nashville, Tenn. “But the people-protection aspect is causing electrical designers to reconsider some of the design practices that they traditionally employed—including their choices of overcurrent protective devices.”

Weigel said the primary strategy to reduce the incident energy released during an arcing fault is to detect and clear the fault as quickly as possible. Because of this, there is often a fine line between optimal arc flash energy reduction and system coordination to provide continuity of service. “For example,” Weigel said, “one way to significantly reduce the arcing fault incident energy release is to lower the ‘instantaneous’ setting on the circuit breaker trip unit (if it has that function). However the instantaneous setting cannot be randomly set at its minimum setting or nuisance tripping is likely to occur as the loads attempt to start. So a time-current coordination study is required, and the coordination study is a critical element of the arc flash hazard analysis for that reason. Once the instantaneous has been properly set based on the coordination study and arc flash analysis, it should never be reset by anyone; unless changes in the arc flash incident energy potential release is considered.”

“Circuit breakers can be used in selectively coordinated electrical systems,” said Kenneth Cybart, senior technical sales engineer at Littelfuse, Des Plaines, Ill. “But specifiers must overlay the time-current curves of all upstream (line side) and downstream (load side) breakers to ensure that the downstream circuit breaker will open under a short-circuit condition before the upstream circuit breaker operates. To ensure that a circuit-breaker-protected system is selectively coordinated, the time-current curves must not overlap at any possible fault current. With fuses, selective coordination is achieved as long as specifiers maintain manufacturer-recommended ratios.”

Common place in high and medium voltage switchgear, zone selective interlocking breakers are making their way into low voltage switchgear as well. Zone selective interlocking uses data network communications between two or more compatible breaker trip units. This technology enables programmed trip unit settings to be altered automatically to respond to different fault conditions and locations. Instantaneous interruption is localized to the specific fault location, while the rest of the electrical system is maintained to provide positive coordination between circuit breakers.

Coordinating electrical systems involves understanding and using time-current curves. “In order to plot the OCPD curves,” said James P. Stroke, PE, a consulting engineer based in Somerset, Mass., “it is necessary to either obtain the OCPD inverse time curves from the manufacturers and plot them out by hand on log paper, or use a software package that plots it all out. I print each OCPD in a different color, which really makes them stand out, and you can more easily spot overlaps and adjust accordingly. In some cases, a ‘perfect’ coordination is impossible.”

Stroke also said that obtaining electrical design software could pose a problem for some. “These software packages are extremely expensive and not every engineer has these,” he said. “So, the implication is that many of the coordination studies probably just don’t get done.” The same applies to short circuit studies, he adds. “What’s really needed is an inexpensive software program for short circuit and coordination studies—either on a CD, or available online so anyone can access and use it.”

“There are many choices that take place during electrical system design,” Weigel said, “and these choices should be diligently consistent with optimizing the design in a way that will also optimize arc flash safety.”

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Overcurrent Protection Devices and their Time Current Curves

• Overcurrent Coordination

Overcurrent Protection Devices

Disadvantages, thermal-magnetic trip device, electro-mechanical or solid-state trip device, instantaneous overcurrent relays (50), definite time-overcurrent relay, inverse definite minimum time (idmt) overcurrent relay, ansi standard curves, iec standard curves.

Understanding overcurrent protection device characteristics is very important in a protection coordination study. To start our discussion on common overcurrent protection devices, let us review the basic considerations of a coordination study. In our previous discussion on overcurrent protection and coordination study , the following considerations were presented:

• Short Circuit Currents
• Load Flow Currents
• Minimum Operating Criteria
• Delta-Wye Transformers

Overcurrent protection devices can be categorized into three main types:

Switching Devices

Fuses are essentially made up of a metal wire or strip that melts when excessive currents flow through. Being such, fuses operate on a continuous-ampere rating. Low-voltage power fuses can withstand 110% of their rating under controlled conditions. while medium- and high-voltage power fuses can withstand currents below 200% of their nominal rating. Low-level overcurrent takes a long time interval to melt the fuse while large overcurrent levels tend to melt fuses very quickly. A typical fuse time-current curve is shown below.

Fuses operate in a time-current band, between

• minimum melting time – the time when the metal strip starts to melt, and
• maximum clearing time – when the strip completely breaks and the arc fully extinguished.
• The difference between these is referred to as the arcing time .

Overcurrent coordination with fuses is a little tricky, especially for a remote backup fuse. The primary device which can be another fuse should clear the fault before the minimum melting time of the remote backup fuse. In other words, for fuse-to-fuse coordination, the maximum clearing time of the primary fuse (also referred to as the downstream fuse or the protecting fuse ) should be lesser than the minimum melting time of the remote backup fuse (also referred to as the upstream fuse or the protected fuse ). In most applications, the rating of the upstream fuse is approximately twice that of the downstream fuse.

The following are some of the advantages and disadvantages of fuses:

• Limits fault energy
• Little or no maintenance
• Difficult coordination
• Limited sensitivity to earth faults
• Single phasing
• Fixed characteristic
• Need replacing following fault clearance

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Switching devices are another basic category for overcurrent protection devices. Miniature Circuit Breakers (MCBs), Molded Case Circuit Breakers (MCCB), Air Circuit Breakers (ACB) fall into this category and are usually used in low voltage applications.

Like fuses, switching devices detects and clears fault but do not need replacement after every fault clearance. The fault interruption is done using an integrated trip device. The trip action may be done mechanically using spring charge or compressed air to separate the contacts, or using the energy of the fault current to separate the contacts through thermal expansion or magnetic field.

Trip Device

The trip devices for low voltage circuit breakers are the following:

• Thermal Magnetic
• Electro-mechanical or Solid-State

Low voltage circuit breakers with a thermal-magnetic trip device allow for the discrimination between an overload from a fault. The thermal element acts as protection from overloading while the magnetic element is for protection from faults. This allows slow operation on overload and fast on fault. A typical time-current curve is shown in figure 6. Thermal Magnetic trip devices may be fixed or adjustable based on the ampere rating.

Electro-mechanical or Solid-State trip devices are more complex than Thermal Magnetic trip devices in that their trip characteristics can be divided into three sections. These are

• Long-Time Element – allows for protection on overloads.
• Short-Time Element – the intermediate protection between overloading and faults
• Instantaneous Element – allows for protection for faults.

Relays detect and isolate faults indirectly. Unlike fuses and switching devices, relays require CT and PT input to detect the fault, and a circuit breaker in order to isolate it. Relays have different functions and use currents, voltages, or their combination (impedance) to identify a fault. Basic overcurrent functions such an instantaneous overcurrent (50) and time-overcurrent (51) are usually common.

These relays operate instantaneously when the current exceeds the pick-up value and reset with no intentional time delay. Most instantaneous overcurrent relays operate on minimum operating time.

These relays operate when the current exceeds the pick-up value after a set time delay. The time delay settings are adjustable and set following an overcurrent coordination study.

These relays operate when the current exceeds the pick-up value and with an operating time that varies inversely the magnitude of the current. This means that the operating time decreases with increasing current magnitude. However, like instantaneous overcurrent relays, IDMT overcurrent relays have a definite minimum operating time . Hence the name Inverse Definite Minimum Time.

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ANSI and IEC Standard Curves

There are different characteristic curves available for Inverse Definite Minimum Time (IDMT) overcurrent relays.

ANSI standard curves are described by the following general equation

Tt is the tripping time

TM is the time multiplier

I is the fault current

Ip is the pick-up current

A, B, p are constants

ANSI standard curves are provided with a disk emulating reset timer described by the following general equation

Rt is the reset time

D is a constant

Imin is the minimum operating current

The ANSI standard curve constants are defined in the table below.

IEC standard curves are described by the following general equation

A, p are constants defined in the table below

IEEE Std 242-2001 [The Buff Book]: IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems.(2001). S.I.: IEEE.

ETAP Enterprise Solution for Electrical Power Systems Online Help

Blackburn, J. (2014). Protective Relaying Principles and Application, 4th ed. Boca Raton, FL: CRC Press.

G. Pradeep Kumar (2006), Power System Protection, notes on Power System Protection Training, Visayan Electric Company, Cebu City, Philippines.

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A viral nine-month world cruise saw plenty of drama, but not the kind you'd expect.

Rachel Treisman

TikToks by passengers on the Ultimate World Cruise — screenshot by NPR — capture the highs and lows of the nine-month journey, which ended last week. @drjennytravels/@anthonyantoine1021/@brooklyntravelstheworld hide caption

When Royal Caribbean’s “ Ultimate World Cruise ” sailed out of Miami and into social media virality in December, it promised an unforgettable nine months:

For passengers, a once-in-a-lifetime journey to all seven continents.

And for viewers, a 360-degree stream of on-board drama, as told from the perspective of a rotating cast of characters (since people could join at any point for one or more segments of the trip ).

Many of the roughly 650 full-timers on board started posting videos to TikTok and Instagram, filming their routines at sea and explorations on shore. And a handful of existing content creators — firmly rooted on land, from New York to California — studied and synthesized those videos to report on the cruise in real time.

How a world cruise became a 'TikTok reality show' — and what happened next

They recapped the week’s events, introduced new characters, chased down rumors using their onboard contacts and speculated about what kinds of spectacles lay ahead. One even made a bingo card, with squares ranging from an early departure to a pirate takeover.

The potential for drama was so high, and the content so ubiquitous, that fans began referring to the cruise as a “TikTok reality show,” even as some worried that online hype would warp or otherwise worsen passengers’ real lives.

Last week — after 274 nights, more than 60 countries and many millions of social media views — the cruise came to an end. Which means it’s finally time to ask: How did it live up to the hype?

“This absolutely should have been a reality TV show. It would have won so many awards,” said Kara Harms, who runs a travel and lifestyle brand and covered the cruise on social media — she created two bingo cards, the first of which took only a couple of weeks to get to bingo.

NPR caught up with several people who were either on board the ship or following it closely on social media.

They all spoke of forming lifelong friendships, marveling at world wonders (either firsthand or vicariously) and generally expanding their cultural horizons. They also confirmed that the experience wasn’t without drama.

“I mean, you cannot stick that many people together in a small space and not have there be drama,” said passenger and TikTokker Jenny Hunnicutt.

But it wasn’t necessarily the kind that people had expected, like romance rumors or interpersonal beefs. The bigger scandals came from much heavier external factors — like politics, war and literal forces of nature — that passengers had to weather during their time together.

The ship rerouted a leg of its trip to avoid the ongoing conflict in the Red Sea, got delayed by climate protesters in Amsterdam and narrowly missed a massive earthquake in Taiwan. Passengers had to be evacuated from Iceland’s Blue Lagoon because of volcanic activity. And on board, many followed — and increasingly fought over — the many twists and turns of the fast-approaching U.S. presidential election.

“I think they definitely started with the more light, fun things like running out of wine or the boat’s flooding from a storm or stuff like that,” Harms explained. “And then they really launched into the realities of what happens when you sail a ship around the world for nine months: You're gonna encounter a lot of real things that happen.”

The real lives behind TikTok’s reality show

When NPR first spoke to content creators in January, just weeks into the cruise, the anticipation — and some anxiety — was apparent.

While passengers were excited about embarking on an adventure, some were apprehensive about their overnight internet fame and how it would affect the dynamics on board.

Many embraced it, said Hunnicutt, who notes that some content creators on board started going viral before they’d even met each other. That buzz brought them together in those early days and set in motion some lasting friendships.

“With the reality show comparison, we definitely leaned into that online in the beginning,” she said. “But this wasn't a reality show that we had signed up for and signed a contract, right? These were real life. So there was always that … you have to be very mindful of others.”

Videos from and about the cruise captivated social media for the first several months of the cruise, which creators say is an impressive amount of time for a trend to hold viewers’ attention.

As interest in the cruise spread across social media, two influencers who had been watching from land got sponsorship deals to come on board for brief stints — and promised to take their many followers behind the scenes with them . By the time both had come and gone, audience interest began to dip (one passenger points out this was around the time the ship arrived in China, where the international version of TikTok is unavailable).

Harms cut back on her cruise content but still kept tabs on what was happening on board.

“By this point, I have spent a lot of time forming these parasocial relationships with all these people on the cruise, but also some actual relationships, like DM’ing some people on there and forming semi-close friendships,” she explains.

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As time passed, and the passengers and creators on land grew closer, the cruise coverage took on less of a gossip-rag feel.

Multiple people told NPR that once the initial frenzy calmed down, TikTokkers realized how the social media speculation could impact passengers’ real lives, for better or worse. The handful of land-based creators ended up working together to decide whether to amplify certain stories or link back to certain videos.

“You just really want to make sure that people trapped on the boat with a bunch of strangers are having the best time possible, because at the end of the day, they paid money to be here, and it's an experience and it's a vacation for them,” Harms added.

Beth Anne Fletcher, a photographer who gained a sizable following covering the cruise from her home in Derbyshire, England (“as far away from an ocean as you can possibly get” in the UK), estimates she’s made some 300 videos about it this year.

Of those, videos about people and the “so-called drama” tended to perform the best. But after hearing from viewers who wanted to follow along with the actual travel, she started making location-focused recaps too.

“For me, it was never about, I want to continually go viral,” Fletcher said. “It was more about, well, these people are living their best lives, and for a lot of us who might never have the opportunity to travel like this, it's a way for us to see the world through all of these different eyes ... And there’s only so much drama that can happen in nine months, surely.”

Fletcher got to board the cruise for a day during its sole stop in England in July and meet the passengers she’d spent so many months getting to know virtually.

She was shocked by how big the ship felt in real life. The 13-deck Serenade of the Seas is 965 feet long and 106 feet wide, according to Royal Caribbean.

“I was like, ‘OK, so this ship is actually big enough that if you don’t like people, you could easily not see them,’” she adds.

Passengers agreed it was fairly easy to steer clear of the more gossip-minded travelers.

Several content creators on board said the highlight of the trip was meeting people and making friends — and while social media played a role in that, it ultimately didn’t make or break their experience.

Take passenger Amike Oosthuizen , who had done some influencing before the cruise and, at the beginning of the year, spoke of pursuing it as a career afterward.

But last week, from her hotel room in Miami, she recounted how her TikTok account had been blocked just weeks before the end of the cruise: Someone reported it for selling counterfeit Chanel goods, which she said she was “obviously not doing.”

It was devastating, she said, especially because she lost a lot of videos — and memories — that she hadn’t saved off the app. She doesn’t regret all the filming, editing and posting she did, which she says taught her practical skills. But in hindsight, she wondered if some of that time would have been better spent living in the moment.

“It just showed me actually how volatile social media is,” Oosthuizen said. “I really do like doing social media, but I don't know if it is always like a thing I would want to do permanently.”

Now, about that drama

The world cruise wasn’t an isolated bubble. Some world events hit passengers especially hard.

“The big one that comes to mind that involved all of us was the big itinerary change and how the ship became a democracy,” Hunnicutt said. “We had to vote on which route we were going to choose when we were unable to sail through the Middle East.”

The ship was originally supposed to sail through the Suez Canal in May, but the cruise line announced earlier this year that it would be rerouted due to disruptions in the Red Sea, where Iranian-backed Houthis have been attacking ships since the start of the Israel-Hamas war.

Royal Caribbean gave passengers two options : “Immersive Africa,” a more scenic route that would stop at multiple ports along the continent, or “Africa & Greece,” which would aim to get the ship around Africa and to Eastern Europe as quickly as possible but involved many more days at sea.

“People were campaigning, people were sharing their opinions,” Hunnicutt recalled. “It was quite dramatic.”

Some of the people opposed to stopping in Africa made generalizations and arguments that their fellow travelers perceived as racist — like classifying the whole continent as a whole and saying there was “nothing to do there.”

The passengers voted overwhelmingly for the first option, to see more of Africa. But Hunnicutt says a fair number of people got off the ship for that portion — some rejoined in Italy, others did not.

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“When you travel for nine months, things are going to change, like the state of the world is going to change,” Fletcher said. “So, many people were, ‘Let’s go with the flow,’ but others weren't. And I guess, again, that’s just representative of real life.”

Another area of growing tension was U.S. politics.

Fletcher said some people had been wearing MAGA hats and shirts on board all along. But the existing political divide on the ship became more apparent in the spring when former President Donald Trump was convicted on felony charges .

Things escalated over the summer after a passenger wore a “ Let’s Go Brandon ” hat. The rhetoric on display made some passengers uncomfortable, sparking a conversation about free speech and whether Royal Caribbean should draw a line.

Fletcher said lots of passengers started responding by wearing shirts and hats in support of Democratic presidential nominee Kamala Harris.

Harris wasn’t even in the running when they first got on the ship, a sign of just how much things changed during the journey. (A passenger picked up the merch in bulk while on shore in the final weeks.)

“Luckily, I guess there wasn't that long left of the cruise, otherwise perhaps it might have become more of an issue,” Fletcher says.

More cruise content is coming soon

Once the cruise ended, there was a sense that the floodgates might open and some newly unencumbered passengers might spill the tea about their neighbors that they’d been sitting on for months.

Some promised they would reveal secrets back on land. In recent days, one TikToker teased, posted and then removed several videos’ worth of anonymous gossip.

The people NPR spoke with said they didn't think there was much tea worth spilling. For the most part, they said people aren’t trying to burn bridges, but maintain the relationships they formed on the cruise.

“I would go as far as saying I met some of my best friends in my life on this cruise,” Oosthuizen said.

Oosthuizen, who is from South Africa, now has offers to crash with people scattered across the U.S. Hunnicutt and her husband plan to stop at new friends’ homes during their road trip from Florida to Las Vegas. Fletcher plans to watch the upcoming live-streamed wedding of the daughter of a cruise couple she became close to from afar.

And then there’s the reunion cruise.

In an onboard announcement just days before the cruise ended, Royal Caribbean International President and CEO Michael Bayley revealed there will be a seven-day reunion cruise in Alaska — which the world cruise did not visit — in September 2025.

“It’s a little bit of a part two of the Ultimate World Cruise because we know circumstances were the way they were,” he said, adding it will be on the same exact ship.

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Oosthuizen and Hunnicutt say they and most of the people they know are planning to go — many bought tickets before they even got off the ship. Fletcher, who says she never dreamed she’d want to go on a cruise before covering this one, may try to make it (otherwise, she says, she’ll “definitely be cruising virtually”).

“The crazy and exciting thing is that anyone can book that cruise; it’s not just for world cruisers,” Hunnicutt said. “So I believe that we will have followers that come join us on that cruise … people are just so excited and engaged with this experience.”

Another reason to look forward to the reunion: That’s where Royal Caribbean plans to announce the details of their next Ultimate World Cruise . Bayley said passengers on the reunion cruise will get first dibs on rooms.

What does that mean for the so-called social media reality show?

Hunnicutt says she still has cruise content to post, like recaps of her favorite places and vlogs from days when she was too busy to edit. Fletcher is already turning her attention to another developing cruise drama: A ship scheduled for a three-year world cruise has been stuck at its home base in Belfast since May.

But Harms is skeptical any future world cruise coverage will reach the same viral heights as the original.

“I think what happened was really special and unique and unprecedented in the way that we create content online, and I honestly don't think it can be replicated again,” she said. “Who knows? But I think that sometimes it's nice to just have a special moment and then button that up and move on.”

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