difference between trip and interlock

• Instrumentation Tools

A blog on field & control instrumentation

Label Links

• Calibration
• Control Systems
• Excel Tools
• Control Valves
• Density Measurement
• Fire & Gas System
• Flow Measurement
• Foundation Fieldbus
• How It Works
• Level Measurement
• Pressure Measurement
• Safety Systems
• Solenoid Valve
• Temperature Measurement
• Vibration Measurement
• BASICS OF TRIPS, INTERLOCKS, PERMISSIVES & SEQUENCES
• High level in a vessel initiates a trip system which stops the pump feeding that vessel, the pump will remain stopped even if the level in the vessel falls to a safe level.
• The Trip must be ‘reset’ by the operator before the pump can be re-started.
• The Trip can only be ‘reset’ if the level in the tank has fallen to a safe level.
• Resetting the Trip will not cause the pump to automatically re-start, however it may be re-started by an operator action or a control system command e.g. part of a sequence.

Permissive:

• Stop the feed pump
• Close the filling valve
• Stop the agitator.
• Wait 30 seconds.
• Open the discharge valve.
• Low level in a vessel opens the filling valve.
• The valve remains open until high level is detected.
• On high level the valve closes.
• The valve remains closed until low level is detected.
• On low level the valve opens and the sequence it repeated.

Combined Functions:

Like to share, post a comment default comments facebook comments.

Latest Articles

Follow us on facebook.

• Interview Questions on Control Valves
• Closed Tank DP Type Level Measurement with Dry leg and transmitter installed below tapping point
• DP Transmitter with two seals applications
• Ultrasonic Level Transmitter Working Principle
• Thermowell Insertion and Immersion Length
• Vibrating Fork Level Switch Working Animation
• DP Flow Transmitter Re-Ranging Calculation
• Non-Contacting Radar Level Transmitter Working Principle
• Interface Level Measurement using DP Transmitter

Instrumentation Android App

Trips and Interlock Systems

Protective tripping systems provide a defense against excursions beyond the safe operating limits by detecting an excursion beyond set points related to the safe operating limits (i.e. the onset of a hazard) and taking timely action to maintain or restore the equipment under control to a safe state.

A trip system may be very straight forward, involving a simple actuation of a final control element when the sensor detected a deviation from a safe operating limit. Often, the trip system is more complicated, with interlocks sequence involving several pieces of equipment.

An example is shown in the Figure for a Heater interlock system. An "interlock table" is also provided to highlight the possible causes and resulting action of a trip.

A more informative interlocks cause-and-effect table as shown provides additional information on the instruments and trip settings.

This method is useful for defining interlocks and basic trip logic, but not adequate for defining shutdown sequences. Instead one uses the logic diagrams , which shows the functional relationship between inputs and outputs of an SIS. One example is shown in the Figure.

Back on Top

• The Fabricator
• Canadian Metalworking
• Our Publications
• E-Newsletter
• Digital Edition
• The Tube and Pipe Journal
• The Fabricator en Español
• Additive Manufacturing
• Aluminum Welding
• Arc Welding
• Assembly and Joining
• Automation and Robotics
• Bending and Forming
• Consumables
• Cutting and Weld Prep
• Electric Vehicles
• Hydroforming
• Laser Cutting
• Laser Welding
• Manufacturing Software
• Materials Handling
• Metals/Materials
• Oxyfuel Cutting
• Plasma Cutting
• Power Tools
• Punching and Other Holemaking
• Roll Forming
• Shop Management
• Testing and Measuring
• Tube and Pipe Fabrication
• Tube and Pipe Production
• Waterjet Cutting

Industry Directory

• Search the Directory (Showrooms)
• Buyers' Guides and Directories
• Product Showcases
• Classified Ads
• Register for the Directory

Our Affiliated Brands

• Fabricators and Manufacturers Association
• Nuts, Bolts & Thingamajigs Foundation

Account Login

• From The Fabricator

A deep dive into a bleeding-edge automation strategy in metal fabrication

Young fabricators ready to step forward at family shop

How laser and TIG welding coexist in the modern job shop

Fabricating favorite childhood memories

• From The Welder

The role of flux in submerged arc welding performance

How welders can stay safe during grinding

Sheffield Forgemasters makes global leap in welding technology

Launching a fleet revival at Seaspan shipyard

• From The Tube and Pipe Journal

Making the move from hard automation to robotic welding

Fabricator achieves extraordinary with simple approach

Three big issues fabricators must prioritize in 2024

Tube 2024 to feature first-ever events

• From The Fabricator en Español

Seguridad en la soldadura desde el inicio

Anidado para la estabilidad del corte láser

Prescripción para la reducción del desperdicio: arreglo esbelto de las instalaciones

Los operadores de prensa dobladora inexpertos hacen indispensable el repensar la seguridad

Interlocks as machine safety devices, multiple configurations, types, and approaches u.s. and abroad.

• By Gary M. Hutter
• March 13, 2007

An interlock can be defined as a device that prevents you from making an inappropriate maneuver, or adjusts the system to a safe state if you make an inappropriate maneuver.

In the context of safety, interlocks can prevent a user from making unsafe actions, or minimize the hazard of unsafe actions by rendering the machine in a safe condition when an unsafe maneuver occurs. For example, a guard may be interlocked to prevent machine operation when a guard is removed, or a control may be interlocked to make it nonoperational if a dangerous condition will result. Safety interlocks may have additional or combined features to reduce hazards.

Interlock Examples

Many product standards mandate interlocks in both industrial equipment and in everyday consumer products. Examples of consumer products incorporating interlocks are:

• Removal of a guard on a food processor prevents the operation of the motor and blade, thereby reducing the opportunity for spinning blade injury (example of guard).
• Removal of the filter access door on a forced-air furnace prevents operation of the blower motor and possible contact with the blower blade and/ or combustion gas recirculation hazard (example of guard and secondary hazard of recirculation).
• The gear shift selector on a car allows the engine to start in Park position only, which prevents the operator from starting the engine in gear and the possible unexpected vehicle movement (control example).
• The inability to open the door on a clothes washer during the high-speed extraction cycle either prevents access to the spinning drum or stops the drum's rotation upon door opening (example of either guard or control feature).

While many often think an "interlock" is simply a safety method that relies on an electromechanical switch (like a limit or magnetic switch) to perform the interlock feature, modern interlocking mechanisms may take the form of other sensors and actuators. Many interlock switch providers currently have multipole magnetic switches, unique-shaped keyswitches, and hidden features buried within structural components.

The single-beam light curtain at the bottom of a garage door acts as an interlock to reverse the door so it can't close on a child or animal. The deadman control on a modern snow thrower (ANSI B71.3) acts as an interlock by placing the snow thrower in a safe condition (engine off or blade brake on) when the user leaves the controls to reach into the discharge chute. The thermocouple on a gas stove prevents the release of unburned gas if there is no ignition of the gas. A light curtain, a captured lever device, and a thermocouple are examples of interlock sensors and actuators.

Several safety standards address both interlocked guarding and interlocked controls on industrial equipment. American National Standards Institute (ANSI) standard B11.1- 1982 for mechanical power presses defines an "Interlocked Press Barrier Guard" as:

2.22.3 A barrier interlocked so that the press stroke cannot be started normally unless the guard itself, or its hinged or movable sections, encloses the point of operation.

Paragraph 3.5.2.10 of the same standard, "Press-Drive Motor Interlock," addresses controls:

The clutch/ brake control shall incorporate an interlock means to prevent initiation or continued activation of the single-stroke or continuous functions unless the press-drive motor is energized and in the forward direction.

The ANSI standard for "Hydraulic Power Presses," B11.2-1995, does not define the term "interlock" while requiring "interlocks" on certain barrier guards. This standard also defines other terms for devices that can act as an interlock safety device. For example, a control that prevents a cycle operation under hazardous conditions could be considered an interlock device. ANSI B11.2 uses the terms below for these interlock functions:

3.3 Antirepeat: The part of the control designed to limit the press to a single cycle even though the actuating mechanism is held in the operated position . 3.10.1 Presence sensing device: A device designed, constructed, and arranged to create a sensing field or area or plane that will detect the presence of the operator's hands or other body parts.

In essence, both antirepeat and presence sensing devices can and do work as safety interlocks.

We often find machine interlocks required by standards or provided based on custom and practice. Additional exemplar domestic and foreign standards regarding interlocks include:

• ANSI B11.19, "Performance Criteria for Safeguarding"
• ANSI/ RIA R15.06, "Safety Requirements for Industrial Robots and Robot systems"
• ISO 14119 (EN 1088), "Safety of Machinery-Interlocking Devices Associated With Guards"

Other Criteria and Risks

Interlocks may be a standard required item for certain features on industrial metalworking machines or an "add on" for other situations. These considerations may be based on U.S. domestic criteria, or criteria from abroad. In the publication Guide to Machinery Safety , (Pilz Automation Technology, 6th edition), electrical control interlocks are discussed both in terms of European Union (EU) standards and custom and practice. That publication states:

Electrical control interlocks are common where rapid or frequent access is required into a machine. (para. 5.1.2)

In the "British Standard Code of Practice for Safety of Machinery," BS 5304, Chapter 9 describes "Interlocking Considerations." This standard addresses both guard and braking interlocking and the failure mode needs of interlocks.

An example of U.S.-based nonmandatory criteria for interlocks is included in the National Safety Council publication Safeguarding Illustrated Concepts (7th edition, 2002), which uses the term "interlocked" as one of the three categories of point-of-operation guard types (other choices are "fixed" or "adjustable") that are recommended. This publication offers examples of interlocked features on industrial machines or devices, several of which are not required by codes. These examples include an interlocked safety prop on a hydraulic press, an interlocked barrier guard on a bagging machine, and an interlocked clean-out door.

This NSC publication also provides a list of interlock guard advantages and disadvantages. It identifies safety upsides of interlocks, such as "maximum protection" and ease of "access"; downsides relate to "reliability" and "defeatability."

The Occupational Safety and Health Administration (OSHA) publication "Concepts and Techniques of Machine Safeguarding," OSHA 3067, discussion and ranking of interlock guarding is similar to the NSC publication (although it lists interlocks second in a grouping of four options for guards). The OSHA publication also lists certain disadvantages of interlocks.

Many machines are not required to have interlocks per OSHA criteria, but interlocks may be required by other voluntary standards or by custom and practice. Certainly, the decision to have a voluntary interlock system, or a mandatory interlock device, relies heavily on a well-designed, high-reliability configuration.

As noted in General Electric's product catalog (3682-5K-0903, 2003, GE Interlogix Industrial), the European Standards EN-954-1 and EN 1050 "Risk assessment of control circuits" references the "likelihood of occurrence [of injury] if a safety interlock fails" in the risk assessment. Those standards discuss the issues of "redundancy," "self-checking," and "redundancy and self-checking." One publication goes so far as to include an interlock in a category of devices that also may "...increase the danger of the protected system." 1 This appreciation of the potential for poorly designed interlocks to fail is recognized in the OSHA criteria for lockouts.

Interlocks for electrical equipment may not be used as a substitute for lockout and tagging procedures. 2

The rationale for this consideration is contained in the National Fire Protection Association "Electrical Standard for Industrial Machinery," NFPA 79.

Collectively these various mandatory codes, voluntary standards, foreign and domestic criteria, and fail-safe considerations identify some philosophical aspects of interlocks on industrial equipment. The reader is encouraged to consult these sources for applying interlocks to industrial equipment.

1 "On Classification of Safeguard Devices," Safety Brief, R.L. Barnett, April 1981, v.1, N.1. 2 29CFR 1910.333(b)(2)(B)

About the Author

Gary m. hutter.

P.E. C.S.P.

See More by Gary M. Hutter

About the Publication

• View the Digital Edition

Subscribe to The Fabricator

Find The Fabricator on Facebook

Find The Fabricator on X

subscribe now

The Fabricator is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The Fabricator has served the industry since 1970.

• Stay connected from anywhere

Easily access valuable industry resources now with full access to the digital edition of The Fabricator.

Easily access valuable industry resources now with full access to the digital edition of The Welder.

Easily access valuable industry resources now with full access to the digital edition of The Tube and Pipe Journal.

Easily access valuable industry resources now with full access to the digital edition of The Fabricator en Español.

Patrick Brunken, VP of Addison Machine Engineering, joins The Fabricator Podcast to talk about the tube and pipe...

• Listen to the Podcast
• Trending Articles

BWXT to expand Ontario manufacturing plant

Material handling automation moves forward at MODEX

BZI opens Iron Depot store in Utah

• Industry Events

World-Class Roll Forming Workshop

• June 5 - 6, 2024
• Louisville, KY

Advanced Laser Application Workshop

• June 25 - 27, 2024

Precision Press Brake Certificate Course

• July 31 - August 1, 2024

Welcome back!

Forgot password?

Not yet registered? Sign up

• My Showrooms
• My Saved Guide Comparisons
• Change Password
• Change Personal Info

About basics of Trip, Interlock, Permissive and Sequences which are regularly used in instrumentation control systems like ESD, DCS, PLC etc.

An Interlock is in essence a ‘self resetting’ Trip. Interlocks are not deemed safety related and can be used for on/off control.

Interlocks are normally initiated by the DCS or PLCs, however if an Interlock is deemed to be safety related it may, depending upon SIL rating, be implemented in the SIS or a Hardwired system.

An interlock will force a device or devices to a pre-determined state e.g. Close valve, stop motor, etc.

Once a device or devices have been forced to a pre-determined state by the action of an Interlock they will remain in that state until the initiating cause returns to a ‘healthy’ condition, the Interlock will then be automatically removed.

Under normal circumstances it shall be possible to ‘override’ Interlocks for operational reasons or ‘defeat’ them for maintenance or other reasons.

‍ CNN Industrial Sdn Bhd ‍ ‍

#processinstrumentation ‍

#controller #interloc #dcs #plc

‍ Reference : https://lnkd.in/gZsPS97W

Latest blog posts

Ultrasonic clamp-on flowmeter

FURTHER ENHANCEMENTS GUIDED TDR RADAR SENSORS

THE NEW EHEDG DESIGN OF THE ROTARY PADDLE LEVEL SWITCH

INSTRUMENTATION AND CONTROL ENGINEERING

Search this blog, basics of trips, interlocks, permissives & sequences.

■ TRIP :

The term trip refers to an action that is initiated by the control system and which forces a device or devices to a pre-determined state. Example of Trip Signals: Close Valve, Open Valve, Stop motor, etc. The Safety Instrument System (SIS) or a Hardwired systems normally initiate trips, however the PLCs or DCS may also initiate trips provided the necessary independence and SIL ratings are met.Once a device or devices have been forced to a pre-determined state by the action of a Trip they will remain in that state until the Trip is manually reset by a conscious operator action.

For example:

High level in a vessel initiates a trip system which stops the pump feeding that vessel, the pump will remain stopped even if the level in the vessel falls to a safe level. The Trip must be ‘reset’ by the operator before the pump can be re-started. The Trip can only be ‘reset’ if the level in the tank has fallen to a safe level. Resetting the Trip will not cause the pump to automatically re-start, however it may be re-started by an operator action or a control system command e.g. part of a sequence. The resetting of Trips is a controlled procedure which will only be possible if the operator is logged in and has the necessary access rights.Under normal circumstances it shall not be possible to ‘override’ or `defeat’ Trips.

■ INTERLOCKS :

An Interlock is in essence a ‘self resetting’ Trip. Interlocks are not deemed safety related and can be used for on/off control.Interlocks are normally initiated by the DCS or PLCs, however if an Interlock is deemed to be safety related it may, depending upon SIL rating, be implemented in the SIS or a Hardwired system. An interlock will force a device or devices to a pre-determined state e.g. Close valve, stop motor, etc.Once a device or devices have been forced to a pre-determined state by the action of an Interlock they will remain in that state until the initiating cause returns to a ‘healthy’ condition, the Interlock will then be automatically removed. Under normal circumstances it shall be possible to ‘override’ Interlocks for operational reasons or ‘defeat’ them for maintenance or other reasons.

■ Permissive:

A Permissive is a patricular type of Interlock used to prevent actions taking place until pre-defined criteria have been satisfied, for example prevents a pump starting until the suction valve is open. Permissives are normally initiated by the DCS or PLCs, however if a Permissive is deemed to be safety related it may, depending upon SIL rating, be implemented in the SIS or a Hardwired system. Once a Permissive has been satisfied and the resulting action implemented it becomes inactive, for example once the suction valve has been opened and the pump started the Permissive takes no further action, even if the suction valve is closed while the pump is running. Under normal circumstances it shall be possible to ‘override’ Permissives for operational reasons or ‘defeat’ them for maintenance or other reasons.

■ Sequence :

A Sequence is defined as a pre arranged action or number of actions which are carried out by the control system. Sequences may be initiated by an event or operator actions. Sequences may be ‘single pass’ or ‘cyclic’. The following is an example of a ‘single pass’ sequence: An agitated vessel reaches a pre-determined level. The operator initiates a sequence that carries out the following actions: - Stop the feed pump - Close the filling valve - Stop the agitator. - Wait 30 seconds. - Open the discharge valve.

The following is an example of a ‘cyclic’ sequence: - Low level in a vessel opens the filling valve. - The valve remains open until high level is detected. - On high level the valve closes. - The valve remains closed until low level is detected. On low level the valve opens and the sequence it repeated.

■ Combined Functions:

It is common for Trip, Interlock, Permissive and Sequences to fulfill combined functions, for example the following pump protection system illustrates how the same system can perform various functions.

Prevent pump starting until suction valve is open.

Pump running – suction valve closed-pump stops.

- High level in vessel-Pump stops -Low level in vessel-Pump starts. - Pump running – suction valve closed – pump stops, - Suction valve re-opened – pump remains stopped. - Operator resets trip. - Pump available for re-start.

Post a Comment

Popular posts from this blog, instrument range selection, what is difference gap control .

• school Campus Bookshelves
• menu_book Bookshelves
• perm_media Learning Objects
• login Login
• how_to_reg Request Instructor Account
• hub Instructor Commons
• Download Page (PDF)
• Download Full Book (PDF)
• Periodic Table
• Physics Constants
• Scientific Calculator
• Reference & Cite
• Tools expand_more
• Readability

selected template will load here

This action is not available.

6.3: Permissive and Interlock Circuits

• Last updated
• Save as PDF
• Page ID 931

• Tony R. Kuphaldt
• Schweitzer Engineering Laboratories via All About Circuits

A practical application of switch and relay logic is in control systems where several process conditions have to be met before a piece of equipment is allowed to start. A good example of this is burner control for large combustion furnaces. In order for the burners in a large furnace to be started safely, the control system requests “permission” from several process switches, including high and low fuel pressure, air fan flow check, exhaust stack damper position, access door position, etc. Each process condition is called a permissive , and each permissive switch contact is wired in series, so that if any one of them detects an unsafe condition, the circuit will be opened:

If all permissive conditions are met, CR 1 will energize and the green lamp will be lit. In real life, more than just a green lamp would be energized: usually, a control relay or fuel valve solenoid would be placed in that rung of the circuit to be energized when all the permissive contacts were “good:” that is, all closed. If any one of the permissive conditions are not met, the series string of switch contacts will be broken, CR 2 will de-energize, and the red lamp will light.

Note that the high fuel pressure contact is normally-closed. This is because we want the switch contact to open if the fuel pressure gets too high. Since the “normal” condition of any pressure switch is when zero (low) pressure is being applied to it, and we want this switch to open with excessive (high) pressure, we must choose a switch that is closed in its normal state.

Another practical application of relay logic is in control systems where we want to ensure two incompatible events cannot occur at the same time. An example of this is in reversible motor control, where two motor contactors are wired to switch polarity (or phase sequence) to an electric motor, and we don’t want the forward and reverse contactors energized simultaneously:

When contactor M 1 is energized, the 3 phases (A, B, and C) are connected directly to terminals 1, 2, and 3 of the motor, respectively. However, when contactor M 2 is energized, phases A and B are reversed, A going to motor terminal 2 and B going to motor terminal 1. This reversal of phase wires results in the motor spinning the opposite direction. Let’s examine the control circuit for these two contactors:

Take note of the normally-closed “OL” contact, which is the thermal overload contact activated by the “heater” elements wired in series with each phase of the AC motor. If the heaters get too hot, the contact will change from its normal (closed) state to being open, which will prevent either contactor from energizing.

This control system will work fine, so long as no one pushes both buttons at the same time. If someone were to do that, phases A and B would be short-circuited together by virtue of the fact that contactor M 1 sends phases A and B straight to the motor and contactor M 2 reverses them; phase A would be shorted to phase B and vice versa. Obviously, this is a bad control system design!

To prevent this occurrence from happening, we can design the circuit so that the energization of one contactor prevents the energization of the other. This is called interlocking , and it is accomplished through the use of auxiliary contacts on each contactor, as such:

Now, when M 1 is energized, the normally-closed auxiliary contact on the second rung will be open, thus preventing M 2 from being energized, even if the “Reverse” pushbutton is actuated. Likewise, M 1 ‘s energization is prevented when M 2 is energized. Note, as well, how additional wire numbers (4 and 5) were added to reflect the wiring changes.

It should be noted that this is not the only way to interlock contactors to prevent a short-circuit condition. Some contactors come equipped with the option of a mechanical interlock: a lever joining the armatures of two contactors together so that they are physically prevented from simultaneous closure. For additional safety, electrical interlocks may still be used, and due to the simplicity of the circuit there is no good reason not to employ them in addition to mechanical interlocks.

• Switch contacts installed in a rung of ladder logic designed to interrupt a circuit if certain physical conditions are not met are called permissive contacts, because the system requires permission from these inputs to activate.
• Switch contacts designed to prevent a control system from taking two incompatible actions at once (such as powering an electric motor forward and backward simultaneously) are called interlocks .

Power plant and calculations

Power plant and calculation site basically includes the detailed study of power plant operation and maintenance, its related all calculations and thumb rules. It also involves detailed troubleshooting guides for operation and maintenance of power plant system/equipments like Boiler, fans, compressors, belt conveyors, ash handling system, ESP, steam turbine, cooling tower, heat exchangers, steam ejectors, condensers WTP. etc. Heat rate, efficiency

Protections & Interlocks in power plants

 Interlocks:   Are the programmed or hardwired control systems to protect systems and improve the operation reliability.

Protections: Are the programmed or hardwired control systems to protect the equipments, man power and systems from failure/harm.

The interlock and protection system is used to ensure safety of equipment and personnel as well as smooth & trouble free operation of the plant

This system initiates automatic corrective actions to stabilize the unit quickly. The protection scheme is developed to trip the equipment automatically with or Class A trip involves a serious electrical fault like differential, stator earth fault etc. and is considered to be the most dangerous in terms of the shock on the unit. Since it involves serious electrical faults, connections from both generator and the HV bus is immediately switched off to limit the damage at the fault point and also to isolate the healthy system. Hence the unit (turbine, generator and boiler) has to be tripped without time delay. Alarm & buzzers are generally used to alert the operator.

POWER PLANT PROTECTIONS & INTERLOCKS AND THEIR SIGNIFICANCE

  Classes of STG Trips:

Class A trip

This involves serious electrical faults and is considered to be the most dangerous in terms of the shock on the unit. Since it involves serious electrical faults, connections from both generator and the EHV bus is immediately switched off to limit the damage at the fault point and also to isolate the healthy system. Hence the whole unit need to be tripped.

Class B trip

Class B primarily relates to mechanical problems. This results in tripping of turbine followed by generator.

Read  Generator and Turbine inter tripping

Class C involves basically external system related problems like frequency, overvoltage etc. This does not involve instant tripping of the unit. CPP unit operates on house load

Classes of Generator protections

  Why do the Boilers explode

What do you mean by Turbine supervisory system???

No comments:

Post a comment.

Hi all, This article is written based on practical experience..If liked, share with others, or any suggestions leave in comment box.

15-Emergencies in power plant operation

Most visited posts.

Basics of Permissive and Interlock Circuits

A practical application of switch and relay logic is in control systems where several process conditions have to be met before a piece of equipment is allowed to start.

Permissive and Interlock Circuits

A good example of this is burner control for large combustion furnaces.

In order for the burners in a large furnace to be started safely, the control system requests “permission” from several process switches, including high and low fuel pressure, air fan flow check, exhaust stack damper position, access door position, etc.

Each process condition is called a permissive , and each permissive switch contact is wired in series, so that if any one of them detects an unsafe condition, the circuit will be opened:

If all permissive conditions are met, CR 1 will energize and the green lamp will be lit. In real life, more than just a green lamp would be energized:

usually a control relay or fuel valve solenoid would be placed in that rung of the circuit to be energized when all the permissive contacts were “good:” that is, all closed.

If any one of the permissive conditions are not met, the series string of switch contacts will be broken, CR 2 will de-energize, and the red lamp will light.

Note that the high fuel pressure contact is normally-closed. This is because we want the switch contact to open if the fuel pressure gets too high. Since the “normal” condition of any pressure switch is when zero (low) pressure is being applied to it, and we want this switch to open with excessive (high) pressure, we must choose a switch that is closed in its normal state.

Another practical application of relay logic is in control systems where we want to ensure two incompatible events cannot occur at the same time.

An example of this is in reversible motor control, where two motor contactors are wired to switch polarity (or phase sequence) to an electric motor, and we don’t want the forward and reverse contactors energized simultaneously:

When contactor M 1 is energized, the 3 phases (A, B, and C) are connected directly to terminals 1, 2, and 3 of the motor, respectively.

However, when contactor M 2 is energized, phases A and B are reversed, A going to motor terminal 2 and B going to motor terminal 1.

This reversal of phase wires results in the motor spinning the opposite direction. Let’s examine the control circuit for these two contactors:

Take note of the normally-closed “OL” contact, which is the thermal overload contact activated by the “heater” elements wired in series with each phase of the AC motor.

If the heaters get too hot, the contact will change from its normal (closed) state to being open, which will prevent either contactor from energizing.

This control system will work fine, so long as no one pushes both buttons at the same time. If someone were to do that, phases A and B would be short-circuited together by virtue of the fact that contactor M 1 sends phases A and B straight to the motor and contactor M 2 reverses them; phase A would be shorted to phase B and vice versa. Obviously, this is a bad control system design!

To prevent this occurrence from happening, we can design the circuit so that the energization of one contactor prevents the energization of the other. This is called interlocking , and it is accomplished through the use of auxiliary contacts on each contactor, as such:

Now, when M 1 is energized, the normally-closed auxiliary contact on the second rung will be open, thus preventing M 2 from being energized, even if the “Reverse” pushbutton is actuated.

Likewise, M 1 ‘s energization is prevented when M 2 is energized. Note, as well, how additional wire numbers (4 and 5) were added to reflect the wiring changes.

It should be noted that this is not the only way to interlock contactors to prevent a short-circuit condition. Some contactors come equipped with the option of a mechanical interlock: a lever joining the armatures of two contactors together so that they are physically prevented from simultaneous closure.

For additional safety, electrical interlocks may still be used, and due to the simplicity of the circuit there is no good reason not to employ them in addition to mechanical interlocks.

• Switch contacts installed in a rung of ladder logic designed to interrupt a circuit if certain physical conditions are not met are called permissive contacts, because the system requires permission from these inputs to activate.
• Switch contacts designed to prevent a control system from taking two incompatible actions at once (such as powering an electric motor forward and backward simultaneously) are called interlocks .

If you liked this article, then please subscribe to our YouTube Channel for PLC and SCADA video tutorials.

You can also follow us on Facebook and Twitter to receive daily updates.

• Basics of Fail-safe Circuits
• Contacts and Coils in PLC
• Pushbutton switches & Types
• Piping &Instrumentation Drawing

Recommended Articles

Ferrules and Cross Ferruling

Is Vacuum affects Field Instruments in Industries?

Pressure Gauge Maintenance and Troubleshooting

Top 5 Things for selecting a Solenoid Valve

Thermowell Design Guidelines

Control Room Design Guidelines

PLC Digital Signals Wiring Techniques

Technical Specification for Uninterrupted Power Supply (UPS) System

Pressure Sensor Specifications

Single and Three Phase Immersion Electrical Heaters Wiring Circuits

Leave a Comment Cancel reply

More articles.

Instrumentation Books Download

Instrumentation Common Measurement Issues

Pneumatic Valves and Cylinders Sizing – Part 2

Cable Routing

Instrumentation Cable trays Installation in vertical orientation

All About Variable Area Flow Meters

Pull Up and Pull Down Switch Circuits

Air Consumption Calculation for ON/OFF Valves

What is Hammer Effect in Gauges ?

How to Minimize Water Vapor in Instrument Air Lines?

• Site Search Search Posts Find A Forum Thread Number Threads by Name Search FAQs
• ENGINEERING.com
• Eng-Tips Forums
• Tek-Tips Forums

Join Eng-Tips ® Today!

Join your peers on the Internet's largest technical engineering professional community. It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!

Already a member? Close this window and log in.

Join Us               Close