Lock-out Relay

Purpose and function

The lockout relay (LOR), designated by ANSI/IEEE code 86, is a crucial component in power system protection. 

Often referred to as a master trip relay, its primary purpose is to act as an intermediary between protective relays and control devices, ensuring comprehensive system protection by expanding control capabilities and providing a latching function that requires manual resetting to resume normal operation

A lockout Relay is not a protective relay but a high speed control relayused as auxilary relay in application requiring many contacts!

Technical characteristics

Typically, Lockout Relays are electromechanical devices

The main components of a LOR include:

• Operating coil
• Resetting coil (optional)
• NO/NC contacts
• Operating shaft
• Operating handle

When a fault is detected, the operating coil energizes, causing the relay to latch and send a trip command to circuit breakers. This latched state remains until the relay is manually reset or the resetting coil is energized.        

To minimize additional delay to the trip time, LORs are designed to operate quickly, typically within 10ms (0.6 cycles)

Schematic of a typical LOR

The LOR has multiple auxiliary contacts (EX. 3 NO and 3NC contacts).

Auxiliary contacts are used for:

• Circuit breaker trip
• Alarm
• Scada communication
• etc.

Customers specify the number of auxiliary contacts needed per their applications.

Operation of the LOR

To better understand how the LOR operates, it is better to assume its initial position to be a No-fault condition with the handle in trip position and the protective relay contact open.

in this position, all the NO contacts of the LOR are closed and all the NC contacts (including the contact 1-2) are open.

The lockout relay is activated by turning the knob from the TRIP position to the RESET position. Once in the RESET position, the knob is blocked and can no longer be manually turned back to the TRIP position. All the NO contacts of the LOR are now open and the NC contacts are closed.

When there is a fault, the protective relay contact closes and the LOR is energized. This later will release the mechanical latch holding the handle at the RESET position and the handle will spring return to the TRIP position.

As long as the protective relay contact remains closed (due to the presence of a fault in te circuit) the handle will not latch at RESET position but will always spring return to TRIP.

Application of LORs

• The LOR is used to provide the protection needed for electrical faults that require human intervention before resetting. (Differential overcurrent, phase, and ground overcurrent, sudden pressure, etc..).
• The LOR is used to provide isolation between the protective relay and the trip coil thus protecting the protective relay from potential faults in the trip coil.
• The LOR is used to multiply contacts. With one trip signal from the protective relay, we can trip multiple circuit breakers, and activate alarms on the relay panel and to the PLC or SCADA.
• The LOR is recommended for breakers with no anti-pumping or trip feature.

Practical examples of application of a LOR

Example 1

In this example, when the Lockout Relay (LOR, 86T) latches, it trips the circuit breaker through its contacts 86T/a on the trip coil circuits.

One issue with this schematic is that, even with the LOR not reset, the breaker can still be commanded to close, but it will trip immediately after contacts 52-1/a and 52-2/a are closed.

Example 2

In this second example, when the Lockout Relay (LOR, 86-C) has latched, the breaker will trip and cannot be closed until the LOR is reset. This is achieved by placing a normally open (NO) contact 86-C on the circuit breaker trip circuit and a normally closed (NC) contact 86-C on the circuit breaker close circuit.

Conclusion

In conclusion, the lockout relay (LOR) is an indispensable component in power system protection. Its ability to serve as a master trip relay, providing a latching function that necessitates manual resetting, ensures comprehensive protection and operational reliability. By isolating protective relays from potential faults and multiplying contacts for extensive control capabilities, LORs enhance the safety and efficiency of electrical systems. Understanding their operation, characteristics, and practical applications is crucial for engineers and technicians to effectively implement and maintain robust power protection schemes.

Authors: Yves Zomebot, PE & Jixil Johnson

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References:

[1]: Comelectric website

[2]: Lockout Relay Fundamentals: Basic Maintenance & Functional Testing Explained

[3]: SEL-787 Transformer Protection Relay Instruction Manual

[4] Comelectric - Lockout Relays Series LR20-LR1S