1. Introduction
A lockout relay (LOR) is a critical component in power system protection schemes, ensuring that once a fault occurs, the affected equipment remains isolated until a manual reset is performed. This mechanism prevents accidental re-energization of a faulted system, ensuring personnel safety and equipment protection. In electrical protection terminology, lockout relays are designated by the ANSI device number 86.
2. Functions of a Lockout Relay
Lockout relays serve multiple functions in electrical substations and industrial power systems:
a) Fault Isolation
Once a protective relay detects a fault, the lockout relay prevents re-energization of the affected circuit by ensuring that all related circuit breakers remain open.
b) Sequential Tripping
Lockout relays coordinate the shutdown of multiple breakers to effectively isolate the fault and protect downstream equipment.
c) Manual Reset
Unlike standard protective relays, a lockout relay requires a manual reset by an operator after a fault clearance check.
d) Alarm and Indication
Lockout relays provide visual and remote alarms via SCADA systems, alerting operators to the occurrence of a major system fault.
e) Protection Scheme Coordination
Lockout relays work in conjunction with various protection relays such as differential (87), overcurrent (50/51), distance (21), and breaker failure (50BF) relays to ensure proper fault clearance.
3. Importance of Lockout Relays
a) Enhancing System Safety
By preventing automatic reclosure in high-voltage networks, lockout relays mitigate potential arc flash hazards and electrical fires.
b) Preventing Equipment Damage
They help protect critical assets like transformers, generators, switchgear, and transmission lines from catastrophic failure by ensuring complete fault clearance before re-energization.
c) Improving Power System Reliability
Lockout relays play a crucial role in ensuring stability and reliability by keeping faulty components offline until inspections confirm they are safe to re-energize.
d) Compliance with Industry Standards
Most electrical power systems must adhere to regulatory requirements regarding protection coordination, and lockout relays meet standards such as:
- IEEE C37.90 – Protective relays and associated systems
- IEC 60255 – Measuring relays and protection equipment
- ANSI C37.2 – Device numbering (LOR is 86)
- NERC PRC Standards – Power system reliability regulations
4. Applications in Power Systems
a) Lockout Relay in Electrical Substations
Substations operate at high voltages and require robust protection mechanisms to prevent equipment damage. Lockout relays ensure that faults in different substation components are effectively isolated.
i) Transformer Protection
- Used with differential protection (87T), overcurrent (50/51), Buchholz relay (63), and temperature relays (49).
- Trips high-voltage and low-voltage breakers when transformer faults occur.
ii) Circuit Breaker Failure Protection
- If a breaker fails to open during a fault, the lockout relay ensures that backup breakers trip to isolate the faulted section.
iii) Busbar Protection
- Works with bus differential relays (87B) to trip multiple breakers in the event of a bus fault.
iv) Generator Protection
- Ensures that faults detected by stator differential protection (87G), overvoltage (59), or loss of excitation (40) relays cause immediate and full isolation of the generator.
b) Lockout Relay in Industrial Power Systems
Lockout relays are also used in industrial applications to protect large motors, switchgear, UPS systems, and backup generators.
i) Motor Protection
- Used with overcurrent (50/51), thermal overload (49), and undervoltage (27) relays to protect large motors from electrical faults.
ii) Switchgear Protection
- Ensures that arc flash protection and feeder protection relays work effectively by keeping faulty feeders offline.
iii) Data Center and UPS Protection
- Prevents system-wide failures in critical backup power infrastructure by coordinating fault isolation in UPS and battery banks.
5. Recommended Lockout Relay Models
Lockout relays are available in electromechanical and digital (microprocessor-based) designs. Selection depends on application requirements such as fault logging, SCADA integration, and reset mechanisms.
a) Electromechanical Lockout Relays (Traditional Type)
- GE Multilin HEA Series – Used in high-voltage substations for robust and reliable tripping.
- ABB RXMS1 / RXMS1-P – Compact and durable, suitable for EHV substations.
- Siemens 7SR86 (Reyrolle) – Offers mechanical flag indication and durability for industrial applications.
Lockout Relay
Lockout Relay
b) Digital / Microprocessor-Based Lockout Relays (Modern Type)
- Schneider Electric Sepam Series – Provides SCADA integration and remote reset capabilities.
- SEL-451 (Schweitzer Engineering Laboratories) – A multi-function relay with IEC 61850 support.
- GE Multilin 850 Feeder Protection Relay – Ideal for distribution networks and industrial switchgear.
6. Integration with Protection Schemes
a) Transformer Protection Scheme
- Primary Relays: 87T (Differential), 50/51 (Overcurrent), 63 (Pressure Relay)
- Lockout Relay Function: Trips both HV and LV breakers to prevent damage.
b) Breaker Failure Protection
- Primary Relays: 50BF (Breaker Failure)
- Lockout Relay Function: Ensures backup breakers operate in case of failure.
c) Generator Protection
- Primary Relays: 87G (Differential), 40 (Loss of Excitation), 59 (Overvoltage)
- Lockout Relay Function: Trips the generator breaker and prevents re-energization.
d) Industrial Motor Protection
- Primary Relays: 49 (Thermal Overload), 50/51 (Overcurrent), 37 (Undercurrent)
- Lockout Relay Function: Prevents motor restart after faults and works with SCADA for alarm indication.
7. SCADA and Remote Monitoring Integration
Modern lockout relays support remote control, event logging, and automation:
- IEC 61850 compliance allows integration with digital substations.
- Event recording and fault logging improve fault analysis and maintenance.
- Remote reset capability eliminates the need for manual intervention in some designs.
8. Conclusion
Lockout relays are indispensable in power system protection, ensuring fault isolation, personnel safety, and equipment reliability. Choosing the right relay depends on system voltage, fault coordination, and automation needs. Traditional electromechanical relays remain widely used in HV substations, while microprocessor-based relays provide remote monitoring, SCADA integration, and enhanced fault analysis.