Maintenance and testing of Overcurrent Protection Relays: ANSI Code 50/51

Overcurrent protection relays play a crucial role in safeguarding electrical power systems by detecting and responding to excessive current conditions. These relays operate based on preset thresholds, ensuring timely isolation of faults to prevent damage to equipment. The ANSI standard classifies overcurrent relays under the following designations:

• ANSI 50 (Instantaneous Overcurrent Relay): Triggers immediately when current surpasses a predefined limit, providing fast fault clearance.

• ANSI 51 (Time Overcurrent Relay): Operates with an intentional time delay, with trip times that are inversely proportional to the fault current, allowing better coordination between protection devices.

Overcurrent relays are widely applied in power transmission lines, transformers, motors, and generators, acting as a primary or backup protection mechanism against short circuits and sustained overloads.

Typical drawing of Overcurrent relay scheme looks like this:

Overcurrent relay scheme

Step-by-Step Guide for Overcurrent relay testing

1. Visual Inspection & Preliminary Checks

• Confirm the relay model, specifications, and protection settings as per the system design.

• Inspect for visible damage, loose wiring, or incorrect terminal connections.

• Verify that the relay’s power supply and communication interfaces are properly connected.

2. Power-On & Self-Test

• Energize the relay and observe its initialization sequence.

• Check for any error messages, alarms, or abnormal indications on the display.

• Ensure successful communication with SCADA or remote monitoring systems.

3. Secondary Injection Testing

Objective: To validate the relay’s response to simulated overcurrent conditions using a test set.

Procedure:

• Connect a secondary injection test set to the relay’s current inputs.

• Inject nominal current and confirm that the relay does not operate incorrectly.

• Conduct Pick-Up Tests:

  • Gradually increase the current level from below the setpoint to the pick-up threshold.

  • Identify the exact current at which the relay detects an overcurrent condition and initiates a trip sequence.

  • Compare the measured pick-up value with the programmed setting to ensure compliance.

• Conduct Drop-Out Tests:

  • Reduce the current after reaching the pick-up point.

  • Determine the level at which the relay deactivates the trip signal and resets.

  • Verify that the drop-out ratio aligns with manufacturer specifications.

• For instantaneous overcurrent (ANSI 50), inject current above the trip setting and confirm an immediate trip response.

  

  For single phase relays, or 3-pahse relays that support testing phases one bye one procedure is reasonably easy as shown above.

• For time-overcurrent (ANSI 51), apply current at varying multiples of the setpoint and measure the time delay against the relay’s inverse time characteristic curve.

• Compare trip times with relay settings and protection coordination requirements.

4. Primary Injection Testing

Objective: To assess the relay’s performance in actual system conditions, including CT performance and breaker operation.

Procedure:

• Inject current through the current transformers (CTs) to simulate fault conditions.

• Monitor the relay’s response and ensure it correctly detects overcurrent events.

• Verify the trip command is executed and the circuit breaker operates accordingly.

  
  
  
  

  

  Example of testing OC relays with primary injection test set Raptor C-05

  
  

Functional Trip Test

Objective: To validate relay operation under simulated real-world conditions.

Procedure:

• Simulate an overcurrent event using a test system or fault injection software like Test Universe by Omicron or ROOTS by EuroSMC.

• Observe whether the relay issues a trip signal as expected.

• Confirm that the circuit breaker successfully trips in response to the relay signal.

• Reset both the relay and breaker to restore normal operation.

6. Settings & Coordination Verification

• Review relay settings and verify alignment with the protection coordination study.

• Confirm that time-current characteristics provide proper grading with upstream and downstream protection devices.

• Simulate different fault scenarios to validate relay response under expected operating conditions.

7. Communication & SCADA Integration Test

• Ensure the relay successfully transmits status, alarms, and trip signals to SCADA or RTU systems.

• Verify the correct operation of control commands sent from the remote system to the relay.

8. Final Review & Reporting

• Record all test parameters, results, and observations.

• Compare measured trip times with expected values and document any deviations.

• Restore relay settings to their designated operational configuration before re-energizing the system.