Overcurrent Relay Malfunctions: Causes, Diagnosis & Fixes

 

In industrial power systems, overcurrent relay malfunctions can lead to severe operational and safety issues — from unexpected shutdowns to unprotected equipment during real faults. These relays play a vital role in protecting feeders, motors, and transformers from excessive current. When they fail to perform correctly, both downtime and equipment damage can occur.

In this article, we’ll explore the most common field questions engineers face when dealing with overcurrent relay problems, understand the root causes, and learn practical diagnostic and maintenance approaches.

 1. Why is the overcurrent relay not tripping during a short circuit?

This is one of the most critical issues.
Possible reasons include incorrect CT polarity, relay not receiving actual current due to open CT secondary, or wrong pickup settings. In some cases, the relay may be healthy, but the trip output contact or breaker coil fails to operate.

Field check:

• Measure actual fault current using test clamp.

• Verify CT secondary continuity and polarity.

• Confirm relay trip circuit health.

Read about: Relay Not Detecting High Impedance Faults | Causes & Solutions

 2. Why does the overcurrent relay trip without any fault (nuisance tripping)?

False tripping often results from harmonics, voltage transients, or CT saturation. If the relay threshold is set too low or if there’s poor earthing, it can interpret normal current spikes as faults.

Solution:

• Filter transient signals.

• Verify coordination curve and pickup settings.

• Inspect grounding integrity.

3. Why is there a delay in relay tripping even when fault current is high?

Incorrect time dial settings or inverse-time curve mismatch can cause delays. Some digital relays also feature intentional time grading for coordination purposes. However, too high a setting can allow damage before tripping.

Tip:
Review the TMS (Time Multiplier Setting) and coordination study to ensure appropriate delay.

 4. Why does the relay fail to trip the circuit breaker during fault?

A healthy relay may still fail to isolate the circuit if the trip circuit wiring, auxiliary relay, or breaker coil is defective. Always test the complete protection loop, not just the relay logic.

Perform a trip circuit supervision test regularly to prevent this.

 5. Why is the overcurrent relay not coordinating with downstream relays?

Improper grading between upstream and downstream protection can lead to both relays tripping simultaneously. Coordination curves must be plotted based on system fault levels and load conditions.

Use manufacturer software (e.g., ETAP, Easypower) to verify coordination.

 6. Why does the relay show pickup indication but no trip?

This often points to logic configuration issues in numerical relays. The pickup element detects the current, but the trip output isn’t enabled or is blocked by another element (like directionality or intertrip logic).

Check:

• Trip logic matrix.

• Interlocks and inhibit conditions.

7. Why is the relay tripping during motor starting?

Motor starting currents can reach 6–8 times the rated current. If the overcurrent relay isn’t set with inrush blocking or time delay, it will interpret this as a fault.

Solution:
Adjust pickup to 1.2× motor FLA and apply a short intentional delay.

 8. How does CT saturation cause wrong relay operation?

During heavy faults, CT cores can saturate, distorting secondary current. The relay receives a lower or delayed signal, causing overcurrent relay malfunctions like missed trips or false trips.

Preventive action:
Use Class 5P or 10P CTs with adequate knee-point voltage.

 9. Why does the instantaneous element not operate while the time element does?

If the fault current is below the instantaneous threshold but above the time delay setting, only the time element trips. Wrong plug setting or scaling error can also cause this mismatch.

Verify current scaling factors and instantaneous pickup in settings.

 10. Why is the relay showing incorrect fault phase?

CT polarity reversal or wiring error between relay terminals can lead to incorrect fault phase identification.
Always cross-check phase sequence and test each CT phase individually during commissioning.

 11. Why does the relay trip below its set current value?

Low pickup or CT ratio mismatch can cause early tripping.
Another reason is the software scaling factor in digital relays being incorrectly configured.

Recalculate the ratio:
Actual Current = (CT Secondary × Relay Reading × CT Ratio)

 12. Why is the overcurrent relay not communicating with SCADA?

This could be due to port configuration errors, wrong protocol mapping (Modbus/IEC 61850), or broken fiber link.
Even if the relay protects correctly, lack of communication affects event logging and remote monitoring.

Recommendation:
Regularly test SCADA connectivity and update firmware.

 13. Why does the relay misoperate during transformer energization?

Transformer inrush current is rich in DC offset and harmonics, which can falsely trigger overcurrent elements.
To prevent this, relays should have second harmonic restraint or inrush blocking features enabled.

 14. Why is the relay tripping due to voltage transients or harmonics?

Voltage dips, switching surges, or frequency variations distort current signals, confusing the relay’s detection logic.
Install surge suppressors and check for grounding loops or VFD-generated harmonics nearby.

 15. Why is the relay not saving fault records or event data?

This is common in numerical relays with corrupted memory or incorrect time synchronization. Without event logs, diagnosing the overcurrent relay malfunctions becomes difficult.

Field fix:

• Reformat or replace memory card.

• Sync relay clock via GPS or SCADA.

• Update relay firmware.

 Field Diagnostic Approach

When diagnosing overcurrent relay malfunctions, engineers should follow a structured method:

1. Visual inspection: Check CT wiring, trip circuit, and grounding.

2. Secondary injection test: Validate relay pickup and timing.

3. Logic review: Inspect configuration, interlocks, and outputs.

4. Communication test: Ensure relay links correctly with SCADA or PLC.

5. Event analysis: Examine last trip record and waveform.

 Preventive & Predictive Maintenance

Regular relay maintenance is crucial to prevent misoperation:

• Perform annual functional testing.

• Verify CT secondary insulation and polarity.

• Update firmware after factory release notes.

• Back up relay configuration before any setting change.

Using digital test kits and integrating relay health monitoring systems can shift from reactive to predictive maintenance — improving protection reliability.

 Conclusion

Overcurrent relay malfunctions are among the most common causes of power system misoperations in plants and substations.
By understanding the real field scenarios — from CT saturation to communication loss — and applying structured diagnostic steps, engineers can significantly reduce unplanned outages and equipment damage.

Strong coordination studies, periodic testing, and continuous relay maintenance ensure your protection system remains both selective and reliable under all fault conditions.