Relay Not Operating During Fault: Causes & Solutions

 When a protection relay fails to operate during a real fault, the consequences can be severe — prolonged fault duration, equipment damage, and major production losses.

The issue of relay not operating during fault is one of the most challenging topics for protection and maintenance engineers, especially in industrial plants like steel factories, cement mills, and water treatment facilities.

Understanding the real reasons behind such failures requires a structured approach that considers wiring integrity, CT/PT performance, relay logic, and trip path health.
Let’s go through the most frequent real-life questions engineers ask on-site and analyze their technical explanations and corrective actions.

 Why Didn’t the Relay Trip Even Though the Fault Occurred?

This is the first and most common question. The relay may appear to “fail,” but the problem often lies in protection coordination.
In many systems, multiple relays share the same fault path — for example, feeder and incomer protection.
If the time grading margin isn’t properly set, the upstream relay trips faster than the downstream one, making it seem like the feeder relay didn’t respond.

Technical Diagnosis:

• Review time-current coordination curves.

• Compare event logs from all relays involved.

• Ensure at least 0.3–0.4 seconds of time selectivity between feeder and incomer.

Corrective Action:
Recalculate protection grading using ETAP or DigSILENT. Adjust TMS or curve type for proper coordination.

 Did the Fault Current Actually Reach the Relay Inputs?

A relay can’t act on what it doesn’t see.
If the CT secondary circuit is open, shorted, or wired to the wrong terminals, the fault current will never reach the relay.
This issue is common during maintenance or panel modifications.

Diagnosis Steps:

• Check CT secondary continuity.

• Inspect terminal blocks for loose or disconnected wires.

• Compare actual secondary current (using clamp meter) with relay metering.

Fix:
Tighten all CT terminals, verify ratio and polarity, and perform a secondary injection to confirm correct measurement.

 Could Wrong CT Polarity Prevent the Relay from Operating?

Yes — especially for directional and differential relays.
If CT polarity is reversed (S1–S2 swapped), the relay will read fault current in the opposite direction, blocking tripping.

Field Example:
In a motor feeder using 67 directional element, wrong CT polarity may make the relay think the fault is “reverse,” preventing the trip command.

Corrective Action:
Conduct a CT polarity test using a secondary injection set or polarity tester before energization. Verify polarity arrows and wiring consistency from CT to relay.

 Are the Relay Settings Too High or Incorrectly Configured?

Incorrect pickup current (I>) or time dial values are among the leading causes of non-operation.
If the fault current is below the pickup threshold, the relay will not initiate tripping.
This often happens when system impedance increases or the CT ratio is selected incorrectly.

Example:
If the relay is set to pick up at 4A secondary current but the actual fault only reaches 3.5A, it will remain idle.

Solution:

• Verify fault levels against relay settings.

• Review curve type (IEC Normal Inverse, Very Inverse, etc.).

• Check that instantaneous (I>>) element thresholds match your coordination study.

Can a CT Secondary Fuse or Disconnection Cause Relay Failure?

Yes.
If the CT secondary circuit opens (due to blown fuse, loose wire, or bad terminal), the relay input current drops to zero — it sees no fault.
Older electromechanical relays often failed silently this way.

Diagnosis:

• Measure CT secondary voltage (should be near zero during normal operation).

• Check for open-circuit voltage spikes (dangerous in open CTs).

• Inspect terminal fuses if used.

Action:
Always use shorting links during maintenance. Replace blown CT fuses and confirm continuity before re-energization.

 Could the Trip Circuit or Coil Be the Real Cause?

Sometimes, the relay does issue a trip command, but the breaker never opens.
The issue lies in the trip path, not the relay itself.
Trip coil burnout, open DC supply, or interposing relay failure are common reasons.

How to Check:

• Verify trip circuit continuity using a multimeter.

• Inspect DC supply voltage (110/220V DC).

• Check for alarms like “Trip circuit open” or “Coil fail.”

Solution:
Implement trip circuit supervision in all critical panels.
It continuously monitors the trip coil and wiring, alerting operators before a failure.

Could Relay Logic or Interlocks Block the Trip Signal?

Modern digital relays use programmable logic.
Interlocks are often configured to prevent false tripping — but sometimes they unintentionally block valid operations.

Example:
Maintenance mode activated, breaker status input reversed, or external block signal from busbar protection.

Diagnosis:

• Review logic diagrams and input/output status from relay HMI.

• Check “Trip not issued due to block” messages in the event report.

Action:
Disable unnecessary interlocks. Ensure all block inputs are documented and tested during commissioning.

 Could Firmware or Configuration Corruption Cause Non-Operation?

In numerical relays, firmware and configuration files define the relay’s entire behavior.
If these files are corrupted or mismatched after a firmware update, protection logic can malfunction.

Symptoms:

• Relay appears normal but doesn’t operate.

• Settings reset to default after reboot.

• Event logs missing or incomplete.

Fix:

• Compare checksum and firmware version with backup files.

• Re-upload configuration and perform a secondary injection test to validate logic.

• Maintain firmware version control as part of your protection relay testing services.

 Could a Power Supply Failure Disable the Relay?

Yes. Relays and trip circuits often share a common DC auxiliary supply.
If the supply voltage drops (battery failure, blown fuse, or loose terminal), the relay cannot issue a trip even if it detects the fault.

Field Test:

• Monitor DC voltage trend during fault conditions.

• Check battery charger alarms or voltage dips.

• Verify trip coil energization current.

Preventive Measure:
Use redundant DC feeds or monitored power supplies for critical relays.

How Can Event Logs and Disturbance Records Help in Analysis?

Every numerical relay maintains event logs and disturbance records (COMTRADE files) that record actual current, voltage, and trip commands during faults.
These are your most powerful diagnostic tools.

Procedure:

1. Download event logs post-fault.

2. Check if the relay detected the fault (pickup flags).

3. Verify whether a trip signal was issued and acknowledged by the breaker.

Benefit:
Disturbance analysis helps distinguish between “relay didn’t sense” vs “relay sensed but breaker failed.

 Conclusion

A relay not operating during fault doesn’t always mean the relay is bad.
In most cases, it’s a chain failure involving CTs, settings, logic, or trip paths.
Through systematic testing — from CT verification to trip circuit inspection engineers can isolate the problem efficiently and restore reliable system protection.

In critical industries, regular protection relay testing services ensure that every device in the protection chain works as intended when the next real fault occurs.