How to Perform Time-Current Characteristic (TCC) Testing Properly?

Time-Current Characteristic (TCC) testing ensures that overcurrent relays trip according to their designed curve. Incorrect timing can result in equipment damage or coordination failure.

Protection coordination depends heavily on accurate TCC performance.

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Why TCC Testing Is Critical?

Every overcurrent relay follows a specific time-current curve. The curve determines:

• How fast the relay trips

• How it coordinates with upstream and downstream devices

• Whether selective protection is achieved

Improper timing may cause:

• Nuisance tripping

• Cascading outages

• Equipment overheating

Step-by-Step TCC Testing Procedure

Step 1: Verify Relay Settings

Confirm pickup current and curve type.

Step 2: Inject Controlled Current

Apply several multiples of pickup current.

Step 3: Record Trip Time

Measure actual trip delay.

Step 4: Compare With Manufacturer Curve

Ensure timing deviation is within tolerance.

Step 5: Adjust if Necessary

Recalibrate settings if discrepancies are found.

Common Mistakes in TCC Testing

• Ignoring CT saturation

• Incorrect current ramp rate

• Not compensating for temperature variation

• Failing to test multiple current levels

Routine TCC testing ensures compliance with protection coordination studies.

For differences between primary injection and secondary injection testing, read Primary vs. Secondary Injection Testing: What’s the Difference?