Secondary injection testing of protection relays: a commissioning checklist

Secondary injection testing proves that a protection relay measures correctly, applies its settings as intended and operates its outputs in the right time. A relay test set replaces the CTs and VTs and injects currents and voltages into the relay's secondary inputs, so every function can be checked without energising the primary plant.

The checklist below follows the order most commissioning engineers use. Adapt it to your utility's test procedures and the relay manufacturer's commissioning instructions.

1. Prepare before you go to site

• Get the approved settings file and confirm the version against the relay on site.
• Collect the schematics: CT and VT ratios, polarities, trip and alarm wiring, interlocks.
• Prepare test plans in the test set software for each protection function, with expected pickup values and operating times calculated from the settings.
• Agree the isolation plan: which trips must be blocked and which test blocks will be used.

2. Isolate safely

• Short-circuit CT secondaries on the CT side before opening any current circuit. An open CT secondary on load is dangerous.
• Open VT circuits so that injected voltages cannot back-feed the VT.
• Isolate trip outputs to the circuit breaker unless you are deliberately testing the trip path.

3. Check the basics

• Measurement accuracy: inject nominal current and voltage on each phase and compare the relay's displayed values with the injected values, including phase angles.
• Binary inputs and outputs: exercise every input from its source and every output to its destination.
• Logic and LEDs: confirm that alarms, indications and event records match the settings.

4. Test each protection function

Overcurrent and earth fault

• Pickup and drop-off for each stage.
• Operating time at several multiples of the setting, typically 2×, 5× and 10×, compared with the IDMT curve defined in IEC 60255-151 or the IEEE equivalent.
• Directional elements: check the forward and reverse boundaries of the characteristic angle.

Distance protection

• Shot tests at points inside and outside each zone for every fault loop (L-E, L-L, three-phase), with operating time.
• Search tests along lines in the impedance plane to verify the reach and shape of the characteristic.
• Power swing blocking, switch-onto-fault and VT fuse failure logic, if enabled.

Transformer and line differential

• Inject currents on both sides with the correct vector group compensation to prove stability for through faults.
• Test points on the bias characteristic slopes, which needs enough current channels to feed both sides at once.
• Second-harmonic restraint, used to block during inrush, and fifth-harmonic restraint, used against overexcitation.

Breaker failure and auto-reclose

• Simulate the trip, keep current flowing and measure the breaker failure time.
• Run complete reclose sequences with the test set simulating breaker status contacts.

5. Prove the complete scheme

• Trip tests to the actual breaker, with the operator's agreement.
• End-to-end tests for line differential and teleprotection schemes, using two GPS-synchronised test sets that inject coordinated fault states at both line ends.
• On-load checks after energisation: measured currents, phase angles and differential currents should match the expected load flow.

6. Record and restore

• Save test reports with injected values, measured times and pass/fail against tolerance.
• Remove all test connections, restore the isolations in the reverse order and check that no alarms remain.
• Download the final settings and keep a copy with the commissioning records.

A three-phase relay test set with automated test modules and report templates removes most of the manual calculation from this list. See how to choose the right test set for the relays you maintain.