Protection relays are valuable only when they operate fast, selectively and reliably at the moment of a fault. Secondary injection lets us apply simulated currents and voltages directly to the relay inputs and verify its behaviour without energising the network. It is performed both at initial commissioning and during periodic maintenance.
What is tested?
- Pickup and drop-off values: the drop-off/pickup ratio of overcurrent functions is typically around 0.95.
- Operating time and its match with the configured curve
- Directionality of directional relays and accuracy of the characteristic angle (RCA)
- Differential characteristic: minimum pickup, slopes, breakpoint and harmonic restraint
- Distance characteristic: zone reaches (Z1/Z2/Z3), angle and time of each zone
- Logic, inputs and outputs: trip, alarm, blocking, breaker failure and links to other equipment
- Relay measurements, buttons and LEDs
Equipment and preparation
- A three-phase current and voltage test set (for example OMICRON, Megger or Doble) with valid calibration
- The relay settings file and the protection coordination study
- Circuit diagrams and access to test blocks
- Saving the relay's original settings before any change
Safety
- Never leave a CT circuit open; short the CT secondary before disconnecting the relay. An open secondary can reach thousands of volts.
- Isolate or lock the breaker trip circuit so the test does not cause an unwanted trip, unless the trip circuit itself is under test.
- Isolate the VT circuit so injected voltage cannot back-feed the network.
- Work permit and tagging according to the operator's procedure.
Test sequence
- Visual inspection and software version: model, firmware version, date and time, internal alarms.
- Settings review: compare the settings in the relay with the approved file.
- Input/output tests and supply circuits.
- Test of each protection function (pickup, time, direction, characteristic).
- Combined logic tests: for example multi-stage tripping, interlocking and breaker failure.
- Restoring circuits: remove shorts, restore the trip circuit and record the "as-left" state.
Inverse-time curves
For overcurrent relays to IEC 60255-151, the operating time follows the formula below, where M is the ratio of current to pickup and TMS is the time multiplier:
| Curve | k | α |
|---|---|---|
| Standard inverse (SI) | 0.14 | 0.02 |
| Very inverse (VI) | 13.5 | 1 |
| Extremely inverse (EI) | 80 | 2 |
| Long-time inverse (LTI) | 120 | 1 |
Example: for SI with TMS = 0.2 and M = 5, t = 0.2 × 0.14 / (5^0.02 − 1) ≈ 0.2 × 0.14 / 0.0327 ≈ 0.86 s. The measured value must lie within the manufacturer's tolerance (usually ±5 % or ±30 ms, whichever is greater).
Acceptance criteria
- Pickup: typically ±2 % to ±5 % of the setting, per the relay datasheet
- Time: ±5 % or ±30 ms, whichever is greater (for definite-time functions)
- Angle: ±3 degrees (directional relays)
- Logic and output behaviour exactly as in the drawings and trip matrix
Why are settings alone not enough?
Many faults are not in the relay settings but in wiring, CT polarity, trip-matrix connections or programmed logic. Secondary injection tests the relay alone; to be sure of the whole chain (CT to breaker) it must be complemented by primary injection and breaker operation tests.
Test report
A professional report contains: equipment and relay details, settings, injected value, measured value, error, acceptance criteria, test equipment and calibration number, date, tester's name and signature, and the final result (pass/fail). An electronic copy of the settings file and the report should be kept for the operator and for later audits.