In short. Secondary injection feeds simulated currents and voltages into a relay to prove its settings and logic. Primary injection passes real current through the primary circuit to prove CTs, wiring and relay together. Use secondary injection for relay function and primary injection for the end-to-end chain.
Comparison
| Aspect | Secondary injection | Primary injection |
|---|---|---|
| Where applied | Relay terminals | Through the primary conductor or busbar |
| Proves | Relay pickup, timing, logic, outputs | CT ratio, polarity, wiring, relay response, trip path |
| Equipment | Relay test set | High-current injection set |
| Typical use | Every relay at commissioning and maintenance | New installations, differential schemes, after CT work |
Secondary injection plan
- Record as-found settings and compare with the approved sheet.
- Test pickup and dropout for each element.
- Test operating time at several multiples of pickup.
- Test directional and sequence elements with correct phase angles.
- Test logic: blocking, interlocks, setting groups, outputs and LEDs.
- Test the trip contact to the breaker, ideally by operating the breaker.
- Restore settings and record as-left values.
Primary injection plan
- Isolate and secure the circuit. Apply a temporary short where required.
- Inject a safe level of current and check each CT’s secondary reading, polarity and ratio.
- For differential schemes, confirm the differential current is near zero for through current and operates for simulated internal conditions.
- Verify that the correct relay and breaker operate.
- Remove all temporary links.
Safety
- Never leave a CT secondary open when current is flowing.
- Isolate trip outputs when testing relays on live equipment unless the breaker operation is intended.
- Use permits and test procedures, with a check list for restoring all links.
Typical findings
- Wrong CT ratio or polarity.
- Trip contacts not wired.
- Setting groups not switching.
- Settings differing from approved sheets.
- Test links left open.
Illustrative example
Primary injection through a transformer differential scheme shows the differential current rising rather than cancelling. One CT’s polarity is reversed. Without the test, the relay would have tripped on the first load.
Two kinds of injection test
| Test | What is injected | What it proves |
|---|---|---|
| Secondary injection | Current and voltage into relay terminals | Relay settings, timing and output logic |
| Primary injection | High current through the primary circuit | CT ratio, polarity, wiring, relay response end to end |
Procedure outline
- Isolate and make safe; disable unintended trips or arrange them.
- Record as-found settings.
- Inject at 1.05, 1.5, 2 and several times pickup and confirm operating times against the curve.
- Check each output contact, alarm and trip.
- For primary tests, inject through the CT and confirm secondary current and relay reading.
- Restore, remove test connections, check CT shorting links are removed, and record as-left settings.
Acceptance (illustrative)
A relay set to trip at 100 A with a given curve is tested at 150 A, expecting a defined time. Measured time within the relay’s declared tolerance passes. Outside it, investigate.
Safety
- Do not leave CT secondaries open.
- Lock out and tag sources; verify absence of voltage.
- Only qualified testers should work on live panels.
Common faults found
Wrong polarity, incorrect ratio, CT shorting links left in, trip contacts not wired, and settings not as specified. See switchgear testing.
Common mistakes
- Testing the relay but not the circuit.
- Failing to restore settings.
- No as-found record.
What to send us
Send relay models and settings. Send your application details. See also switchgear testing.
Questions engineers ask
Is secondary injection enough?
For routine checks of the relay, yes. New installations also need primary injection or equivalent checks.
Why record as-found values?
To show drift or unauthorised changes and keep a history.
Can injection be done live?
Secondary injection on live equipment requires strict isolation of trip and CT circuits. Often planned outages are used.
How often should relays be tested?
At commissioning and at periodic intervals set by policy and the relay type.
This guide gives general principles. Follow equipment and test-set instructions.