Protection testing uses two different techniques, primary injection and secondary injection, and the words are often used loosely. They test different parts of the protection chain, and a complete commissioning usually needs both. This guide explains the difference and when each is used.
The Protection Chain
A protection scheme has several links. The current transformer (CT) senses the primary current, wiring carries the secondary current to a relay, the relay decides and issues a trip, and the trip circuit operates the breaker. A fault in any link defeats the protection.
Secondary Injection
Secondary injection feeds a controlled current directly into the relay's current input, bypassing the CT. It tests the relay itself: its pick-up, its time characteristic and its output contacts. Because the current is small, the equipment is portable and the test is repeatable. The CIE/RTS-5 and RTS-33 relay test sets work in this way. See IDMT relay curves explained for what is verified.
Primary Injection
Primary injection passes a large current through the actual primary conductor, such as a busbar or cable, so that it flows through the CT, the secondary wiring and the relay. It proves the whole chain, including the CT ratio, polarity and wiring. It needs a high-current, low-voltage source, which is heavy, and it must be done with the circuit isolated and set up carefully.
| Secondary injection | Primary injection | |
|---|---|---|
| Where current is applied | Relay current input | Primary conductor through the CT |
| What it proves | Relay settings, timing and contacts | CT ratio, polarity, wiring and the relay together |
| Equipment | Portable, lower current | Heavy, very high current |
| Effort | Quick and repeatable | Slower, more preparation |
CT secondaries must never be open-circuited
Which Do You Need?
- Routine maintenance on an existing relay: secondary injection is usually enough.
- New installation or replaced CT or wiring: primary injection is normally added to prove the whole path.
- Fault investigation: start with secondary to rule out the relay, then primary if the cause is not found.
What CIE Supplies
CIE's relay test sets are secondary injection instruments, and we do not supply primary injection sets. They cover single-phase over-current and earth fault testing and three-phase testing with phase angle control for directional and distance relays. See relay test sets, and read how to choose a relay test set and the substation pre-commissioning checklist.
Contact CIE to discuss your protection testing.