Secondary injection applies currents and voltages directly to the relay, at its terminals or at the test block, and verifies the relay and its logic. Primary injection drives a high current through the primary of the current transformer or of the breaker, and verifies the whole chain: CT ratio and polarity, wiring, relay and, if enabled, the breaker trip. In commissioning they complement each other.
The two ways to test a protection
A protection is a chain: the current transformer (CT) or voltage transformer (VT) in the switchyard, the wiring to the panel, the relay, its logic, the trip circuit and the breaker. The two tests differ in the point of that chain where the signal is applied.
| Secondary injection | Primary injection | |
|---|---|---|
| Where it is applied | At the relay terminals or at the panel test block | At the CT primary, or across the terminals of the low-voltage breaker |
| Current magnitude | CT secondary level | Primary current, high, with a test set able to deliver it |
| What it verifies | Relay: pickups, times, characteristics, logic and outputs | CT ratio and polarity, secondary wiring, phase assignment in the relay, metering, trip |
| What it does not verify | The CT or the wiring from the switchyard to the panel | The complete relay characteristics at fault currents |
| Equipment | Three-phase secondary injection test set | High-current test set or primary injection source |
What secondary injection verifies
A secondary injection test set, such as the OMICRON CMC, replaces the CTs and VTs with controlled currents and voltages. It allows every relay function to be covered precisely: pickups, times, curves, distance zones, differential slopes, logic, reclosing and communications. It is the main relay test in commissioning and maintenance, with the scope set by the relay test plan, and the standards that govern it are in standards for protection testing and secondary injection.
Its limit is the connection point. If a CT has reversed polarity, if two phases are crossed at a switchyard terminal block or if the CT is connected to the wrong ratio tap, the relay will pass secondary injection and fail in service.
What primary injection verifies
With primary injection the current flows through the real chain. In general terms it verifies:
- CT ratio, by comparing the primary current with what the relay measures.
- Polarity, by checking the direction of the current in the relay against the primary.
- Wiring and phase assignment, by injecting phase by phase and between phases.
- CT circuits shared by several functions, such as differential, metering and backup protection sharing cores or terminal blocks.
- Differential stability, by passing current through the zone and checking that the relay sees no differential current.
- Breaker trip, if the current reaches the function pickup and the trip circuit is enabled.
The CT tests themselves, such as ratio, polarity, excitation and knee point, are described in CT excitation and knee point and in current transformer.
When each one is used
- Do you want to verify the relay, its settings or its logic?
- YesSecondary injection at the test block
- NoDo you want to verify the chain from the primary (CT, wiring, phases)?
- YesIs the equipment de-energized and the primary accessible?
- YesPrimary injection before energizing
- NoOn-load checks at energization
- NoReview the test scope with the commissioning lead
- In the commissioning of a new bay, both are used: secondary for the relays, and primary, or equivalent CT and wiring tests, for the chain.
- In relay maintenance, secondary injection is the usual test. Primary injection is repeated when a CT is changed, a panel is rewired or a relay is replaced.
- In low-voltage breakers with an electronic trip unit, primary injection is the one that verifies sensors and unit together. Schneider, for example, documents that its secondary injection test kit for Masterpact NT and NW does not test the current transformers or their connections. We expand on this in low-voltage trip unit and in low-voltage circuit breaker testing.
Limits of primary injection
- Available current. The test set delivers a fraction of the fault current. It serves to verify low pickups and the chain, not to cover curves at high multiples.
- Logistics. The test sets are heavy, the high-current leads are short and thick, and the primary must be accessible and de-energized.
- Wear. Every test with a trip is a breaker operation.
- Safety. There are high currents and, on the secondary side, the risk of opening an energized CT circuit.
The energized complement: on-load checks
Neither injection tests the installation with its real service voltages and currents. That is why commissioning ends with hot commissioning, when the equipment is energized and takes load. In general terms the checks are:
- Phase sequence and voltage magnitudes at every relay and meter.
- Directionality: directional and distance functions see the load in the expected direction.
- Differential currents close to zero in transformer, bus and line differentials.
- SCADA measurements consistent with those of the relay and the meters.
These checks confirm what primary injection should already have ensured. If primary injection was not done, they are the first chance to find a reversed CT, with the risk that the error is discovered through a misoperation.
We do both tests
At Tiza Group we perform secondary injection testing and current and voltage transformer testing in the commissioning of medium, high and extra-high voltage substations and industrial plants, in Colombia, or elsewhere on request.
Related guides
- OMICRON CMC 356, CMC 430 and Test Universe
- Doble F6150e, Megger SMRT410 and Freja test sets
- Test blocks and bay isolation for testing
- Ratio, polarity and vector group test (TTR)
- Low-voltage circuit breakers: ACB, MCCB and MCB, parts and tests
- Commissioning standards: FAT and SAT
- Cold and hot commissioning: de-energized and energized
References
- IEEE C37.233-2023, Guide for Power System Protection Testing. https://standards.ieee.org/ieee/C37.233/6676/
- IEEE C37.110-2023, Guide for the Application of Current Transformers Used for Protective Relaying Purposes.
- ANSI/NETA ATS-2025, Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems.
- Schneider Electric, MasterPact NT and NW catalog, test equipment (the secondary injection kit does not test the CTs or their connections). https://www.productinfo.schneider-electric.com/0613ct0001mp/
- OMICRON, CPC 100 brochure (injection at the CT primary for ratio, polarity and burden).
Frequently asked questions
Does primary injection replace secondary injection?
No. Primary injection only reaches moderate currents compared with fault currents, and it is hard to cover a relay's complete characteristics with it. Secondary injection tests the relay and its logic thoroughly; primary injection confirms that the chain from the primary is correctly connected. Commissioning uses both.
Why is primary injection preferred for low-voltage breakers with an electronic trip unit?
Because the manufacturer's secondary injection test goes straight into the trip unit and does not test the current sensors or their connection. Primary injection passes current through the breaker poles and verifies sensors, internal wiring, trip unit and opening in a single test.
Can primary injection be replaced by on-load checks?
Partly. With the installation energized and enough load, directionality, differential currents and the sign of the measurements are verified. But those checks come late, when the equipment is already in service, and they depend on there being load. Primary injection finds wiring errors before energization.