VA-TEK Engineering Secondary injection tests the relay at its terminals. Primary injection drives current through the complete primary circuit, including the CT, its ratio and polarity, the secondary wiring, the relay and the trip output. They are not alternatives; they answer different questions, and a commissioning program normally needs both.
Key takeaways
- Secondary injection proves the relay, its settings and the panel wiring from the injection point onward.
- Primary injection proves the CT ratio and polarity, the secondary current circuit and the overall trip path.
- Sequence: secondary first to prove the relay and logic, primary second to prove the current chain, then a trip test to prove the breaker.
What each test proves
| Question | Secondary injection | Primary injection |
|---|---|---|
| Where the quantity is applied | At the relay current and voltage terminals | Through the primary conductor or switchgear circuit |
| Relay measurement and logic | Yes, directly | Yes, through the CT and secondary circuit |
| Panel wiring from the injection point | Yes, up to the relay terminals | Yes, the complete secondary current circuit |
| CT ratio and polarity | No | Yes |
| CT saturation behaviour | Only if the test set reproduces the waveform | Yes, within the current the source can deliver |
| Trip coil and breaker operation | Only if the trip circuit is included | Yes, when the trip circuit is restored for the test |
| Typical stage | Commissioning and periodic maintenance | New switchgear, CT replacement, differential stability checks, owner requirement |
The boundary is the injection point. Secondary injection cannot prove a CT ratio or polarity, because the CT is not in the circuit. Primary injection can prove the whole chain, but it cannot easily separate a relay error from a CT error once the current is flowing.
When primary injection is required
- New switchgear or a new CT installation: prove ratio, polarity and phase identity before the protection is relied upon.
- CT replacement or re-termination: a secondary test cannot detect a reversed or misidentified CT.
- Differential schemes: a through-fault primary test is the most direct check of stability and CT group matching.
- After cable or busbar work: confirm that the current path and the secondary circuit still correspond to the drawings.
- Owner or utility specification: some specifications require primary injection for all current-operated relays, independent of the secondary test result.
How a primary injection test is staged
- Plan the current path. Decide which primary conductor will carry the current, where the source will be connected, and how the return path is completed. For switchgear, this may mean removing the breaker or using a service-position connection.
- Isolate the trip. Disconnect the trip coil or disable the trip output so an injected current does not trip the breaker. Confirm the isolation point and tag it.
- Confirm the circuit is dead. Use a rated voltage detector, open the breaker, confirm the position indicator and discharge the mechanism. Apply lockout/tagout and arc-flash precautions.
- Verify the relay settings and CT parameters. Check the CT ratio, phase identity and relay scaling against the rating plate and setting sheet before injecting.
- Inject one phase at a time. Raise the current to the planned value and compare the relay reading with the injected value. The tested phase should read correctly; the other phases should read zero.
- Record the result at each phase. Note the injected current, the relay reading, the phase angle and any deviation. Repeat at a second point if the ratio or linearity is in question.
- Restore and prove the trip. Remove the source, restore the trip circuit, and perform a trip test from the relay output to the breaker. Confirm the breaker opens and the indication is correct.
For a differential scheme, the through-fault test is the key step: inject current through one side and out the other so the relay sees a restraint quantity with no differential current. The scheme should remain stable. A second test with the current path deliberately unbalanced proves the operating characteristic.
The limits of primary injection
Available current. A portable primary injection set may not reach the fault current the relay would see in service. The test is still useful at a lower multiple if the relay characteristic is linear over the tested range, but the report should state the achieved current and the reason for the limit.
Transformer impedance. Where CTs are mounted in transformer bushings, primary current may have to be injected through the transformer winding. The winding impedance limits the current, and the test may need a short circuit on another winding. Plan the current path before the outage, not during it.
Access. Busbar and switchgear geometry may require the breaker to be removed or a test shutter to be used. The available method determines the maximum current and the phases that can be tested.
Safety. Primary injection uses low voltage but high current. Loose connections, hot joints and unexpected current paths are the main hazards. Treat every conductor as live until the circuit is proven dead, and keep the test area clear.
How to combine both tests
| Stage | Test | What it establishes |
|---|---|---|
| 1 | Secondary injection | Relay settings, measurement, characteristic and logic |
| 2 | Primary injection | CT ratio, polarity, secondary current circuit and differential stability |
| 3 | Trip and scheme test | Trip coil, breaker operation, interlocking and alarms |
| 4 | On-load check | Phase angle and magnitude against the actual load |
Doing primary injection first is possible, but it is inefficient: a relay that is misconfigured will fail the primary test for a reason that a secondary test would have found in minutes. Do the secondary test first, fix the relay, then prove the current chain.
Frequently asked questions
Can secondary injection replace primary injection?
No. Secondary injection cannot prove the CT ratio, polarity or the primary current path. It proves the relay and the wiring from the injection point, which is a different scope.
Is primary injection required for every relay?
Not universally. It is most important for new CT installations, differential schemes, CT replacements and wherever the owner’s specification requires it. For routine maintenance, secondary injection plus a trip test is common where the CT circuit is known and unchanged.
How much current is needed for primary injection?
Enough to prove the ratio and the relay response at a meaningful point. The exact value depends on the test purpose: a ratio check may need only rated current, while a differential stability test needs a through-fault multiple. Record the achieved current and do not imply a higher value.
Can primary injection prove CT saturation?
It can, if the source can deliver enough current and the secondary burden is realistic. Where the source cannot reach the saturation region, the limitation should be stated; a secondary test with a reproduced waveform can supplement, but it does not prove the CT itself.
Where VA-TEK test sets fit
Secondary injection is the core use of the VAE-430: 3×0–40 A AC, 4×0–120 V AC, 8 binary inputs and 4 binary outputs, with up to 1×0–120 A in parallel for high-burden relays. The VAE-660 adds six current channels for differential schemes tested from several ends at once. CT and PT ratio, polarity and excitation are measured with the VACTP200P, which covers ratio to ±0.05% and knee-point voltage up to 40 kV. The full range is listed on the relay test set hub.
VA-TEK does not supply a primary injection set; the primary test is performed with the contractor’s high-current source. The secondary test procedure is covered in detail in the secondary injection testing guide. For the CT/PT characteristics that the primary test confirms, see CT/PT excitation and knee point.
Free whitepaper: the standards, test items and test-set requirements discussed in this article are consolidated in Relay Protection Testing: Standards, Test Items and Test-Set Requirements — a 29-page PDF, free to download without registration.
Standards and references
- DL/T 995-2016, field inspection items for CT/VT circuits and whole-scheme trip tests.
- IEEE C37.233-2023, Guide for Power System Protection Testing: standards.ieee.org.
- NETA ATS-2025 / MTS-2023, Chapter 7.9: netaworld.org.
