Secondary injection testing verifies the protection relay and its input circuits by injecting test signals at the relay’s secondary terminals, while primary injection testing verifies the whole chain by injecting current through the current transformer and the primary circuit. The methods differ in scope, equipment, outage requirements and the evidence they produce, and a complete commissioning plan usually includes both.
The Two Methods Verify Different Parts of the Protection Chain
The protection chain runs from the primary conductor, through the current transformer (CT), the wiring, the relay input and the relay logic, to the trip circuit. Secondary injection starts at the relay terminals, so it verifies everything downstream of that point: the relay settings, timing, logic and output. Primary injection starts at the primary side, so it verifies the CT, its ratio and polarity, the wiring, and the relay response together.
The practical difference is where the evidence begins. A relay that passes secondary injection can still fail in service if the CT or the wiring is wrong, and a CT with the correct ratio can still leave the relay unprotected if the settings or logic are wrong. The two methods close different gaps in the chain.
The scheme type also shapes the test plan. An overcurrent scheme needs verification of pickup, time-current characteristics and timing; a differential scheme needs verification of the CT ratios, polarities and the relay’s restraint behaviour; a distance scheme needs impedance, angle and logic verification. Both injection methods contribute to each scheme, but the balance between them follows the scheme’s failure modes.
What Secondary Injection Testing Verifies
Secondary injection applies accurate current and voltage signals to the relay’s secondary terminals, simulating fault conditions to verify pickup, timing, characteristic curves, logic and trip outputs. It is the standard method for testing relay settings and performance, and it follows the test requirements defined for the relay type, typically within the IEC 60255 series for measuring relays and protection equipment.
Secondary injection requires the CT secondary circuit to be isolated or bypassed at the relay, and it can be performed with the CT in place. Its advantage is precision and control: the injected signals are known, repeatable and can be stepped through the relay’s full operating range without energising the primary circuit. Its limit is that it does not prove the CT, the wiring from the CT to the relay, or the polarity of the installation.
The test set for secondary injection should provide accurate, synchronised current and voltage channels with the range and phase control the relay’s characteristics require, and record the injected values with the measured response. The accuracy of the test set is part of the evidence, so its calibration status belongs in the report with the relay results.
Document the test plan before the work: the settings to verify, the points on the characteristic to measure, the tolerances applied and the acceptance criteria. A written plan prevents the common gap where a relay is tested but the evidence does not cover the settings that are actually in service.
What Primary Injection Testing Adds
Primary injection passes a current through the CT primary winding, large enough to produce a secondary current that operates the relay, thereby verifying the complete path: CT ratio and polarity, secondary wiring, the relay input and the relay operation. It is the definitive test for CT ratio and polarity correctness, and it proves that the measured current actually reaches the relay with the expected magnitude and phase.
Primary injection requires a high-current test source and typically a short isolation of the circuit, because the current is injected into the primary conductor. It is used at commissioning and after CT or wiring changes, and it complements secondary injection by verifying the parts of the chain that secondary testing cannot reach. The current level is set by the test plan and the CT ratio, and the operation is subject to strict safety control because of the high current involved.
The evidence produced is the secondary current seen by the relay: its magnitude confirms the CT ratio, and its phase relationship confirms the polarity and the wiring from the CT to the relay. For a multi-ratio CT, the test may be repeated at the ratio in service, and the results are compared with the CT’s nameplate and the scheme design.
Outage, Current and Safety Requirements
The two methods differ in operational impact. Secondary injection is performed on an isolated relay circuit with modest test equipment, and it is the routine method for setting verification and periodic testing. Primary injection needs the primary circuit isolated and a high-current source, which increases the outage requirement, the equipment footprint and the safety controls, so it is typically reserved for commissioning, major changes and periodic verification of the CT chain.
Both methods require the protection to be taken out of service with the appropriate work permits, and the test leads and sources must be rated for the currents used. Primary injection in particular demands a documented safety plan: the high current path, the exclusion zone, the earth connections and the communication between the injection point and the relay point.
Because the CT secondary is live during primary injection, the safety rules for CT secondary circuits apply: never open the secondary under load, verify the shorting and grounding arrangements, and confirm that personnel are clear of the circuit before the current is applied. The test director and the relay point operator should agree on the command sequence and the stop conditions in advance.
When Both Methods Belong in One Commissioning Plan
A complete commissioning or verification plan normally includes both: primary injection to prove the CT ratio, polarity and wiring, followed by secondary injection to prove the relay settings, characteristics and logic. The order matters, because primary injection first proves the sensing chain, and secondary injection then verifies the relay against that proven input.
| Scope element | Secondary injection | Primary injection | Evidence produced |
|---|---|---|---|
| Relay settings and logic | Yes | Partial | Pickup, timing, characteristic, trip output |
| CT ratio and polarity | No | Yes | Secondary current magnitude and phase |
| CT to relay wiring | No | Yes | Continuity and polarity through the circuit |
| Overall chain response | Partial | Yes | End-to-end operation from primary current |
The table summarises the scope of each method. The final plan should be written for the specific scheme, using the applicable standards and the utility’s commissioning procedure, and the results of both methods recorded in the same test report so the chain evidence is complete.
Acceptance should be based on the complete chain: the primary-injection results show that the current reaches the relay with the correct magnitude and phase, and the secondary-injection results show that the relay responds with the correct timing, logic and output. The report should reference both sets of evidence against the scheme’s protection settings and the applicable standards.
Choosing the Method for the Required Evidence
Choose the method by the evidence gap you need to close. For a relay setting change on a proven CT circuit, secondary injection is sufficient. After a CT replacement, a wiring change or a new installation, primary injection is required to prove the sensing chain, with secondary injection completing the relay verification. For periodic maintenance, the plan defined by the utility’s procedure decides the balance between the two.
Record the reason for each method in the report, together with the settings, the injected values, the measured response and the acceptance criteria. The report should show that the protection chain was verified end to end, and that each method covered the part it is responsible for.
For periodic maintenance, the utility’s procedure defines the balance: secondary injection on a defined interval for setting verification, with primary injection at commissioning, after CT or wiring changes, and at longer intervals for the sensing chain. The maintenance record should show which method was used and why, so the scheme’s evidence history is complete.
Frequently Asked Questions
Why is primary injection done first in commissioning?
Because primary injection proves the sensing chain from the CT to the relay, and the relay settings are then verified against that proven input with secondary injection. Doing secondary injection first would verify the relay against an assumed CT and wiring path.
Can secondary injection replace primary injection?
No. Secondary injection verifies the relay and its input, but it cannot prove the CT ratio, polarity or the wiring between the CT and the relay. Primary injection covers those parts, so the two methods are complementary.
What outage is needed for each method?
Secondary injection requires the relay circuit to be isolated but not the primary circuit; primary injection requires the primary circuit to be isolated and a high-current source connected. Primary injection therefore carries the larger outage and safety requirement.
For the relay-test-set selection framework, see the relay protection test equipment guide. When you need to specify injection test systems for your protection schemes, review relay protection testing solutions and request a technical proposal.
The evidence gap each method closes is the deciding factor: use secondary injection to prove the relay, primary injection to prove the sensing chain, and both together to prove the protection scheme end to end.