Primary versus secondary injection testing of circuit-breakers — what each test method does and doesn't verify

The guide on circuit-breaker LSI(G) settings covers how the trip parameters (Ir/tr, Isd/tsd, Ii, Ig/tg) of an electronic trip unit are configured, and the guide on MCCB versus miniature circuit-breaker covers the distinction between the two breaker types. This article covers a follow-on question: once configured, how is it actually verified that the circuit-breaker and its protection function correctly, and why are there two fundamentally different test methods for this?

Secondary injection testing: only the relay and its wiring

In a secondary injection test, a test signal (current or voltage) is fed directly into the secondary side of the protection circuit — that is, straight into the input of the protection relay or electronic trip unit, downstream of the current transformer (CT). This verifies whether the relay itself responds correctly to the applied test signal (for example: does the Ii setting trip at the correct current value), and whether the wiring between the relay and the trip coil functions. A secondary injection test does not test the CT itself, the wiring between the CT and the relay, or the actual physical tripping of the circuit-breaker under a real, high current.

Primary injection testing: the complete protection chain

In a primary injection test, a real, high test current (often hundreds to thousands of amps, supplied by a specialised primary injection test set) is driven through the entire primary current path: through the CT's primary winding, via the secondary winding and wiring to the relay, and finally to the trip coil that actually causes the circuit-breaker to open. This is the only test method that demonstrates the entire chain — including any wiring errors between the CT and the relay, an incorrectly connected or incorrectly sized CT, and the breaker's actual mechanical operation — in a single test.

Why not always test primary?

A primary injection test requires specialised, heavy and expensive test equipment (capable of delivering the required high test current), and also requires that the relevant current path be taken completely out of service — it is therefore generally not practical for routine periodic maintenance. A secondary injection test is faster, requires simpler and more compact equipment, and focuses specifically on the relay itself — which is why this method is more often used in practice for periodic checks, while a primary injection test is generally reserved for commissioning (to catch wiring errors between the CT and relay that may have been made during new construction or a modification) or for a thorough fault investigation.

Practical relevance

When drawing up a test plan for circuit-breakers, the method must be explicitly chosen to match the purpose of the test: a primary injection test at commissioning of a new or modified protection chain (to rule out CT-wiring errors that a secondary test cannot detect), and a secondary injection test for periodic checks of the relay settings themselves. Using both methods together at the right time gives the most complete assurance about how the protection chain performs.

Common mistakes

Assuming a secondary injection test demonstrates the entire protection chain — this test completely skips the CT and the wiring between the CT and the relay.
Not performing a primary injection test at commissioning of a new installation — this is precisely when wiring errors between the CT and relay are most likely, and a secondary test cannot reveal them.
Trying to perform a primary injection test as a routine periodic check without accounting for the required outage and the heavier test equipment — in practice this is often not feasible for periodic maintenance.
Interpreting the results of a secondary injection test as proof that the circuit-breaker will actually trip during a real fault — the mechanical, physical tripping is only actually exercised by a primary injection test (or a real fault).