Primary/Secondary Injection Test (Protection Relay)

Two test methods used to verify the correct operation of a protection relay (for example an overcurrent, differential or earth-fault relay) in a medium-voltage or large low-voltage installation. In a secondary injection test, a test set is connected directly to the secondary side of the current and voltage transformers (i.e. directly to the relay input), and only the relay itself is tested for its setting and time characteristics, without loading the primary current transformers, the wiring to the relay panel or the tripping circuit itself. In a primary injection test, by contrast, a large test current is actually driven through the primary side of the installation (the power circuit itself), so that not only the relay but the entire measuring chain — current transformers, wiring, terminal connections and the relay — is validated as a whole, including any saturation effects or wiring errors that a secondary test would not reveal. Primary injection requires specialised, heavy test equipment and a full outage of the circuit under test, and is therefore applied less frequently than secondary injection, typically only at commissioning, after major modifications, or when the integrity of the entire measuring chain is in doubt.