How to connect the test leads for relay protection

Relay protection test leads should be connected to the relay terminals according to the type of test: primary injection leads connect to the CT primary circuit, while secondary injection leads connect directly to the relay's secondary terminals.

Primary Injection Test Lead Connection

For primary injection testing, the test set is connected to the primary side of the current transformer (CT) in the circuit. The output terminals of the injection set are placed in series with the CT primary circuit, allowing the injected current to flow through the CT and the relay simultaneously. This method tests the entire protection system, including CTs, wiring, and the relay itself, and is essential during commissioning or new installations to detect incorrect CT polarity or relay wiring errors (S.

Rao, 2025).

Secondary Injection Test Lead Connection

For secondary injection testing, the test leads are connected directly to the relay's secondary terminals. This method bypasses the CT primary circuit and focuses on verifying the relay's operation, settings, and timing characteristics. Secondary injection is commonly used for routine maintenance because it is sufficient to check relay functionality without stressing the primary circuit (S.

Rao, 2025).

Safety and Preparation

Before connecting test leads: Isolate the relay from the live circuit to prevent accidental energization. Verify the correct polarity of CTs and PTs. Use insulated test leads and wear appropriate protective gear.

Follow the relay manufacturer's instructions and standard testing procedures (IEEE, IEC) to ensure accurate results (CED Engineering, 2025).

Summary

Primary injection: Leads connected to CT primary; tests the full system including CTs and wiring. Secondary injection: Leads connected to relay secondary terminals; tests relay operation and settings. Always ensure safety, correct polarity, and adherence to standards during testing.

Proper connection of test leads ensures reliable relay operation and prevents equipment damage or system instability.