Circuit Breakers and the Importance of Testing

Circuit breakers are automatically operated electrical switches designed to protect an electrical circuit from damage caused by excess current from an overload or short circuit.
The basic function of a circuit breaker is to interrupt the current flow when a fault is detected. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.
Circuit Breakers play a critical role in protecting expensive equipment in your plant. However, they can go long periods without activation, so it is essential that they are regularly maintained and tested. If they fail to activate when needed it could have catastrophic consequences for your staff and plant if an arc flash were to occur and would result in extensive damage to your plant’s electrical systems.

Primary & Secondary injection testing of your circuit breakers should be included as part of your Preventative Maintenance plan to ensure their reliability and safety.
Primary Injection Testing vs Secondary Injection Testing
Primary Injection Testing injects a multiple of the fault rated current of the breaker through the breaker to measure the trip time. 

Secondary Injection Testing injects a ratio of the fault rated current directly into the trip unit and measures how long the breaker takes to trip.

The primary tests of the breaker are closer to a real-world scenario while secondary tests the timing and functioning of the trip unit.

Primary injection testing is usually the preferred test method as it tests the entire circuit including the current sensors, wiring and the current conduction path in the circuit breaker as part of the test. However, it is also more expensive, so is generally conducted at commissioning and on circuit breakers that are part of a critical process or engineered safety system. Other than trip testing other tests that can be conducted as part of your primary injection testing include:
– Power Transformers (Through Faults)
– Relay Testing
– Bus-work, Switchgear and HV Breakers
– Low voltage breakers
– Switchgear Testing
– Heat runs
– Stability tests
– Loose Connections
– Core Identification
If you need help incorporating primary and secondary injection testing as part of your preventative maintenance programme get in touch with the EAS team today on 07 834 0505 or [email protected].


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