MV Circuit Breaker Timing Test

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1. Timing tests of a circuit breaker are usually done with the breaker isolated from the HV or MV system, but with normal DC control power. The test set has leads that you connect to the trip or close coil circuits to sense the (externally applied) trip or close command. It also has leads that connect to each pole of the circuit breaker to detect the opening or closing of the main contacts, and often a transducer that connects to the operating mechanism to measure the travel & velocity of the mech.

2. When there are two trip coils on HV or MV circuit breakers, it is so that two independent relay systems can be used for redundant tripping in high-reliability situations such as bulk power transmission. To verify both systems, each trip coil's operation should be tested separately.

Thank you, I want to add one more question. I saw the test analyzer that the coil sensor leads require to be connected in parallel/across the close/trip coils. The other type the coil sensor leads to be connected across the close/trip switches in the control circuit. Even from the same manufacturer Vanguard model DigiTMR the coil sensor leads to be connected in series with the coil (or across the trip/close switches). But the Vanguard CT-3500 the coil sensor leads to be connected in parallel across the trip/close coil. Do you know what would be the reason.

also the "Timing tests of a circuit breaker are usually done with the breaker isolated from the HV or MV system, but with normal DC control power" is that the same as being on the test position for metal clad 5KV switchgears.

The answer to the first question is that it depends on the technology the test set manufacturer is using to sense the trip or close signal. Always connect the test set per the manufacturer's instructions. If the sensor is connected in series with the coil, it is probably to measure accurately the current required to trip or close the breaker. A parallel connection probably (I'm speculating here) only is concerned with sensing when the coil is fired, not necessarily what current is drawn by the coil.

The second answer depends on what the manufacturer determines is the function of the "Test" position. Most commonly in Test position the breaker is still connected to the secondary control circuits (trip & close switches), but disconnected from the live bus, so that it does not have actual load current flowing through it. That's what I meant by isolated - the breaker is disconnected from the switchgear bus, load cables, or any other MV/HV connections.

Thank you, I appreciate the clarification