Q; What is the first trip test how is it used to evaluate my circuit breaker?
A: The first trip test uses small clamp-on current transformers that connect to the coil circuit and the load, or protection transformers on the circuit breaker while the breaker is still in service.
The breaker is then tripped and the coil current is measured along with the voltage drop. The current extinction times of the three phases are also measured. The coil current trace and other parameters can be compared to previous measurements to see if the breaker is operating normally. This test ensures that no operation is left unmeasured and actually gives a picture of “real life” conditions and how the circuit breaker operates after remaining closed for months or even years.
Q: Since first trip evaluates “real life” conditions, do I still need to perform traditional offline timing on my circuit breaker?
A: Because first trip is relatively easy and quick, some people have tried to replace traditional timing tests with first trip testing. However, it is important to keep in mind that first trip testing complements but does not replace offline time and travel analysis. With first trip you are comparing previous measurements and trending, but time and travel analysis allows you to not only compare and trend results, but to actually verify that the circuit breaker is operating within manufacturer and IEEE/IEC specifications.
Q: You mention time and travel analysis, but connecting the transducer is often difficult. With this in mind, are travel measurements really necessary? Why can’t I just perform timing?
A: Timing ensures that the three phases are synchronized and that the contacts are opening at the correct time, but travel measurements provide a lot more information on how the contacts are actually performing. Travel verifies the stroke of the circuit breaker, as well as the velocity of the contacts. The circuit breaker times can be out of specification, but as long as the velocity of the interrupter is correct it will still be able to clear the fault. Additionally, travel will reveal mechanical issues such as overtravel and over damping. In order to simplify transducer connections, Megger provides a variety of transducers and connection adapters that fit multiple circuit breakers.