Q and A: Circuit breakers

Circuit breakers are at the heart of every modern power system and, to ensure those systems are properly protected, it is essential that  the breakers should operate correctly when called upon to do so. This means that regular testing is vital, so it’s unsurprising that our technical support team often receives questions on topics relating to breaker testing. These are a few of the things we’re asked most frequently.

Q: When measuring the resistance of circuit breaker contacts, the results obtained are often considerably higher than the figures quoted by the manufacturer. Why is this and what can be done about it?

A: By far the most common reason for these unexpectedly high readings is that the test current used in making the measurement is too low. To obtain dependable results, it’s essential to use a test current of 50 A or more. The breaker manufacturer’s literature, or the standard to which the breaker is being tested, will provide more detailed guidance. This need for high current testing  means that a general-purpose low resistance ohmmeter that uses only a modest test current is unlikely to be suitable for measuring the resistance of circuit breaker contacts. A 100 A microhmmeter is perfect for the job.

Q: Can contact resistance be measured with the safety earths still connected to the breaker?

A: Yes, if the test equipment used is confirmed by the manufacturer as being suitable for dual-ground testing. It is important to check this point carefully, however, as by no means all circuit breaker test sets meet this requirement. This is because, as we’ve seen, the instrument must inject large currents into the breaker in order to produce dependable results. For all but the best specified instruments this can be challenging in the case of dual-ground working, where a substantial proportion of the available current is typically shunted via the path provided by the two ground connections.

Q: What is dynamic resistance measurement (DRM) for circuit breaker contacts and what can it reveal about the condition of the contacts?

A: DRM essentially means measuring the resistance of the circuit breaker contacts throughout the opening and closing operations, and then plotting the resistance against time. The plot obtained during the opening operation is particularly informative. It will show a step change in resistance as the main contacts open, since at this point all of the test current is carried by the arcing contacts.

A short time later, the resistance will increase almost to infinity as the arcing contacts open. By noting the time between the operation of the main contacts and the arcing contacts, it is possible to deduce the remaining length of the arcing contacts – something that could otherwise only be determined by dismantling the breaker. This technique does, of course, rely on the availability of dependable information about the motion of the breaker contacts during operation, but Megger circuit breaker analysers like the TM1800 provide facilities for accurate motion analysis as well as for DRM and support for dual-ground testing.