The most important parameter in the operation of large circuit breakers is the timing of the main contacts. Timing errors indicate that the breaker needs servicing and have an adverse effect on synchronisation. For these reasons, all circuit breaker test sets provide facilities for measuring contact timing. The conventional test method is simply to apply a DC test voltage across the contacts and monitor the resulting current, so that the instant of opening and closing can be determined.
A major safety hazard
With this method only one side of the breaker can be grounded. This may not appear to be a big problem, but leaving one side of the circuit breaker ungrounded creates a major safety hazard as the ungrounded side of the breaker may be energised by induction from nearby circuits.
Dual ground testing
Clearly, testing with both sides of the breaker grounded – dual-ground testing – is highly desirable, and a number of methods have been devised to permit this. One is the indirect measurement of contact timing, by performing the tests on the circuit breaker auxiliary contacts. Unfortunately, the values obtained in this way are not necessarily reliable.
Another approach is dynamic resistance measurement, which involves the injection of large currents into the circuit breaker under test, but this method involves the use of test equipment that is inconveniently heavy and bulky.
A new test method
To address the shortcomings of these techniques, Programma, a part of the Megger organisation, has developed a new method of circuit breaker testing. This is based on viewing the circuit breaker contacts as two plates of a capacitor that, in conjunction with stray inductances, form a resonant circuit.
The key to the operation of this method is that, as the contacts move the resonant frequency of the circuit changes. The Programma TM1800 test set first applies a sweep frequency in the megahertz range to the contact assembly. This enables the baseline resonant frequency to be determined.
The instrument then tracks the changes in this frequency throughout the breaker operating cycle, and uses this information to determine the exact contact timing.
At the high test frequencies used, the external circuits connected to the breaker appear as high impedances. Grounding these circuits therefore has no effect on the accuracy of the test results. In other words, dual-ground testing, with its inherently enhanced safety, is fully supported.
Additionally, because the impedance of the external circuits effectively isolates them from the contacts under test, it is never necessary to disconnect the breaker’s main circuits – a big advantage where bolted busbar connections are used.
Results from the high-frequency circuit breaker test set are presented in exactly the same way as those from a traditional circuit breaker tester, making them easy to interpret. The test is also compact, light and easily portable.
Cut costs and save lives
Dual-ground testing of circuit breakers cuts costs and has the potential to save lives. Until now, it has been difficult and inconvenient to implement but, with the advent of the Programma TM1800 circuit breaker test sets from Megger, these shortcomings have been completely overcome.