In nuclear installations, molded case circuit breakers (MCCBs) play a key role in protecting vital equipment. Nevertheless, in the past they’ve rarely been tested. It’s time for this to change, says Megger’s John Strange.
It’s often hard to know that an MCCB has developed a defect until it is called upon to operate. Unfortunately, by then, it’s too late to do anything about the problem. In nuclear installations this is a particularly serious issue, as the correct, or at least predictable operation of virtually every item of plant has a bearing on overall safety.
The obvious solution is to test MCCBs regularly but testing MCCBs by traditional methods is costly, inconvenient and time consuming. In addition, it often yields dubious results. Let’s see why.
Eight hours per test
The standard approach to MCCB testing involves removing the breaker from the equipment in which it is installed, and transferring it to a dedicated test assembly. Research by Megger in conjunction with engineers at Pacific Gas & Electric Company’s Diablo Canyon nuclear power plant, shows that this process typically takes a full eight-hour shift to test just one breaker.
Here’s the justification. Taking the motor control center in which the breaker is used out of service requires around an hour. Removing the breaker takes another hour, followed by 15 minutes of radiation screening, and 15 minutes to transport the breaker to the test facility. The test takes two hours, with a further hour to return the breaker to the panel, re-install put the panel back in service.
And time is not the only problem. Testing the breaker outside its normal operating environment often affects the accuracy of the results.
Testing in place
Clearly, it would be much better if it were possible to test the breaker without removing it from the MCC or switchboard. This is now possible, thanks to the introduction of the Oden MCCB test set from Programma, a part of the Megger organization.
For testing in place, many factors have to be considered. For example, the connections to the breaker under test cannot be conveniently made using a pre-defined length and size of wire, as is recommended by NEMA.
Instead, the Oden test set uses a handheld probe that is insulated and rated for the working voltage of the equipment under test, therefore eliminating the need for users to wear special safety gear.
The Oden test set also has an I/30 function that injects one-thirtieth of the normal test current. This avoids pre-conditioning before the test is carried out, and also eliminates the 20-minute cooling down period.
Big time savings
The time needed for each test is greatly reduced, as a practical example at Diablo Canyon clearly shows – 77 breakers were checked in six hours, allowing the panel to be returned to service before the end of the shift.
With the new test regime, it has been possible to implement a greatly improved MCCB testing program at Diablo Canyon, with the result that the out-of-tolerance (OOT) rate of breakers on test at the plant has fallen from 25% to just 1%.
Considering the vital role they play in nuclear installations, molded case circuit breakers rarely receive the regular attention they deserve. The latest test technology, however, is making it easy to address this issue, as demonstrated by the experience of the engineers at Diablo Canyon nuclear facility.