MIDWEST asked our Switchgear Engineering Technicians for an example of a molded case circuit breaker that had a hidden defect that could only be found by experienced testing. We find many used circuit breakers that have defects that can be found by a close visual inspection. Sometimes the defect may not be obvious but an experienced Engineering Technician knows where to look and would find the problem. In the business of repairing and reconditioning circuit breakers, we find about every possible problem that can occur. Whether we are reconditioning a replacement Westinghouse, General Electric or Siemens circuit breaker, many of the problems are the same. But we also frequently find hidden defects that could only be detected using proper testing.
For this example, let’s just use a replacement Square D PAL362000 circuit breaker. The breaker was reconditioned, which involved removing the cover to thoroughly inspect and maintain the interior of the Square D circuit breaker. The technician was immediately suspicious because he detected the slight smell of an overheated circuit breaker. Once you get that smell in your nose, you never forget it in the future. Even a small whiff tells you there may be a problem. In this case everything visible was in very good condition. There were no signs of overheating at the contacts or the line or load side breaker terminals. But the contact resistance test results indicated very high resistance on the center pole. A DLRO, digital low resistance ohmmeter, was used to locate the problem. The usual location is either the contacts, the bolted connections to the trip device, or the line or load side terminals or lugs. In this case the defect was inside the trip device. And a close sniff of the trip element confirmed it. MIDWEST performed an additional test, a high current test, to determine the voltage drop and therefore the resistance in the trip device on the center pole. Even though this replacement Square D circuit breaker looked just great, the trip device was defect and had to be replaced.