Sometimes MIDWEST’s Engineering Technicians in our Switchgear Shop like to do what the Engineers call “Play.” The technicians call it serious research. Sometimes their adventures are quite worthwhile, even enlightening. Recently an Engineering Technician dropped a circuit breaker test form on an Engineer’s desk and said, “Would you take a look at that and tell me if you think it’s okay.” The test results were excellent. The contact resistance, insulation resistance, over current test results, reset tests, all the test results were excellent. So why in the world was the Engineering Technician dropping it on the Engineer’s desk when the test results were so good? The results looked like those for a new circuit breaker. What they had done was test an old Westinghouse circuit breaker that had been practically crushed. The case was broken and the arc chutes damaged. It looked like it had been dropped from 50 feet and hit on one corner. But, oddly enough, against all reason, it operated mechanically and the test results were all good. But, of course, it failed the visual inspection. When the Technician dragged the Engineer to the Switchgear Shop to evaluate the breaker, everyone had a good laugh. This was good fun and the technical evaluation was unanimous, POJ, Piece of Junk. One of our favorite highly technical terms. In this case it was very obvious. But frequently we find defects in used circuit breakers and in brand new circuit breakers, and other electrical equipment, that would never be revealed by testing alone. That’s why the experience, training and knowledge of the Technicians and Engineers are extremely important. There is no perfect test standard for every POJ.
MIDWEST frequently gets calls about brand new circuit breakers nuisance tripping. The owner often believes they have a
defective new breaker, because it just should not be tripping. For example, a 2000 amp Square D or Cutler Hammer or General Electric main circuit breaker for new switchgear suddenly trips in the middle of the day two weeks after the new switchgear is energized for the first time. It’s agonizing
when an owner spends a lot of money for new equipment and suddenly they have a problem they never experienced with the old equipment which had been around for 30 years. If he or she is in charge of the new project, or maybe just in charge of maintenance, they can be in big trouble with non technical management. Sometimes they are so panicky it is hard to get the information needed to determine what the problem may be. We calm them down by telling them they probably don’t need a new replacement 2000 amp Siemens or Cutler Hammer circuit breaker. Usually the problem is the pickup settings of the circuit breaker have not been properly set. If an analysis was performed to determine the proper breaker settings, it may have never been given to the installation contractor. Equipment delivered from the manufacturer is often set in minimums. Minimum pickups and minimum trips times. We might see a 2000 amp main circuit breaker set to pickup at 50%, 1000
amps. Or we might see the ground fault protection set at 100 amps pickup, range may be 100 to 1200 amps, and possibly 0.1 seconds (minimum) delay. All the settings on adjustable circuit breakers should be determined by an engineering analysis and set up accordingly. Very low ground fault pick up settings are most insidious because a small ground fault deep in the power distribution system may trip a new Siemens circuit breaker for the whole facility. So once we get the manufacturer and catalog number, we can tell them what to look for, as far as settings on their brand new circuit breakers. Usually a few quick phone
calls will locate the analysis data and on site instructions to their electrical contractor will make the problem go away. If an analysis was not performed, temporary conservative settings might be used until a proper engineering analysis can be performed. The National Electrical Code does recommend testing the ground fault “system” of new main switchboards.