A contractor called MIDWEST for a little free advice. His customer had a 400 amp molded case circuit breaker. This happened to be a 400 amp Westinghouse circuit breaker, but it could just as well have been Square D or General Electric. He was very worried because his customer’s 400 amp circuit breaker was rattling, making a terrible humming and rattling noise. He had never heard this before and didn’t know if the circuit breaker was going to blow up, fall apart or what. He said this was a very old breaker, 1960s. We recommended he measure the load on the feeder cables from the breaker, but to do this very safely. Measure the current at the load end of the feeder, if possible, not at the circuit breaker. It is not totally unusual for an old circuit breaker to rattle and hum away when the load is near or over the rating of the over current device. When the breaker’s internal over current device is picking up because of the load. It depends on the breaker. The noise might be an indication the over current device is picking up and may trip the circuit breaker if the load doesn’t drop down below pickup soon enough. Either way, it may be a good idea to replace the circuit breaker. Sometimes these breakers will nuisance trip at less than the pickup current level. If possible retrofit the old circuit breaker with a newer replacement model. Sometimes the rattling is from the metal arc dividers in the circuit breaker arc chutes. Either way, it is not a noise you want to hear. Check the load. If that is not the problem, change the circuit breaker. Safely.
On a field service project, the customer was amazed at how much equipment we had on the large service vans. Besides the test equipment for old and newer circuit breakers and for oil filled power transformers and switchgear, we had the equipment and tools to maintain the switchgear and make many potential repairs. Plus generators, fuel and lights and much more. The customer asked, kind of as a joke, if we had anything on the trucks that was very important but wasn’t technical. This was a shutdown project where the power was turned off at 5:00 AM and had to be back on by 11:00 AM. A lot of work in six hours, including replacing one of the circuit breakers.
The immediate simultaneous response from two Engineering Technicians was, “Hornet Spray.” Each truck has at least one can of hornet, actually wasp spray, in a can that will spray a stream 10 to 15 feet. We learned decades ago that it can be painful if you have a short shutdown project and open up the switchgear to access your favorite Square D circuit breakers or Westinghouse circuit breakers or new Siemens circuit breakers and you find the switchgear to be a hotel for a bunch of wasp nests. Hard to find a volunteer to take the bite clearing out wasp nests so you can replace circuit breakers. Instead, a little stream of spray here and there and you’re ready to go. No customer wants their project put at risk because of a few bees, even if they are mad. The bees that is. It’s a magic solution for a non technical problem. So that’s something non technical but extremely important for an outdoor project to inspect, test and maintain circuit breakers and electrical switchgear. Don’t leave home without it.
MIDWEST recently sold a 2500 amp 15 Kv old used air circuit breaker for $5000.00. It was in excellent condition and years ago it would have sold for $25,000.00. One might think the reason for the big drop in price is because of the age. But the biggest reason is the use of vacuum technology in 15 Kv class circuit breakers. Vacuum breakers are used to replace air circuit breakers in new 15 Kv switchgear. And many old medium voltage air circuit breakers have been replaced with new vacuum breakers or retrofitted with vacuum technology.
Maintaining and Testing 5 Kv and 15 Kv vacuum circuit breakers is a lot easier than the old air circuit breakers. And the vacuum breakers are a lot easier to handle. An old or obsolete air circuit breaker could weigh 1500 pounds. The new vacuum equipment is half that. The biggest difference is the simple replacement of the old heavy arc chutes with the simple vacuum bottles. For an historical perspective, the early 1900s saw the use of big old oil circuit breakers. The mid 1900s began the use of air circuit breakers. And the late 1900s began the use of vacuum breakers. MIDWEST has worked on all of them and the vacuum breakers are just so much simpler to service. Some older electricians and switchgear service technicians do not trust the vacuum circuit breakers as a safe circuit open device. I wouldn’t trust vacuum circuit breakers or old air circuit breakers or obsolete oil circuit breakers as open circuit protection. The circuit breaker must be racked out and, of course, the circuit grounded before any circuit work or equipment maintenance. By racking the breaker out, one has a “visible open.” Medium voltage air circuit breakers are an example of equipment becoming obsolete long before they actually wear out, all due to new technology.