You’ve been called in to consult on a problem that has plagued the 480V circuit breakers for quite some time. Three electricians have worked on them, and all three say the breakers behave erratically. They have similar accounts of the behavior.
One 400A breaker, for example, was showing 402A during a routine PM in which one step was to measure the current using an inductive clamp. Some sources of overload were identified and corrected for, dropping the current to 350A. During a planned shutdown, that electrician was able to manually operate the breaker. He then used a breaker tester and found the breaker tripped exactly as it was supposed to.
What’s going on?
The first rule of breaker testing is never operate the breaker before testing it. The reason is you must also test the lubricant. Grease hardens over time because of oxidation and other factors; in that condition it will impede the movement of parts. Manually operating the breaker cracks the hardened film of the grease but doesn’t restore the lubricant to its required properties.
It isn’t that the breakers are behaving erratically. They are being tested improperly and they aren’t being maintained. The examples are similar, and the one example clearly indicates a sticking mechanism.
The solution is to schedule every 480V breaker for a complete PM, including a lubrication change. Have this done by a qualified electrical testing firm; the in-house electricians lack the skills and would also lack the time during the shutdown window.
