A feeder breaker tripped three times in as many weeks, taking down a large area of the plant. The downtime for each event causes a revenue loss of almost one million dollars. Additional costs of scrap materials and idled operators are not trivial, either.
The breaker was replaced, but the new breaker does the same thing. The responding electricians are not always the same ones. In each case, no fault was found, and the breaker was reset. Equipment was restarted, and nothing seemed to be wrong. Amp readings taken with a DMM and current clamp looked normal.
That plant manager had talked about this problem with your plant manager. The result of that conversation was to send you to that plant and figure out what is causing the expensive problem and fix it. The plant manager at this plant told the plant engineer to give you whatever you need.
How might you proceed?
If a large area of the plant is affected when this feeder breaker trips, then we know this breaker feeds several branch circuits. It’s odd that no fault was found, but it’s also a clue. It could mean you have a temporary fault that is caused by undetected insulation degradation. To solve this correctly, you must proceed methodically. And that means starting with breaker coordination.
So, the first thing to do is ask them to dig up the last coordination study for this feeder and its branch circuits. If there is no coordination study or if equipment moves have been done since the last one, take the plant manager up on his “whatever you need” offer and order a new coordination study.
While you are waiting for the coordination study, photograph the feeder breaker and if it has any type of adjustable trip settings, make sure to have a clear shot of those. Write them down, too. Then go to each branch circuit panel and document the size and (if any) settings with photos and written notes.
Once you have the coordination study results, assume for now the problem may just be the breaker settings do not match the coordination study settings. If they don’t, then make them match. If they do match, then the next step is breaker testing. And that will be of the branch circuit breakers. If one of them is not opening when it should, the feeder breaker is doing its job.
When you first mention the need for breaker testing, you’ll get a reply that it’s a new breaker. Make it clear you aren’t talking about the feeder breaker, although it should be tested also. If all of the breakers match the coordination study results, the problem is probably (but not certainly) a bad branch circuit breaker — one that sees a fault but doesn’t open.
Before you replace that breaker, you need to find out what the fault is and why it goes away. There was an actual case in an automotive plant where conductors with cracked and aged insulation would conduct leakage current between each other. And do that inconsistently. Rapidly changing current can cause conductors to physically move; they may even appear to “jump” a short distance. In this case, two phase conductors would sometimes move just enough to allow particularly bad segments of insulation on each one to line up and touch. It’s not actually a dead short (e.g., bolted short), but it’s enough to trip the breaker. And then when the cables relax again, the fault goes away. Which is why nobody can find it before reclosing the breaker.
A similar temporary fault can be caused by moisture entering the raceway. Initially, you may get a fault that seems to go away. Each time this happens, however, the insulation degrades. Eventually, you’re going to have a serious “high temp” problem, not a temporary problem, in that raceway.
If all of the breakers tested good, the cable fault is between a branch circuit breaker and the feeder. Perform cable testing on the conductors of those circuits. At the very least, this should include high-pot testing. Since the only test data reported by maintenance was current readings using an adapter with a DMM, hire a qualified testing firm to handle this.
When the problem conductors are identified and replaced, write a report that shows the cost of scheduled cable testing versus repeated downtime events. No maintenance deficiency should ever need this kind of problem before it is corrected.
