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 Post subject: What arre the causes for a normal looking CB to blow up?
PostPosted: Tue Jan 10, 2017 9:30 am 

Joined: Mon Jan 09, 2017 1:47 pm
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Jim,
This question may seem rhetorical -but I can't help but wonder, after watching PPE-clad or not, personnel, as they operate (draw out) a seemingly normally operating CB, and the piece simply blows up in their face. Do you know of a site with forensic data that would explain different circumstances?
I'm looking for something other than a wrench falling across the buss or accidentally brushing the board's frame, etc.


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 Post subject: Re: What arre the causes for a normal looking CB to blow up?
PostPosted: Thu Jan 12, 2017 1:17 pm 
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Location: Maple Valley, WA.
It can be a number of things. If it is Low Voltage, water or other form of moisture is the cause. Sometimes closing a circuit breaker can cause a mechanical failure. Linkages or springs break that can then cause a fault inside the breaker. Also, solid state trip units or shunt trips that have failed and can not trip the breaker, can cause the breaker contacts to overheat and fail.

For Medium and High Voltage breakers, the failures are usually due to contamination. These breakers need to be free from moisture and other contaminants such as chemical atmospheres. If not, then flash overs can occur. Another problem is mechanical issues where one or two poles fail to open for a fault down stream. These poles can eventually overheat and fail, causing an arc flash event at the breaker.

This is why electrical equipment inspection, testing, and maintenance are extremely important to perform. Usually, you can not tell when a circuit breaker is going to fail. This is why NFPA 70E added verbiage about the importance electrical equipment maintenance.

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 Post subject: Re: What arre the causes for a normal looking CB to blow up?
PostPosted: Thu Jan 12, 2017 1:31 pm 
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I would suggest viewing this video. https://www.youtube.com/watch?v=tglwN4AfDL0

The technicians were racking in a breaker. Obviously, something went wrong, it may have been a mechanical failure or contamination issue. Medium and High voltage breakers must be very clean, contaminant and moisture free. Otherwise, tracking and flash overs can occur.

Low voltage breakers can fail due to mechanical linkages or springs that fail. Breakers that have solid state trip units and fault and blow up if the solid state trip unit fails to trip for a fault downstream. The breaker contacts will melt and potentially causing an arc flash event at the breaker.

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 Post subject: Re: What arre the causes for a normal looking CB to blow up?
PostPosted: Sat Jan 14, 2017 5:39 pm 
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I honestly don't recall any molded case breakers that passed the visual inspection in NEMA AB-4 and yet failed on re-energizing, and specifically that it "exploded" (ie, arcing fault). I can think of tons of breakers that have failed and several that had obvious visual signs of failure that did fail after trying them, and a few cases of breakers that "exploded" (arcing fault) but were not inspected prior to the event. But none that were inspected and faulted in an arcing fault immediately after inspection.

But it seems that we're looking for a needle in a haystack:
1. Inspection is done but failed to detect a fault. I ca't really put a number on accuracy of inspections but I'd guess it's pretty high.
2. Arcing fault resulting in injury. In general this seems to be about 1 in 100,000 workers per year based on ESFI data.

However this is not just a case of the probability of each in isolation because we need the probability of the first case with the second case depending on the first case. My guess is that this is going to be very rare indeed.


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 Post subject: Re: What arre the causes for a normal looking CB to blow up?
PostPosted: Thu Jan 19, 2017 10:07 am 
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Interesting question however, the idea of "normal looking" is somewhat subjective. I've seen failures of breakers, but the failure is often because the breaker was not switched off and on at least once a year. Add the environmental conditions breakers are subjected to, then consider the history of the load fed by the breaker. Do that and you'll get a better sense about if or why the breaker fails.

The videos we see of breaker failures are often conducted in labs. The lab bolts bus across the load side lugs and faults with the breaker with very high fault currents. These are worse case tests and most breakers aren't subjected to this sort of treatment. These test breakers generally clear the fault but are damaged by the test. Subjecting a breaker to high fault currents generally makes them unusable, but do they get replaced? Just consider what we do with a fuse that's subjected to high fault current. It trips and is replaced. With breakers, we often reset them and expect the problem is fixed.

The failures I've seen are generally caused by poor or none existent maintenance. Breakers get ignored for years and yet we still expect them to safely operate during worst case events. Breakers are often subjected to high ambient temperatures, become overheated and trip. And when they trip, they're reset without bothering to learn about the cause(s). And if they're switched off (after years without ever being operated) instead of being switched off with the handle, they are tripped using the trip button - without bothering to switch off the connected loads. The electrician trips the breaker and when it won't reset, he panics. Have you seen breakers with the handle broken off? Broken handles are often the result when someone has improperly switched off a large breaker. The breaker trip mechanism won't catch and the electrician, now under pressure to get power on, gets crazy and ends up breaking the handle. That's often when bad things happen.

No, circuit breaker failures are not common. I'd recommend that you take another look at the explosive failures and learn about the conditions behind the breaker failure. Large amperage circuit breakers are both a blessing and a curse. They're terrific devices, however, as it the case with all mechanical devices, they need periodic maintenance performed by competent people.


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 Post subject: Re: What arre the causes for a normal looking CB to blow up?
PostPosted: Sat Jan 21, 2017 3:31 pm 
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egurdian wrote:
Jim,
This question may seem rhetorical -but I can't help but wonder, after watching PPE-clad or not, personnel, as they operate (draw out) a seemingly normally operating CB, and the piece simply blows up in their face. Do you know of a site with forensic data that would explain different circumstances?
I'm looking for something other than a wrench falling across the buss or accidentally brushing the board's frame, etc.


After reading this carefully again I noticed the key words "draw out". This looks like you are actually asking about the draw out mechanism and NOT the circuit breaker itself. I have personally had issues with drawout breakers where the drawout mechanism is damaged (gearing) and the breaker will not come out. I haven't seen any cases of failures during the drawing out operation. The only two practical ways I can see this happening is if in shifting it out of the cell, it allows something previously broken to fall down onto the bus bars, or if someone forgets to trip/open it manually and the fingers on the bottom also don't force a trip during draw out, AND the loads are still connected and energized.

Inserting a drawout breaker on the other hand has a really high failure rate. In addition to the above problems of damaged/modified fingers where the breaker is closed/disabled so that it is shorted, or any number of defects within the breaker (which again should all be obvious/visible before insertion), once in a while especially with certain brands and models, the stab arms are so long that if misalignment occurs, one stab can be bent over sideways and come close enough to another phase to arc over. Obviously there is some force involved here and I'm not going to comment on what went on in desperation to try to get a breaker into a cell when the mechanism is racked from for instance building settling that it simply won't budget. This is not just paranoia or some rare case...almost all "large" plants have had this happen at least once.


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