They definitely do
Loss of circuit
- Thread starter davecooper
- Start date
I'm not doubting/disputing that - I was asking 'in what way' they fail.
Signs of burnt housing mainly
Dunno ive never opened one up after the failure, but ive been to many jobs where there's no voltage coming out of the mcb.....its the first thing I check.
Would that be in the vicinity of (probably loose) terminals?
Interesting. As I recently wrote, I would be surprised if the contacts of an MCB which were virtually always closed would suddenly open spontaneously. When you see that, do you think that it may have often occurred as I suggested - inability to re-set after 'deliberate' opening of the contacts (manual or due to overload)?
Maybe the toggle doesnt switch it back on, not really sure, next time it happens I'll break it open.
Obviously MCBs do fail.
Yes they also char but in my experience that is when they regularly run above their rated current for long periods or due to poorly tightened terminations. My own observations are MK MCBs noticably discolour more than other brands, however I'll also make an observation they have a different finish/texture to most other brands. As to charing/burning I'll have to say Merlin and Eaton including their associated name changes - however I'll have to also mention they are probably the 2 most popular brands in commercial buildings (where the vast majority of my work is).
Yes they also char but in my experience that is when they regularly run above their rated current for long periods or due to poorly tightened terminations. My own observations are MK MCBs noticably discolour more than other brands, however I'll also make an observation they have a different finish/texture to most other brands. As to charing/burning I'll have to say Merlin and Eaton including their associated name changes - however I'll have to also mention they are probably the 2 most popular brands in commercial buildings (where the vast majority of my work is).
Older MEM MCCBs are known for never being able to switch them back on again after they have been switched off. A colleague tripped a 400A(!) Dorman Smith MCCB a while back. We couldn't reset it until it was dismantled and the mechanism freed. The grease used on the moving parts had turned to hard wax so the small catch that operates under overload/short circuit conditions got stuck in the tripped position, preventing resetting. Seen 3 Federal Electric MCBs not trip under fault conditons, 2 of those the faults were cleared by the cuout fuse! Have seen a few fail due to internal overheating. Also seen some external overheating causing damage:
That's what I keep suggesting - that (other than for incidents of over-heating), the 'failures' people are talking about are perhaps failures to be able to reset after a legitimate opening of the contacts (manually or by an over-current trip) - since, as I've said, I find it hard to see how/why a device with almost permanently closed contacts should/could suddenly and spontaneously open
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Indeed. I imagine we have all seen it - but, as I've said, virtually always manifested as an inability to be re-set after a legitimate opening of contacts (manual or due to over-current), not a sudden spontaneous and inappropriate opening of closed sockets (which seems pretty unlikely)
Indeed so. I'm not sure I've ever seen significant thermal damage in a device which had not been 'abused' and in which the terminations were satisfactory.
IFAIC I've known one which 'switched off' and wouldn't reset, it felt mechanically incorrect (can't remember in what way) and was discoloured, it fed only 2 double sockets being used for TV etc customer had been there for only a short time. I guessed the sockets had been used by previous occupants for something very different as I 'saw' no reason for failure.
However I have known of a number of failures which seemed very similar but they were all due to persistant overloads (commonly 10.5KW showers on 4mm² cable and 32/30A MCBs)
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Ah yes another but abused and certainly not left on indefinitely, a Dorman Loadmaster just about fully loaded with linear flood lights on a councils theatre stage (something like a 15A running 3KW but again far too long ago for detail) being used as the switch so regularly operated during the course of a show.
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