What would cause this MCB problem?

Paul.M · May 17, 2011

Something I haven't come across before and now its bugging me. Here is the situation.

3 phase type C mcb protecting an air con unit. The area gets a blackout for 30 mins or so, when the DNO restore power the mcb didn't trip but when tested no current was going through any of the phases on the mcb but continuity from busbar lug to the wire nut connector is present?

I took off the mcb and tested for continuity and it was fine. Replaced it back on the board and now it works? Is there something I'm missing here or something I'm not thinking of? It's strange fixing a fault when you don't know what the fault was.

telectrix · May 17, 2011

beats me. gremlins? could the MCB have just been a bit stuck with not having tripped for ages, just flicking it off and on cured problem?

Paul.M · May 17, 2011

Thanks Tel that was my first thought that the mcb is a bit old and maybe the mechanical parts inside are stuck.

Ponty Massive · May 17, 2011

telectrix said:
beats me. gremlins? could the MCB have just been a bit stuck with not having tripped for ages, just flicking it off and on cured problem?

Nice...thinking outside the box !!

RWJ · May 17, 2011

Was my thought too.

The lever might have been stuck closed but the MCB may have tripped as the internal mechanics will still trip even if the lever is "locked on". Some MCB's let you lock the lever on so it can only be reset by a competent person (or the person with the key).
Some alarm companies do this with intruder / fire alarm MCBs to stop them being turned off, quite how your meant to isolate if you needed to....

Engineer54 · May 17, 2011

RWJ said:
Was my thought too.

The lever might have been stuck closed but the MCB may have tripped as the internal mechanics will still trip even if the lever is "locked on". Some MCB's let you lock the lever on so it can only be reset by a competent person (or the person with the key).
Some alarm companies do this with intruder / fire alarm MCBs to stop them being turned off, quite how your meant to isolate if you needed to....

Think you'll find that this facility is and has been standard on all lever operated MCBs since there introduction, and a very useful facility it is too....