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Circuit breakers flashing over or exploding

Johno12345

choosing my words carefully here.

 

Resetting MCBs in a DB or in a machine control panel.  

We have people who are trained in MCB resetting, contactor trips etc inside control panels, and the Schneider MCBs in a dis board are rated for operation by ordinary persons. 

 

I recently lectured someone on the danger of resetting something with the power energised in a control panel, and reminded him that it must be isolated before opening the door (even interlocked isolators can be turned on with the door open. 

 

Well, low and behold, a breaker tripped, panel was isolated correctly, breaker reset, panel turned back on and the MCB and contactor blew up

 

At least it illustrated my point but does this happen much? I have seen it a few times in my years and never in a dis board. Reading the thread about arc flashes has made me think, and if he had his hand on it as it blew up, there would have been an injury of some sort.

 

I'm just thinking about things really, id be interested in your experience of this

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    It should be very rare for an MCB to fail in a dangerous way - unless the fault current is very high it is supposed to fail in a way that contains all the bits, and the standard tests for type approvals do test for  this - actually with a ‘cheese cloth’ is placed over it  that must not get singed. 

    (This is really a piece of light cotton fabric - real cheese comes in plastic or foil these days if you like dairy-lea, but the test methods pre-date that)

    Repeatedly closing an MCB onto a fault is unwise, as the contacts gradually get burnt, but normally I'd still expect a failure in the sense of no longer connecting, rather than welding in the permanently on position or blowing up.

    In this case I might be wondering if the supply PSSC and the breaker type are inappropriate, or if it was a phase to phase fault on a single phase breaker.

    Mike.

     

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  • 0

    It should be very rare for an MCB to fail in a dangerous way - unless the fault current is very high it is supposed to fail in a way that contains all the bits, and the standard tests for type approvals do test for  this - actually with a ‘cheese cloth’ is placed over it  that must not get singed. 

    (This is really a piece of light cotton fabric - real cheese comes in plastic or foil these days if you like dairy-lea, but the test methods pre-date that)

    Repeatedly closing an MCB onto a fault is unwise, as the contacts gradually get burnt, but normally I'd still expect a failure in the sense of no longer connecting, rather than welding in the permanently on position or blowing up.

    In this case I might be wondering if the supply PSSC and the breaker type are inappropriate, or if it was a phase to phase fault on a single phase breaker.

    Mike.

     

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