I do hope that the MCB is not simply turned back on without investigating the cause of the trip.

Well, thats another issue, but generally, a machine operator would be able to say that its tripped because of a jam or maybe something faulty connected, that's been unplugged since. 

I do know exactly what you mean though.

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.

It was considered safer years ago, to have suitably rated backup H.R.C. fuses, motor type if required, to clear high current faults. Is this not done now?

Example of vintage types

x3 MEM 20A amp HRC Fuse INDUSTRIAL FUSES 20SB3 breaker fuse | eBay

Z.

mapj1: 
(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)

Real cheese comes in real cheese cloth!

I remember some years ago buying some real Wensleydale at Sainsbury's. The lass behind the counter was having a real job getting the cheese wire to cut until I advised her to remove the muslin.

True story guv!

I've only ever known one breaker fail this way. It was a breaker controlling 4 3 phase fan motors  it flashed over when being reset and tripped a 300 amp breaker in the main panel. 

Kelly Marie Angel: 
 

I've only ever known one breaker fail this way. It was a breaker controlling 4 3 phase fan motors  it flashed over when being reset and tripped a 300 amp breaker in the main panel. 

Possibly due to the inductive loads all working together causing a high Voltage arc.

Z.

funnily enough, it was controlling a large fan.

we do have plenty of those HRC fuses in the place, I wouldn't call them vintage though!

Johno12345: 
 

funnily enough, it was controlling a large fan.

 

we do have plenty of those HRC fuses in the place, I wouldn't call them vintage though!

No, the H.R.C. cartridge fuses are excellent. I was only referring to the vintage fuses shown in the link, not to modern day types still in use. Solid, reliable and with high rupturing capacity.

Z.

ill take a photo of my fuse stores some time, we have plenty as old and older than those!