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K

KFH

Had a call from a plumber I know "I think I have blown the main fuse, there is no electric and no MCBs have tripped".

As he was nearby I went out the main fuse had blown and no MCBs had tripped. He was fitting a shower cubical and was screwing a length of aluminium section to the wall when the top slipped under the cover of the extractor fan when bang and everything went dark.

I checked extractor fan and it was wired OK but his section had touched an earthed metal frame under the cover. After a few checks I suspected that he had put a screw through a cable I got him to put in the screws (power still off) and one had gone through a cable (L + E) but the MCB had not tripped (One of the old Wylex rewirable fuse replacements).

So he had been holding a live 6ft length of aluminium and if he had not contacted the frame of the extractor he would have left a live shower enclosure for the customer to find.

He is going to buy a lottery ticket this week and he was still shaking the following day.

When he dug the cable out it was running diagonally across the wall!

I have the MCB and will disassemble it to see if there is an obvious fault.

I have the job of replacing the CU to include RCDs but am going to have to sort out the wiring/accessories first as the earth leakage was over 100mA.

The cable was imperial 1.0 mm equivalant for an imersion heater and was connected to a 32A MCB for the ring.

It could have so easily ended differently.
 
Thank god for rubber soled boots!

The MCB was either faulty, or your plumber mate caused a fault current so high to flow that it simply fried the MCB! Who knows? Who really even cares? One thing is for sure though, that's one lucky escape!
 
I will be interested to hear what you find in the MCB. I have seen one of the early pushbutton ones that I was suspected of jamming mechanically, although it had suffered some abuse and I was unable to prove it.

Alternatively, reversed polarity at the meter tails - leaving all the MCBs in the neutral? I once fixed a small installation that had been like this for years.
 
Could even be a weakened DNO fuse through past issues .... What size DNO fuse is it (not the carrier rating).

To note most household MCB's only provide partial discrimination to the DNO cutout so it is plausible that on occasion the DNO goes first although as i dont have the DNO fuse info i cant say the odds of the MCB acting first.
 
It's probably just a time related issue where under a fault current of maybe a couple of kA the fuse separated a few milliseconds before the MCB mag operated.
 
Viewed a similar installation recently,with the potential for that scenario. Dno head from the 30's,rewireables,pair of 30 amp fuses in line and neutral with a Bill switch fuse,fitted in the 70's for a shower with a rewireable 40 amp. Must have had issues in the past as carrier beds were splattered ...
 
Alternatively, reversed polarity at the meter tails - leaving all the MCBs in the neutral? I once fixed a small installation that had been like this for years.

One of the few times I've damaged a pair of cutters was on an installation with reversed polarity on the tails. Had to shell out for a new pair of cutters.

Thinking I was safe, having opened the relevant circuit breaker and seen/heard the light and fan switch off, I cut the cable to the fan. A lesson learnt.
 
I've had it once where I blew the DNO fuse when drilling through a cable.

Moving a thermostat, needed 2 holes, no accessories anywhere close, drilled through and a great big bang and flash.
I expected to turn on the CB or RCD and continue, but they hadnt tripped at all, but the main fuse had.
Turns out there was a 11kA PEFC, as the transformer was over the road, which I think is why the fuse went so fast.
Again, a DIY job had put a cable at 45 degrees from top corner to opposite bottom corner of the room.
 
I had the same thing many years ago,electric kettle went faulty,didn't take the 13A plug top fuse,not even the 30A rewireable in the CU, oh no this took the service fuse out,apparently these things are more common than you think.
 
I had not realised how often it happens. I have never had one go like that before, often had MCBs tripping before 13a plug fuse blows. Fault current was 1.2kA, DNO fuse was 60A and the fuse carrier, supply cable and meter were recent changes so the fuse may have been new. I have taken the MCB to pieces and it looks OK but the contacts are very pitted so they may have welded together although they are free now. The magnetic operation operates a trip to allow a small spring to pull the contacts apart, if they were welded the spring may not have been strong enough. The lever on the front will not trip until the contacts break so manual operation may have broken the weld.
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The plumber still thinks he was very very lucky, I will have to ask him if he has bought his lottery ticket :)

Plumber report that DNO man was fully dressed up in PPE including glow in the dark jacket when changing the fuse. The DNO man was very reluctant to allow the instalation to be energised, plumber had to promise on his life that an electrician was comming the next morning to see what was wrong.
 

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