Non tripping MCB\\\'s

jma1801 · Posted: Wed Jun 27, 2007 7:05 pm Post Subject: Non tripping MCB\\\'s

The house has an upstairs & downstairs ring mains. these are both on the same RCD. The upstairs mcb is 16A the downstairs one is 32A.
Recently my Wife was drying her hair upstairs when the dryer sparked, and ignited where the dryer wire goes into the hairdryer housing. i.e the bit where the wire flexes most on use.
She managed to unplug it, and the wire was completely burnt out.
However what concerns me is that neither the MCB or RCD tripped out, and the ring main remained live.
Should this be the case.

JohnD · Posted: Wed Jun 27, 2007 7:10 pm Post Subject:

did the 13A fuse in the plug blow first?

jma1801 · Posted: Wed Jun 27, 2007 7:47 pm Post Subject:

Unfortunately we didn't check and threw the hairdryer in the bin, and it's been emptied today.

plugwash · Posted: Wed Jun 27, 2007 8:35 pm Post Subject:

fuses and breakers will only go if the current is high enough for long enough. If the fault isn't quite that bad or is a series fault rather than a paralell fault it can produce a lot of heat in the wrong places without tripping anything.

ricicle · Posted: Wed Jun 27, 2007 8:54 pm Post Subject:

As most, if not all, domestic hairdriers are double insulated there is no earth wire in the flex so any short on that would not have operated an RCD.It should operate though (hopefully) before you receive a fatal shock if you had touched the flex and come into contact with the phase wire or possibly the neutral.
As has been implied above, you would need a fairly substantial fault to have operated an MCB instantly.If the flex had a low resistance short between phase and neutral there could have been enough heat generated to start a fire/melt the flex but not enough current to operate the MCB.
The plug-top fuse could have operated by then but alas you did not check

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plugwash · Posted: Wed Jun 27, 2007 10:21 pm Post Subject:

worse is a fairly high resistance fault or a series fault with a powerfull appliance, you can easilly get enough concentration of heat to burn stuff without drawing enough current to blow any fuses.

lukilly electrical goods tend to be made of self extinguishing plastics so you tend to get bangs and smoke but very rarely actual fire.

JohnD · Posted: Thu Jun 28, 2007 9:56 am Post Subject:

I've found steam irons to be the worst culprit, with the flex constantly bending. The 13A fuse will allow quite a substantial fault current (think how much heat a 3kW electric heater can throw out).

When it's happened here the RCD has always tripped (as there is an earth conductor), but not before substantial visible heat damage at the cracked flex.

"FizzBang!"