I was asked by a friend to investigate a seemingly faulty 20A DPswitch for a washing machine today.In hindsight slightly unwisely, I unscrewed the switch and pulled itforward to be rewarded with a flash, bang and the appropriate cloud ofnoxious smoke ! Hey isn't it great when you get the pyrotecniksrather than just something going dead.Anyway my guess is that the line terminal had not been tightenedproperly originally, had over heated, the plastic broken down and myapproach had resulted in the line terminal breaking free and coming incontact with the earth lead.The fuse in the CU did not rupture !!The switch was replaced.The question arises now is should I strongly advise my friend to havehis fuses CU replaced with a modern one; the emphasis is on the'strongly' as my feeling is that despite a large flash/short as thiswas, the fuse did not protect the system as an RCD or MCB would havedone. In fact an RCD would probably have tripped out long before Iwas asked to look and would have given the protection that the houseshould have (said having just has my own RCD trip and finding that itwas due to the oven fan becoming leaky).Rob

Helps liven things up! ;-)It might not on a very brief arc over. Much the same happens when a filament lamp blows in some cases.Well there are pros and cons. Have a look at the lists and see which apply most in this case:

Replacing a fused CU ?

Jul 31, 2008 41 Replies

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Terry Fields 17 years ago

One can imagine the conversation:

"How effective has Part P been?"

"Oh, I wouldn't know, we only set policy, we don't monitor it!"

"So you can't say whether it has done any good?"

"Of course it's done good, we wouldn't have brought it in, otherwise!".

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John Rumm 17 years ago

IIRC there was a question asked in the house recently on the apparent rise coincident with part P. A search of Hansard will probably turn up something.

There are figures available and well documented for the effects of shock and the implications of shock current and duration of exposure:

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RCD protection offers very much more than a "feel good factor" - unless you include the ability to breathe and absence of ventricular fibrillation as "feel good" ;-)

Note also RCDs are used not only to lessen shock risks, but also as a earth fault protection mechanism. A TT install with a 6 ohms fault loop impedance could end up sinking 40A for an extended period of time into a protective conductor without tripping a typical 32A MCB on a power circuit. This would (in the absence of the RCD) result in at best cable damage, and possibly fire.

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