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Discuss RCD didn't trip when live conductors cut in the Electrical Forum area at ElectriciansForums.net

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I recently cut through the live 240V power cable of a plane the tool is double insulated, the blade exposed and combined both conductors. The supply is protected by a 30ma RCD and 30Amp MCB (10 yera old installation), the RCD didn't trip, it does trip when the test button is pressed. Could it be the case that in this situation the RCB won't trip or is it likely there is some sort of fault in the system? Thanks
 
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RCDs fail (get stuck) , I have tested several RCDs that have failed to trip under test...
they are not perfect
The very reason they have a test button! Not that most folk do...

Compared to the over-current protection of a MCB, the electronics for the RCD action is significant more complex and so more likely to fail.
 

davesparks

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Thanks - so I guess if I'd picked the exposed live up it would have?
If you received an electric shock of high enough magnitude it should have done yes.

A 30mA RCD is designed to disconnect the power if a person receives a bad electric shock before it becomes fatal for the majority of healthy people.
 
Also the RCD action only protects you if you touch live and are otherwise grounded (typically folk chopping cables while in the garden, previously a very common way to get electrocuted).

If you are unlucky enough to touch both live and neutral the RCD sees nothing wrong, so you fry. If you are lucky and it is only on one hand you probably will be able to get away and injury is probably quite local, but hand-to-hand is often fatal.

In the old days of working on valve radios, etc, with their 200-400V HT supply it was common advice to keep one hand in your pocket!
 
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