DIY Doctor

Fuse box or fused plug??

Postby Sharp » Fri Apr 23, 2010 9:44 am

What's the difference between a fused plug and a fuse box. I have mains voltage garden lights which plug into a shed. I was thinkin of getting a fuse box installed in the shed to stop the main house fuse box tripping if something happens to the lights, but will the fused plug not do the same thing
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Simply Build It

Postby ericmark » Sat Apr 24, 2010 9:27 am

A 13A fuse will have same curve in a plug as a FCU however the curve for a fuse is very different to that of a circuit breaker and it has been know for a B32 MCB to trip before a 13A fuse.
The tripping time for a fuse is a curve against current but a trip has both magnetic and thermal parts so have a straight line then a curve and as electricians we measure the ELI and the PSC to determine what automatic disconnection device to use.
The RCD is also a consideration and although marked 30ma they can trip between 1/2 of 30ma and full 30ma so each unit should be a factor of 3 to next so with house at 30ma the shed would be feed with 10ma.
Older properties may have S type which has a delay and may be rated 100ma and so what may seem a simple question is quite complex.
Even when the correct disconnection devices are used a nail through the cable for example may go back through 3 levels of protection and I have seem with a cabin the 30ma trip and B32 MCB go plus B63A MCB and 100ma S type in local board and the 300A moulded breaker in main board since connected to earth leakage device it could be either current or leakage.
Cure is of course not to drive nails in wall to hang ones coat.
And in your case it is to ensure the shed is safe where SWA cables should be used they are and I would supply any pond with 10ma RCD. And outside lights would always be 30ma or less as would whole of house so only method is to use RCBO's in consumer unit instead of just one pair of RCD's.
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