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Shed electrical supplies.

Postby BobQ » Fri Apr 09, 2010 3:01 pm

I have moved to a house which already has a wooden garden shed with an electrical supply. The installation comprises a fused spur off the house ring main, starting with a switched and fused unit, then an RCD. The cable (metal sheathed) is buried under a patio and feeds a double socket mounted on a fixed wooden board, into which is plugged a lamp.

The previous owner told me that the installation was done by a qualified electrician about 10 years ago. He explained that the spur was fused at 3A and only intended to supply a 100W light in the shed and a transformer for some low voltage garden lights. He explained that were I to plug a power tool into the socket it would blow the fuse and/or trip the RCD as it was not intended they were used for power tools.

As the shed is rotting in places I am contemplating replacing it with a plastic shed (these have a steel frame). My question is what do the regs say about this? Can I just fix the wooden board in the plastic shed and carry on using it like this? Do I need to earth the shed frame? Is the installation non-compliant? Like the previous owner I am not interested in power tools and just want a light in the shed.
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Postby sparx » Fri Apr 09, 2010 7:47 pm

seems like a good installation to me.

If as you say the cable is a steel wire armoured type, the smallest size available is 1.5mm2, which when buried in ground has a current max rating of 22A.

so can see no reason for limiting supply to 3A, since it is feeding a socket outlet it would seem pointless restricting the supply to less than a 13A fuse.

Any light plugged in could have a 2 or 3A plug fuse protecting it and would allow use of tools etc of up to 3kW/13A.

Incidently the RCD does not give over current protection only earth leakage, which is why there is also a fused connection unit in line.

Normal way is by use of combined RCD-FCU which is neat way of doing it with single unit.

I would do as you intend and once socket board mounted in new shed consider connecting a 4mm2 earth wire from socket earth to shed frame.

regards Sparx
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Postby ericmark » Sat Apr 10, 2010 10:36 am

As Sparx says normally one would not need such a low rating on a SWA supply so I wonder how long it is?

Sheds close to the house rarely cause problems and normally can be feed from house supply without any real problem.

However as the distance between the house and shed increases the possible potential difference between house and shed earth increases. With a wooden shed with on earthed exterior parts this is still not normally a problem but with a metal frame then consideration must be made as to if under fault conditions either some one outside touching shed could get a shock or someone inside touching socket earth and shed frame could get a shock.

It is unlikely this would be a problem but if the shed is some distance from the house especially if out in country then I would advise you at least get it checked by an electrician in case it needs it's own earth rather than house earth connected.

The tests required are not hard but the meters are too expensive for DIY to be worth buying them.

I am not sure on Part P as your not moving socket only replacing shed so I would not think it needs reporting but I am no expert on this and you should read up yourself and make up your own mind.
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Postby TheDoctor4 » Tue Jun 29, 2010 7:52 pm

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