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What's the difference between running an extra socket from a spur and plugging in an extension lead with 6 sockets?

Postby DIYelf » Thu Jan 29, 2015 8:57 am

Can someone please explain the difference to me between running cable from a spur socket to create another socket, apparently not allowed, and plugging in an extension lead with, say, 6 sockets on into the spur socket, to run office equipment, which is ok? If a radial circuit just runs one socket to another with just one source cable from the consumer unit, why can't the same be done from a spur socket ? Any response gratefully received.
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Postby ericmark » Thu Jan 29, 2015 5:28 pm

It's all down to the overload device (Fuse, MCB, RCBO) with a 4 way extension lead you have a fuse in the plug.

With a ring final circuit you have a MCB in the consumer unit which is the right size for two cables but too large for one cable.

So if you have one cable from a socket with two cables (three in all) then that extra socket is only protected by the fuse in the plug. So with one or two plugs that's OK but if you can plug in 10 plugs it can draw too much current.

So the normal way is to use a fused connection unit (FCU) which has a 13A fuse and from that you can have as many sockets as you like.

So with an un-fused spur only one device (the device can be a socket or a FCU) but with a fused spur you can have as many sockets as you like.

With a radial rather than a ring final you have two types. Some use thicker cable and still have a 30/32 amp fuse/MCB others use 2.5mm and have a 16 - 20 amp fuse/MCB.

With the thicker cable it's just like the ring either just one socket or use a FCU but with the 2.5mm and 16A MCB then you can add as many as you like.

As you can see you have to first work out what you already have before you can know what is allowed. The problem is people do break the rules so having two cables at a socket does not automatic mean it's a ring final. One has to test the cables to see it is actually a ring.

All too often we have faults which means the ring is broken so every time there is a new occupant or every 10 year less in Scotland you need to have the electrics tested to see if it still complies.

Keeping to the regulations is a problem in Jan 2016 my house will be non compliant and other it requires some changes. One hopes after the EICR or PIR is completed you can add without too much of a problem as all faults should have been identified.

The amount of houses where the ring turns out not to be a ring is quite high easy way out is fit a 20A fuse/MCB instead of the 32A one but clearly with high loads it can then trip.
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Postby DIYelf » Fri Jan 30, 2015 8:22 am

Ah. Got it. Thanks for the clear and detailed explanation. I like to understand the whys and wherefores.
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