14-10-2015, 09:06 AM
(This post was last modified: 14-10-2015, 09:09 AM by ppppenguin.)
We'll ignore the fact that this work needs to be certified by somebody in a registered scheme (NICEIC, NAPIT ELECSA etc) and assume he's reasonably competent to do a decent job. Strictly speaking he shouldn't be doing this work himself.
Exporting earths is a slightly tricky area. This can often be done safely, usually if there are no extraneous conductive parts in the shed. Generally this means no water pipes or large lumps of metal connected to local earth. There are other issues too. You have already told us that the circuit is protected by an RCD (the modern term for ELCB) so that's good. A 32A MCB will protect 4mm cable though I haven't done the volt drop and earth loop calculations to ensure that the MCB will in fact trip satisfactorily in case of short circuit. Assuming that the earth is being exported I hope that one core of the cable is being used for earth. The armour should be bonded at both ends.
Next is discrimination. A 20A MCB at the shed will just about give discrimination wrt the 32A MCB at the house. A 32A MCB at the shed is not recommended.
The installation must be tested to ensure low enough earth loop impedance and correct RCD tripping.
If you decide not to export the earth then all sorts of things change. Won't cover that in this post.
As an example my own detached garage is fed via a dedicated 30mA RCD and a 20A MCB via 4mm 3 core armoured cable. Sockets are fed directly, as an ordinary 20A radial circuit. The lighting is via a FCU (fused spur in old terminology) with a 5A fuse. There is no signifcant earthed metal in my garage so I reckoned it was OK to export the earth.
Exporting earths is a slightly tricky area. This can often be done safely, usually if there are no extraneous conductive parts in the shed. Generally this means no water pipes or large lumps of metal connected to local earth. There are other issues too. You have already told us that the circuit is protected by an RCD (the modern term for ELCB) so that's good. A 32A MCB will protect 4mm cable though I haven't done the volt drop and earth loop calculations to ensure that the MCB will in fact trip satisfactorily in case of short circuit. Assuming that the earth is being exported I hope that one core of the cable is being used for earth. The armour should be bonded at both ends.
Next is discrimination. A 20A MCB at the shed will just about give discrimination wrt the 32A MCB at the house. A 32A MCB at the shed is not recommended.
The installation must be tested to ensure low enough earth loop impedance and correct RCD tripping.
If you decide not to export the earth then all sorts of things change. Won't cover that in this post.
As an example my own detached garage is fed via a dedicated 30mA RCD and a 20A MCB via 4mm 3 core armoured cable. Sockets are fed directly, as an ordinary 20A radial circuit. The lighting is via a FCU (fused spur in old terminology) with a 5A fuse. There is no signifcant earthed metal in my garage so I reckoned it was OK to export the earth.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
