My spark has let me down - didnt turn up to look at the job this weekend (Grrr)
SO... I am looking to get a parts list together so that I can order what I need for my garage electric install.
I have 4mm2 2 core SWA with glands and a bare 6 way contactum CU for the garage already.
I want a lighting circuit in the garage and a ring / radial for sockets that will run various DIY power tools (not all at the same time) and I would like a circuit for garden lights too.
The garage is about 10 meters away from the house CU and there is a spare way in the house CU.
With this info, can anyone tell me what rating of breakers etc that I should order for the house CU and what breakers RCDs I should order for the garage?
The would mean that you could probably export the house earth to the garage since it is not that far away, and you won't need to be quite as exacting maintaining the equipotential zone as you would with a TN-C-S system. I take it the garage has a concrete floor or other solid floor?
If you are planning on using the space as a workshop then you need to think about how you will achieve discrimination between the power and lighting circuits. (i.e. the power circuit ought to have RCD protection, but you probably don't want to lose the lights while you have a spinning power tool in your hands). There are several options here: A split load CU would do this, alternatively you could use RCBOs for the power circuits, or you could use a "whole CU" RCD and then provide some battery backed up emergency lighting.
Since it is not a TT install, you won't need RCD protection at the head end of the SWA - you can feed this from a spare non RCD protected way on the house CU. You will need to think carefully about the size of protective devices you use with regard to the discrimination issue again, not much point in having (for example) a nice split load CU in the garage if a short on the power circuit in there takes out the MCB in the house at the same time (or instead of) the one for power the circuit in the garage. One solution is to ensure the cascaded MCB is at least two ratings down from its parent (refer to the manufactures data sheets however since these characteristics vary between brands). Another way is to use a HRC fuse carrier in the house CU.
You may also want to protect the feed that goes off into the garden on a separate RCD since this circuit is the most likely to cause spurious trip problems should dampness get into it.
Could you explain the significance of this please ? I've a TT system with the garage something like 16m away fed off a 15A fuse in the house (seperate CU from the main split load CU with RCD). In the garage there is a split load CU wirh RCD and an earth rod effectively paralleled with the house rod.
If you have a TT system at the house, but the SWA submain is fed from a non RCD protected CU, then you have a potential problem should either the SWA be damaged in such a way as to cause a phase to earth fault, or the downstream CU suffer a similar fault (say melting in a fire in the outbuilding). You would then probably not get sufficient fault current flowing to the (high impedance) earth to open the protective device at the head end.
Having TN-S at the house, and TT in the garage however, is fine even without RCD protection on the submain from the house.
Wise to use RCBOs in the garage rather than MCBs, otherwise any earth leakage fault will leave you taken by surprise and plunged into darkness while the power tool youre holding is running.
Lighting inside: 6A mcb or prferably 5A fuse, less likely to nuisance trip.
Garden lighting 6A mcb unless youve got a very large property
garage sockets: if 13A sockets, 32A mcb with ring or 20A mcb with radial.
The cheapest solution might be to replace the incomer of the CU in the garage with a 30mA trip RCD, fit 20A type B MCB to a radial for the sockets, a 6A type C MCB for the lights, and a 6A type B for the external lights. Then fit a non maintained emergency light. User a 32A HRC fuse carrier in the CU in the house for the submain.
Would something like the split load board below be a wiser choice at the garage end? Then I could have the sockets and outside lights etc on the RCD side of the board with the interior lighting coming off the non rdc side?
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refer to the building as a garage as it is garage sized, however it is actually a shed so it has a wooden floor and not a concrete one.
The material the floor is made from becomes more significant if you have a TN-C-S earth in the house and wish to export that. You then have the usual potential problem of what happens when the supply neutral is disconnected[1]. This means that you have to make sure that the same Faraday cage earthing principle is employed in the outbuilding as is used in the house to prevent this situation becoming very dangerous for the occupants. With an earth floor for example (say in a greenhouse, or cheap shed) it impossible to prevent easy access to a local earth which would be at a very different potential to the exported one in a fault condition. In these circumstances it is normal to make the outbuilding a TT install in its own right and not export the main earth.
[1] With the house neutral disconnected no current flows, no voltage is dropped over the electrical loads in the house and the neutral rises toward 240V. The earth, being connected to the neutral at the house, and therefore all exposed metalwork hence also rises to 240V
Google for "IEE garage earthing" and you will get an article returned on garage/outbuilding earthing. Its not so much the floor material as the presence or absence of "extraneous conductive parts" that defines the earthing required.
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