My latest c*ck-up: Having dug 15M trench, put SWA cable in and re-filled, I've now discovered it's a 3 core rather than 2 core cable. Doh! (It did seem a little thick, but I hadn't used any SWA for a while...) I could just go ahead and use it anyway I suppose, but since the core colours are brown, black and grey as for 3-phase system, I can't use the correct single phase colour scheme - there's no blue.
Is there a way round this without digging the whole bloody thing up again?
Don't worry - that's normal. I've not seen any 2 core myself, most if the sheds sell 3 core anyway.
The consensus of opinion on the IEE forums suggest this (old and new 3 phase SWA colours used in single phase installations) (with some differences of opinion):
RED OR BROWN - LIVE YELLOW OR BLACK - EARTH (SLEEVED GREEN/YELLOW) BLUE OR GREY - NEUTRAL (SLEEVED BLUE)
The whole thread is here, so best check it out yourself:
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important bit is that you sleeve the conductors at each termination correctly to indicate their true meaning, eg brown sleeve=live, blue=neutral, yellow/green=CPC earth
I'm not an expert though I have laid SWA before. Although in many cases the armour should make a sufficiently good earth connection, you should in theory prove it. The easy way around is to, as is commonly done, use one of the cores as an earth as shown above. But you must also connect the armour to this earth CPC via the solder ring that comes as part of the gland kit. I suspect you knew that, sorry, just being careful.
This assumes that you are exporting the supply end earth to the destination, rather than using a local rod.
Don;t forget, if this is making your installation mixed old and harmonised colours, there's a sticker you are required by the IEE regs to put on the CU.
Are you mad? Who is going to see it unless you put a big poster on the house. No one will know unless YOU tell them, so I don't see the problem. A bit of common sense is required. Or you could just dig the trench up, pay out loads more money, throw the perfectly good cable in the bin and buy new cable. Do you really need someone to give you the answer?
You probably have used the right cable. It is bad idea to rely on the cable armour for earthing (obviously it should be earthed, but just not used as the main earthing conductor). It is much better to have a proper copper conductor for the earth. I used 3 core for my shed.
You should sleeve the ends of the cable with the appropriate colours where they are terminated.
Sleeve the black as blue and use as a neutral. Use the brown as live directly. Sleeve the grey as yellow/green and use as earth.
Connect the armour to earth at the house end or both ends according to preference and ease.
If you are TT earthing the other end, then you don't need an earth conductor. Just sleeve the black as blue and don't connect the grey conductor to anythying.
It would be better to connect to grey to the armoured at both ends in this case rather than just leaving a floating cable. The house end of couse being connected to the house earth.
The accepted colours for such wiring is grey (sleeved with blue) used as neutral and black (sleeved with green/yellow) used as earth.
Indeed they do, but the point to be made is that using the armour as earth (CPC) will always require design calculations to verify earth fault loop impedance (for disconnection time compliance) and that the armour will not overheat before a fault clears (adiabatic compliance).
OTOH if a copper cable core is used as the CPC you have a CPC which is equal in size to the live circuit conductors and calculations aren't required (provided the feeding fuse/MCB rating doesn't exceed the current rating of the cable). The maximum circuit length (to meet both voltage drop and disconnection time requirements can simply be looked up in Table 7.1 of the OSG.
I was talking about the incoming supply. I certainly wasn't proposing TN-C.
It's more of a problem with the termination, where rusting can lead to high earth loop impedence at the connection point. The armour itself shouldn't be a problem.
Thanks Adam and others. I wasn't aware it was permissable to sleve the tails to different colours. Makes life far easier. My previous involvements with SWA cable had all used 2 core and the armour for earth
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