The main complexity sounds like it is going to be dealing with earthing and possibly main equipotential bonding. What earthing scheme do you have in the house?
I have to say, I wouldn't recommend it on principle (i.e. run both services on a single cable). From a safety point of view, it would not be usual or expected that you would have a source of energy at each end of a single cable. If you do choose to go this route, then make sure your labelling is permanent, clear and unambiguous.
The idea was to use the SWA armour as the earth for both and have an additional earthing rod next to the generator point connected via 10mm cable (Generator is only 7.5KVA).
The generator transfer switch This one -
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not have the facility to switch the earthing, so I am assuming all the earth connections can stay connected all the time?
My main earth is supplied by the armour of the incoming supply cable (TN-S)
Is there any reason the earth connection to this can't be left connected in the event of a power failure with the generator running? (I assumed it is OK as the transfer switch does not have the facility to switch the earth connection) The generator has one leg of it's output tied to ground, so there will be no potential difference between neutral and earth in the system. If live found it's way to earth, this would just create a short, tripping the MCB, so, as far as I can see it, there is no risk to anyone working on the power grid here (just like the power company ties their neutral to earth everywhere)
Also any reason the earth rod can't stay "in circuit" during normal operation (considering the lead water pipe is connected to the earthing system 24/7 via 10mm cable and this will be acting the same, if not better)
In reality the generator won't be running very often, as the mains power supply should be reliable, but I already have the generator, so am just making provision for it's use "just in case" - If I move out, it will be coming with me too, so nothing to worry about later on either, it will simply be disconnected.
I think John is alluding to concerns about "exporting" the earth from your TN-S main supply out to the garage and "re-importing" the generator earth back from the garage to the house.
see here for more info:
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that you have a TN-S supply and are proposing properly buried SWA cabling and this garage is presumably a building of substance rather than a rickety shed, then I would probably be inclined to (permanently) connect all the earths together and run an additional main-bonding conductor alongside your SWA cables. You should consider getting a duct to run your cables rather than direct burial, because you would need to upgrade your main bonding earth wire further if it was direct buried.
The garage is indeed a proper building (brick) and is only about 6m from the house, so this is why I am choosing to export the house earth there
- if it were down the other end of the garden, then I expect a separate TT system would be better for the garage. - It will have an earthing rod for the generator too, so it will be both TN-S and TT I suppose!
I was planning on using 4mm XPLE SWA cable, and having the feed to the garage via a 32A MCB (Not RCD protected) and then have a 5 way CU in the garage, with a main switch, RCBO's for sockets and exterior lights and a MCB for the freezer circuit (All the wiring in the garage will be done in surface mount conduit, so nothing will be buried in the walls)
How do I find out the CSA of the armour in the 4mm SWA cable - is there a chart somewhere showing the values?, I am happy to use 6mm if I need to, to get the CSA up on this cable, and save having to run another duct (I will be running a green duct for comms cables anyway, but would rather not run another duct just for a separate earth cable if I can help it.
I was more concern about the difficulty that may occur should one end up with a TN-S system that has to become a TT for use with the genset. That would them mean the need for RCD protection on all circuits to maintain the fault protection.
However - more info from Toby suggests that the genset will be configured to supply a TN-C-S style supply.
The transfer switch can also have an RCD fitted if I need it - do I (I would rather not if I can get away with it to be honest) but if I do need one, can I fit a 100mA one?
my view is that thought through properly then 2 circuits in one SWA cable is fine when suitably labeled. Also, provided you gland properly then it should not be necessary to run a separate earth wire. And finally, SWA is suitable for direct burial - ducting should not be necessary if you think everyrhing through.
For a line CSA of 4mm2, the minimum earth CSA is 10mm2 if using the steel armour of a buried SWA cable, provided that there is mechanical protection[1]. (Table 54.1).
For a 4mm2 2-core SWA cable, the CSA of the armour is about 20mm2, so you are OK in principle and don't need an additional earth wire.
[1] - dig a trench at least 550 deep. Lay about 50mm of sand blinding[2]. Lay SWA on sand blinding (if laying two cables, keep them separated laterally[3]). Check that top of cable is at least 450 deep. Add 150mm sand blinding on top of cable; place mechanical protection on top of sand blinding (roof slate, concrete tiles, clay half pipe, etc), back fill another 150 or so; Place yellow "Electrical Service Below" tape; Backfill to top.
[2] Use soil in lieu of sand if soil is "free of sharp stones and other materials that might damage the cable".
[3] Note that two SWA cables sharing the trench incurs a current rating factor of:
Remember that if you are exporting a TN-C-S earth, then you must also export the equipotential zone. Hence it must be large enough to support operation as an earth and a main bonding conductor. That can eat into your copper equivalent mm of armour CSA quite quickly.
That's not allowed - using any combined neutral and earth (CNE) conductor in a consumer's installation is illegal in the UK, under the ESQC regulations [1].
Configure the generator to make a TN-S system. Here's a blast from the past which includes an ASCII wiring diagram:
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would very strongly discourage the 4-core SWA approach here, because its unconventional nature could be quite dangerous for the unwary. Using two 2-core SWAs will bolster up the armour CSA nicely too.
[1]
I think the opposite is true from the point of view of CSA size. 4 core SWA meets the Cu equivalent of 10mm2 from 4mm2 cables upwards whereas with 2 core cables you need to be using 10mm2 cable before the Cu equivalent hits
10mm2. (I am assuming you might want the SWA to act as main equipotential bonding). See
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is not necessarily unsafe. Labels and diagrams......
I was thinking of the total armour CSA, assuming that the two armours are in parallel. I know there are arguments about how fault current will divide between the two, but if you design so that each cable is separately compliant then the parallel path will only help lower the Zs.
4 mm^2 SWA has armour CSA of 19 mm^2 for 2-core and 23 mm^2 for 4-core.
For 6 mm^2 the corresponding values are 22 and 36 mm^2, so in both cases
2x 2-core gives you considerably more total steel area than 1x 4-core.
Hmm, the requirement for bonding conductors is "equivalent conductance" to copper, not equivalent thermal fault capacity. Since the resistivity of steel is over 8 times higher than that of copper you're looking for over 80 mm^2 of armour for main bonding on a PME installation, which takes you up to _much_ larger SWA sizes. A separate (paralleled) copper bonding conductor is probably a more practical proposition. Of course this is only relevant if there are actually any extraneous-conductive-parts in the outbuilding that need bonding.
In general, yes. In this case no, IMHO. See the 314 group of regulations in BS 7671.
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