Could you clarify what I've read on Google groups?
* For an outbuilding, a separate TT earth is required/recommended,
provided by an earth rod.
* The power itself is provided by a 2 core SWA cable carrying live
* The armour of the SWA is connected to earth.
Is this correct, and to which earth should the SWA armour be
connected, the house, or the garage?
yup. Exporting the house earth is OK for a garage nice and close to the
house (nearly-attached or just a yard or two away); much further and
the TT route is somewhat better.
You connect the SWA armour to the house earth: that'll give you the
best (lowest-impedance) path back in the unlikely event of a live-to-earth
fault in the body of the cable (spade, mole with titanium false teeth,
or similar). At the outbuiling end you keep the SWA armour isolated from
the consumer unit - glands to do this are available, or you can just
terminate at an all-insulated CU.
That's the accumulated wisdom I've gathered from this 'ere group!
"Stephen Gower" wrote
| Could you clarify what I've read on Google groups?
| * For an outbuilding, a separate TT earth is required/recommended,
| provided by an earth rod.
Required if the supply to the house is PME or the outbuilding is more than a
certain distance away.
| * The power itself is provided by a 2 core SWA cable carrying live
| and neutral.
| * The armour of the SWA is connected to earth.
| Is this correct, and to which earth should the SWA armour be
| connected, the house, or the garage?
Yes, and the SWA is earthed at the house end. At the garage end you can
either use an insulated CU or a special gland which insulates the armour.
This is an interesting matter which has puzzled me for a while.
According to an IEE source, when I enquired a while ago, the main
requirement for whether or not to use a local earth rod, is if the
earth fault loop impedance (Zs) becomes too high, at the outbuilding.
This is related to the distance, size of cable, and MCB used, and Ze
reading of the source.
If Zs is low enough then I was advised to use the main house earth.
I have not come across the PME factor?
Interestingly I am in negotiations to fit a new supply to a barn/
workshop from a house - which presently is on a TT supply, but will
soon be PME (It will be PME by the time the new feed is in use). The
barn is some 20m away, and will probably have a 6mm XLPE cable. I am
planning to have a local RCD in the barn. The supply cable (in the
house) won't be on an RCD.
I'd value some guidance on this.
Unless of course you 'export' the house PME earth satisfactorily to the
garage (it would have to be with more than just the CPC conductor of an
ordinary cable) and connect all exposed metalwork there to it as well.
Not a very practical approach in a domestic situation but the more one
moves towards a serious workshop the more sense it might make.
Is there a specification or a general rule as to how far away an outbuilding
can be before it requires it's own local earthing arrangement? I've looked
through past posts but couldn't find such a figure mentioned.
email me at
richard at olifant d-ot co do-t uk
As I understand it, you'll have to ensure the earth loop impedence is within
spec, which gets increasingly difficult with longer cable runs and increased
power. Also, you have to run main equipotential bonding back to the earthing
terminal by the consumer unit in UNBROKEN thick earthing cable. This is an
issue if you have metallic services, or structural metalwork.
OK, so reading into this answer, it will depend upon the expected usage in
the outbuilding along with the construction. For instance, for a supply to
a wooden shed that has outlets used for portable power tools it may be
acceptable to export the house earth, but if there's structural metalwork,
and fixed machinery (eg tablesaws) then a separate earthing arrangement
would be the only acceptable solution?
In my case, it's a wooden shed about 12m from the house, with portable power
tools being used in it...
For the equipotential bonding issue your answer implies that the SWA would
have to be run right back to the CU and terminated there, rather than at the
point of entry into the house, or are we talking about a bonding earth cable
completely separate to the SWA sheathing here?
email me at
richard at olifant d-ot co do-t uk
As I understand it, it must be completely separate, not run in the same
cable at all. It has to be pretty thick, too. I can't remember exactly, but
10mm rings a bell, and it needs mechanical protection for burying, too. If
it gets chopped, it must be relaid end to end, unlike the SWA supply, which
can use buried junction boxes.
Yes - one or the other ;-) You really have to do the earth-loop-impedance
calcs to be sure. The "unbroken" requirement, though, does mean that
if you can get away with using the steel (note, steel, not copper, so
a rather higher resistance) armour as the bonding conductor, the SWA needs
to run unbroken to the CU and have its sheath made off directly at your
main earthing terminal - possible but messy (weird pigtailing of the
sheath, wrapping the wrapped-and-collapsed sheath in green-and-yellow
heatshrink, and bringing it through the CU to the main earth terminal).
And at the garage end, full adherence to the "unbroken" requirement
if you need to cross-bond structural metalwork or water/gas pipes
makes it impractical. Hence the more common route is to end up with a
hefty separate bonding conductor, likely thicknesses off the top of my
head being 10 or 16mmsq, cable-tied to the outside of the SWA, doing
the bonding-in-the-outbuilding-thing, "visiting" (looping through) the
earthing point in the garage, continuing along the route of the SWA,
and then running direct to the PME earthing point. Heavy...
All of which sounds, thank goodness, like it's going to be unnecessary
*for**your**particular**installation* (no generalisations to
bottom-of-the-garden brick-built workshops with 63A submains and
a water main connection, please!): you've a *wooden* shed, so presumably
no fixed metalwork which would need equipotencial bonding; and with a,
what, 20A? 30A? breaker in the CU which feeds your 12m of, what, 4mmsq?
6mmsq if you want minimum voltage loss and lower earth-loop-impedance
figures, you're likely (but I haven't done the calcs!) to be in good shape
to meet the 0.4s disconnection time requirement even without an RCD for
the whole shed. (You may find it sensible to use 3-core SWA with its own
4mmsq/6mmsq earth - sorry, circuit protective - conductor, rather'n
using the sheath for the main earth; you'd still earth the sheath at
the house end, but now you're using the sheath for mechanical protection
only. It'd be nicer to keep the shed lights off the 30mA RCD you'll want
for the shed sockets; that way you'll still see what the spinning power
tool is doing after you slice through its supply cable or whatever ;-).
Hope this helps rather than confusing... Stefek
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