It is the "MUST" part above that needs explaining. We established in a previous thread that in a TN supply/TT generator switchover system, that the TT earthing rod required for generator TT operation may be connected to the TN supplier's earth. Why does the same not apply here, provided that the earth rod is tested to below 200 ohms and the RCD requirements are met? Is it some sort of earth loop current issue?
Christian.
[...]
I would vote for the inclusion of that in the electrical section of the uk.d-i-y FAQ, if it is sufficiently generic to deal with a lot of the export/don't export earth questions.
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
Nice write up. As this comes up a lot, how about sending to Phil to go into the FAQ....
I've used both of the described methods for different applications.
Do you have any particular suggestions where the provisioning is for a workshop and heavier equipment is in use - say 30A single phase, where the workshop is dry and the house supply is TN-C-S?
.andy
To email, substitute .nospam with .gl
2m is likely to be excessive. The recommendations say at least 450mm but more if it is likely to be heavily dug over - say 800mm.
Normal practice is to bury the cable - back fill 150mm or so, and then lay a warning tape, then backfill the rest. That way if someone does get carried away recreating their very own air raid shelter in the future they should reach the tape before they get chance to damage the cable.
"Andy Wade" wrote | Briefly, there are two main earthing options: | 1. Exported house earth | ----------------------- | Simply 'export' the house earth via the armour of the SWA cable (the | 'submain'). This will be fine for a dry timber shed with its floor | raised well off the ground, so that it remains dry. ...
| Do NOT use this option if the house earthing is TN-C-S (PME) and ^^^
Should that 'and' be an 'or' (PME earths should never be exported)?
| For a tidy looking job use metalclad wiring accessories mounted | side-by-side and coupled together using 20mm conduit bushes and | lock-rings.
And I'd suggest running a green/yellow earth wire from the SWA gland through each box and connecting to the earth terminal in each box and on each faceplate, rather than relying on conduit-box-faceplate touching contacts. Maybe you assumed this would be done, but better to be explicit.
Owain
in sand
- back fill 150mm or so, and then
Sorry for a rather delayed follow-up. If you don't break the earth then you have effectively exported the house earth and your shed is a TN system, not TT. And, unless you've sunk a massive earthing system, your local earth electrode will be bugger-all use in preventing a rise in voltage above the local ground potential if a supply network fault is trying to pull it up toward 230V.
The need to get away from the supplier's TN earth is relevant where the equipotential zone concept is difficult or impossible to apply - hence the reference to greenhouses and Class I equipment used outdoors. Remember that TN earths can rise to potentially dangerous voltages due to (for example) cable faults on TN-S systems, or the classic broken service neutral on a PME supply. OK, these things are pretty rare, but they do happen, which is one reason why equipotential bonding is required. The big advantage of TT earthing, properly implemented and maintained, is that your earth really is at the local ground potential, and will stay there. Even if the earth electrode has the maximum recommended resistance value of 200 ohms (see OSG), then to trip a 100mA RCD shouldn't lift the earth by more than 20V.
The downside of TT is the reliance on RCDs, which don't always fail safe, and the need to ensure that the earth system is maintained. Hence in dry conditions it's probably best to stick with TN.
An alternative protective measure for PME installations now being recommended by the IEE [1] is to provide your own earth electrode and connect it to the main earth teminal in the _house_ installation - i.e. in parallel with the supplier's multiple earthing. The recommended maximum earth resistance in this case is, for domestic installations, 20 ohms - not necessarily easy to achieve with DIY efforts in some soils. Then, in the event of a break in the supply neutral, there is a path to earth which will tend to reduce the touch voltage on your earthing. If you only have a few lights switched on this will be a quite effective safety measure, but it won't help a great deal if an electic shower is in use at the time of the fault - that's when you really rely on the bathroom's supplementary bonding to save your life.
I hope that's clearer now from the above. A standby genny on your own premises can always be operated as a TN system - why would you want use TT for that? You do have to provide your own independent earthing of course - and the parallel earth electrode approach referred to above would kill two birds with one stone.
[1] See, for example, section 12.5.4 of the current /commentary/ on BS 7671, ISBN 0852962371. (Recommended reading for all wiring regs geeks.)
No, I meant "or" - but you are free to disagree ;-). I wouldn't say "should _never_ be eported" - "never, without due consideration" perhaps.
Yes I agree entirely - Reg. 543-02-07 demands that anyway.
Would you care to explain that a bit more?
I am considering a small genny to power my central heating plus a couple of freezers and some lighting in the event of a power cut. All devices fed by the genny would be unplugged from their usual mains sockets and plugged into the genny instead - and would thus be totally isolated from the house supply. [I have wired my CH through a 13A plug rather than a FCU to facilitate this.] What do I need to do about earthing these appliances when they are being powered by the genny?
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.