Online assistance for electrical trade people Login  |  Register  |   Forgot Password
Assistance for electrical trade people




Click here to send Ron a pdf document for publication on this Topic

Documents must be less than 200k in pdf format

Posted By Topic: New supply to outbuilding

Feb 27 2018 23:11

Hi guys - existing main dwelling with men switchboard.

Have a outbuilding which was planning to run a new sub circuit off main switchboard to. Approx 40m from main dwelling.

Originally thinking of running 32A supply with voltage drop i am going to run a 16mm tps.

I assume as long as ELFI is all ok then no problems should apply - assuming a run like this shouldn’t affect ELFI too much.

Am wondering what advantages (if any) of not using the PEC on the sub main TPS & installing an earth stake instead??

All I can gather is it’s just going to complicate things as it will then need inspection.

However is it a safer method?

Any constructive comments/ideas appreciated!


Feb 28 2018 00:01

IMO, it doesn't make a lot of difference really. The earthing system in the two buildings are either tied together with the (larger) PEN neutral, or the PEC.

Using PEN, the only issue could be a broken neutral, where the earth stake comes into play.

Probably, this could be viewed as an over simplification, but...

And also to set your mind at ease, installing a PEN submain and earth stake at an outbuilding is not mains work because it is not the main earthing system, so it does not require an inspection.

Feb 28 2018 08:36 (b) iii "shall be regarded as the main earthing conductor"

ESR 4 (Definitions) Mains Work (a) ii "work on main earthing systems (including connecting the conductors of the main earthing systems..."

High risk work
(not being low risk PEW) that -
(b) is mains work on an installation

ESR 70 (1) High risk work to be inspected

SO I would suggest such an earth installed would require inspection.

Feb 28 2018 08:41

The advantage of using a PEN submain is cost. However it is generally regarded as inferior to running a PEC in the submain.

The earth electrode, MEC, and MEN link in an outbuilding perform exactly the same function as those at the MSB. Clause (b) explains how to do it, and points out that when using a PEN submain the outbuilding switchboard "shall be regarded as a main switchboard for the purpose of effecting the MEN connection."
Also the earthing conductor to the outbuollding's electrode "shall be regarded as a main earthing conductor for purposes of earthing the electrical installation in the outbuilding".

As long as there are no other services connecting the two structures, a PEN submain should be adequate. Problems can arise if there are any alternative fault current paths, such as through structural components or water pipes that are earthed or bonded (as
required) to MSB in main building - see item (vi) of the clause, and the Note.

On the matter of inspection, these same components meet (exactly) the definition of a "main earthing system" in ESR 4, and work on any "main earthing system" is "mains work"; which is (mostly) classified as "high risk by ESR 6A- and therefore does need an inspection under ESR 70.

Whether or not this was intended by the writers of ESRs can be debated, but makes no difference. What matters is what the words say; and how EWRB or courts interpret the words when / if a case comes before them.

Most recent news from EWRB was that they prosecuted someone for failing to have work inspected; when - in the Board's view - the work done went beyond replacement of a switchboard. They didn't give is enough info as to how they decided this; but they did warn that a "cautionary" approach should be adopted when deciding whether or not to have work inspected. So the question becomes: are you confident you can justify your decision not to have the work inspected?


Feb 28 2018 09:38

I can see how that could be the interpretation, because the definition of mains work (a(ii)) it says ".. at a MEN switchboard" meaning any MEN switchboard I guess.

I'll have to discuss it with the inspectors I have been using, because they don't inspect the MEN switchboards in outbuildings.

Feb 28 2018 10:20

"ask any 3 inspectors, and get 4 opinions."
Sad but (almost) true.

Which in part is due to an almost complete lack of explanation from Energy safety when ESRs change; making it hard to grasp the underlying "system" the ESRs are supposed to implement.
And in part to some Regs that are not written clearly enough, so are capable of multiple interpretations.

That said; sometimes the interpretation taken - even some taken by Energy Safety and published as guidance -just isn't there in the words at all. Some have more to do with what the writer thinks it ought to say, than what it actually does say. The one that tried to make Inspectors responsible for errors or omissions on CoCs being a prime example. Some of the EWRB's "findings" are similarly flawed - but it would take deep pockets to appeal a decision.


Feb 28 2018 14:58

"Yes" when i have been asked if an outbuilding with its own earth and electrode requires inspection.

Feb 28 2018 20:37

I did a PEN submain to a sleepout recently. I didn't have a choice as the submain was already in the ground. I read the rules as per AlecK description and arranged inspection.

The inspector was bemused. Agreed that it was consistent with the rules but said he couldn't recall the last time he'd inspected anything similar. His conclusion, that others must be certifying such without inspection.

Mar 04 2018 14:33

thanks very much for all your replies - good to get others opinions on the matter.

I'll opt for a PEC in the sub-main (one less inspection i have to book!)

Puts my mind at ease if i ever decide to do a job in future with an earth stake at outbuilding.

Many thanks!