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Posted By Topic: Earth peg requirement

Dec 05 2018 06:15

Have a new outbuilding with it's own 6-way switchboard consisting of 1 x 63A MCB (main), 2 x 16A MCBs, 2 x 10A MCBs & 1 KWh meter.

As this new outbuilding is 120m from the supply it is fed with a 16mm NS which in turn is supplied via an RCD & 63A MCB from a previously signed off sub-board in another building which does have it's own earth peg onto the earth bar.

Now, the question is does this new install require its own earth peg? The sparky that installed it says no but the an inspector friend (who's not involved in doing the job) says yes.

Be appreciative of the input of someone more knowledgeable. By the way, none of the bords on the entire property (except the primary board) have a NE link.

Dec 05 2018 06:20

What you havent told us is, is there a PEC run as part of the 16mm NS?

If yes then no earth peg required, if no PEC then a earth peg is required along with an MEN link in the sub board.

Dec 05 2018 06:52

And with the new main earth is the requirement for inspection ESR 6A (2 b) mains work. I’m open to being corrected as I have herd recent discussion regarding if this is in fact mains work (even tho it’s a sub main it has a main earth and a MEN link). I hope I’m not hijacking your post :)

Dec 05 2018 09:16

gregmcc is correct; whether the outbuilding requires an earth electrode (and an MEN link) depends on whether the submain includes a PEC.

There is no doubt (despite widespread "discussion") that an outbuilding DB that has an earth electrode & MEN instead of a PEC in the submain requires inspection of the outbuilding's "main earthing system".
All you have to do is read the definitions of "mains work" & "main earthing system".

But this set-up has a bigger problem. In fact several.
There's NO way that you can use that combination of cable & protection over that distance at 63 A. There's NO way you'll have any sort of discrimination from upstream device(s) using a 63 A mcb. And (if no PEC) it's NOT OK to run a PEN submain off another PEN submain without changing the way the first PEN submain is connected.

Single-phase volt drop for 16 mm2 is 2.54 mV/A/m (at conductor temp 45 C; ie well under fully -loaded). So over the distance, it's good for only 18.9 A (2.5% VD). But this is the second submain in series, so it's very unlikely there's 2.5 % available. After allowing for mains & first submain, and leaving something for the final subcircuits, a more realistic VD allowance would be 1.25 %; which would permit only around 10 A before volt drop exceeds 5% overall. The fact that the DB has 2 x 16A & 2 x 10 A circuits, and has a (check?) meter, indicates a high likelihood of drawing more than 20 A from time to time.

Discrimination isn't absolutely required between mcbs for downstream mcb < 250A [ (a)(iii)] (assuming first submain is protected by an mcb); but using a 63 A is ridiculous; especially when you won't ever be able to draw anything like that.

To get useful current, you really need a bigger cable; but at least you need to change the submain protection down to 25A max. Should be even lower, but you want at least partial discrimination downstream of it as well. But if I was the occupant of the 2nd outbuilding, I certainly wouldn't want to trudge back to outbuilding 1, let alone to MSB, just because I plugged in a heater too many (overload curve) or something suffered earth fault / short circuit (likely to trip three devices in three places unless discrimination is provided). You'll generally get reasonable discrimination with 2 "standard ratings" difference.

Most people use the PEN submain option for an outbuilding solely to save the "cost" of running the PEC. And - as seems probable here - they don't bother to read the conditions of using it; let alone consider whether it will provide adequate earthing. One of those conditions [listed at] is item (v), that the continuity of the PEN conductor to the outbuilding DB must NOT depend on any terminals of any intermediate switchboard. So the incoming N to first DB has to be joined directly to the outgoing N feeding 2nd DB, NOT (or rather only indirectly) to the DB1 N-bar.

At least this one is fixable (eg using a "line-tap", or even better a sheer-bolt connector; with a tee-off to DB1 bar).

Probably too late to put in an adequately-sized submain.


Dec 05 2018 09:34

Also there's this bit - "By the way, none of the boards on the entire property (except the primary board) have a NE link."

If the switchboard in the first outbuilding has "it's own earth peg onto the earth bar." - then it must have a MEN link - or, it didn't need the earth stake, because there's an earth back to the main board?

As Alec said, the voltage drop is the biggest problem. Even limiting that 120m run to 25A, you're still at the 3% mark.
Another option would be to just have 1x16A and 1x10A (or a 20A and a 6A) circuit at the new outbuilding.

You should also look at the RCD configuration. You're sort of saying there's an RCD on the feed from the first outbuilding, but you didn't mention it in the new building?

Also are you sure these "63A MCBs" Are MCBs? Or RCDs? Or just switches?

I'm assuming you're not an electrician?

Dec 05 2018 11:13

Depending on age - and area, different rules may have applied when the first submain was installed. Eg in the early 1980's in Waitaki valley (alluvial gravel) all outbuilding submains were required to be MEN + electrode; in an effort to drag down the impedance between distribution N & mass of earth. And under 1993 Regs, it was OK to supply an MEN swbd from another MEN swbd, regardless whether either was in an outbuilding.

That said, having an electrode but no link has never been an accepted methodology, and this may well have had the link removed by some idiot who believed that "we don't have links in DBs any more". Possibly the same idiot who specified the cable? And lined up a series of protective devices going {supply fuse} > ?? 1st submain protection > {RCD + 63 A 2nd submain mcb} > 63A mcb DB2 isolator > 16 A / 10 A mcbs for final subcircuits?

If there's no PEC in that 1st submain, having no link makes it - and everything connected to it - VERY unsafe. And from the sound of it there may be other submains similarly unsafe.
I strongly suggest getting your Inspector friend over to have a good look at the entire installation.

Dec 05 2018 16:27

OK, here we go - will answer as many queries as possible.

No, I'm not a sparky but own the property and have done the mechanical work, burying the cable etc. A local sparky has installed the new switchboard (the second submain) and lights/sockets and will provide the CoC.

The total draw will be no more than 10A ever so voltage drop shouldn't be an issue. The supply from the main board to the first submain is 22m and is 50mm2 NS.

There is no PEC in the run.

Yes, they are 63A switches at each end (sorry, I called them MCB's).

I confirm that the first sub-board has it's own earth peg but that N & E are not linked.

Looking around at the other sub-bords here, they all do have a NE link and they all do have their own earth peg.

Yes, a total inspection is on the cards for sure. Might also employ a different sparky.


Dec 05 2018 17:28

The most urgent thing, as in RIGHT NOW, is to get a link back into that existing DB.

With the earthed items supplied from that DB connected only to mass of earth via the DBV's electrode, it's unlikely to be able to pass enough current for (earth) fault protection to operate. So when an earth fault happens it'll be livening the ground around the electrode relative to the N conductor, and to anything earthed / bonded / otherwise connected to the main switchboard; while leaving whoever is holding the faulty appliance / broken wire subjected to a prolonged shock current. Not guaranteed to kill every time; but pretty high up the 'unsafe' scale.

The other option, for older DBs with electrode connected to N bar, is that there will be no fault current at all, instead everything will be raised to supply voltage compared to ground & N. Again not guaranteed to kill every time; but equally dangerous.

The same thing happens when idiots remove links from older caravans "to stop it tripping the RCD on the supply"; but fail to do the mandated tests to prove the earthing is effective.

Unfortunately, some of those idiots hold "Electrician" and even "Inspector" licences.

Once that's fixed, you can get on with the new submain.

If the device at the upstream end of the submain is just a switch, it needs to be changed to an overcurrent device []. Best option probably a 25 A mcb.

Plus all the other bits already mentioned.

Can't do a full V-drop calc without knowing total load at each point, and conductor route distances rather than simple distances; but the figure for 50 mm2 (again at conductor temp 45 C) is 0.87 mV/A/m, so over 22m will be 19.14 mV/A.
Add on the 304.8 mV/A for 2nd submain, and even if the 10 A at DSB2 is the ONLY load in the entire installation, you've used up 3.24 V in the 2 submains. Which might sound OK until you add some load supplied form MSB and some from DB1, and maybe some form any other DBs supplied by other submains.
It's not hard to run out of mV, you only get 11,500; which is probably why whoever installed 1st submain use 50 mm2 even though the distance was short. And why using 16 mm2 over 120m is pushing boundaries (to be polite).


Dec 08 2018 00:06

OK an update.

As had buried the 16mm NS in a hunk of 50mm poly tubing, have been able to pull it backwards and replace it with 50mm NS. Three cheers for talcum powder and tractors! So, in fact, we now have 50mm all the way from the Main Board - 140m in total.

New sparky has connected it to live feed from Main Board and fitted to board in new outbuilding: (in order) 63A switch -> kWh meter -> RCD -> MCBs for each circuit. Earth peg onto earth bar.

All seems OK except that linking NE results in the RCD not working with a 30mA tester on any of the sockets. Disconnecting the NE link on this new board results in the RCD working as it should at 30mA. So at the moment, he's left the link off pending further enquiries but he is stumped. Any suggestions?

Dec 08 2018 08:37

I can only surmise that he's wired it wrong in the outbuilding switchboard. Probably the neutral wiring.

It's sort of basic stuff that any electrician should be able to work out though..

The incoming neutral should go to a neutral bar, then to the line side of the RCD.
The load side of the RCD should go to a smaller bar then to the outgoing circuit neutral, or possibly the outgoing circuit neutral is directly connected to the RCD (although that's not correct).

The E-N link, must be on the first bar where the incoming neutral is connected.

Can you post a photo of the outbuilding switchboard with the cover off?

The "picture" link on this site doesn't allow hi res images.
I usually use this free site, then post the link here.

Dec 08 2018 12:39

Aha ha. The EN link is off the second E bar, ie the load side of the RCD. I agree, is basic stuff that a sparky would know. Will delicately advise, a tad worrying about the skill-levels that I've been paying for.....

Dec 09 2018 07:26

When testing RCDs, need to understand the test circuit being used. It's different for a "push-button" test than for an external RCD tester.
The tester takes active and provides a path via PEC to earthing system.
The on-board push button doesn't use any earth connection, just connects active from load side to N on line side.
Where everything is wired correctly, both tests work. But for some fault scenarios they'll work - or sometimes not work - differently due to using fundamentally different circuits.

Most RCD faults for new RCDs are not a problem with the actual RCDs, but a problem with how they have been selected and / or installed.

Dec 10 2018 12:40

It could also be an issue with where the neutral to the meter is terminated, i.e. load side of RCD would cause tripping if feed to meter is before the RCD and vice versa. Also some RCDs must be installed with due consideration to load and feed side.