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Posted By Topic: Earthing Arrangement

Nickg
Jul 30 2019 23:33

Hi

I've got an issue with an earthing arrangement I've come across recently. I'm completing the inspection work, and aren't satisfied with how the the electrician has completed the work.

We've got a commercial building, with multiple tenancies within the one building. There has been one earth electrode driven. Each tenancy has an MEN switchboard. Each MEN switchboard has an earthing conductor running to the driven earth electrode, so there are multiple earthing conductors connected to the one electrode. Not an arrangement I've come across before, so it set off a few alarm bells for me.

I can't find any wording in Section 5 of 3000 that would support such an arrangement. In fact I completed some load testing at one of the MEN switchboards, as I assumed current would likely be flowing in the earthing conductor, which was the case. Seemed about 25% (approx) of the load current was flowing through the earthing conductor. There's parallel paths in the earthing conductors for load current to flow, due to all the earthing conductors connected together at the electrode plus the MEN links within each switchboard.

...So that's what I'm faced with, and believe it needs to change.

I'm working in with the electrician to try find the simplest solution. There's been some discussion of driving multiple earth electrodes (one for each MEN switchboard), not an arrangement I'm fond of, as it means driving a stupid amount of earth stakes. Also the fact that all the MEN switchboards are within the one building might be an issue I think, I'm not actually sure if this is permitted? Although I have seen it done like is in the past.

My suggestion was to install an outside Central Supply/MEN Switchboard cabinet and convert all the tendency switchboards to DBs, as is the usual arrangement. However, I was led to believe this might not be an option due to customer specifications.

My next thought is can the 'Network Distribution Box' be converted into the MEN point? Obviously Network approval would need to be sought. I'm wondering whether the main earthing conductor could be connected directly to the neutral bar within the Network box, and the earthing conductors going to each DB connected there too. or to simplify it further could the frame/chassis of the network box actually be considered to be the electrode? There's a lot of metal in those boxes, and would likely make a better earth than a driven earth stake. It seems a bit ad hoc, but effectively it'll function the same as it would if there were a Central Supply/MEN Switchboard installed. There just won't be a 'main switch', or removable MEN link, but as I say it'll still function just the same. I've been reading through the 'exceptions' in 3000:5.5.1.1, and are thinking possibly this arrangement would fall into 'exception 2'?

Thanks for reading! Any thoughts/feedback much appreciated! :)

   

DougP
Jul 31 2019 07:35

There's nothing that disallows multiple MECs connecting to one earth stake or other compliant main earth. For example, each tenancy could connect the main earth to the reinforcing in the slab, and your result would be the same.

As you have found, there is some current flowing in the MEC, and that's not uncommon in commercial installations, and current flowing in the MEC is also not disallowed (that's only for PECs).

As long as each tenancy has their own connection to the stake (preferably identified), and complies with the 0.5ohm requirement, I don't see a problem with it.
   

AlecK
Jul 31 2019 09:06

Nor do I; in principle.

There's certainly nothing that prohibits sharing of the earth electrode - and if using a foundation electrode, no-one would think twice about it. Using the network box as an installation electrode might be effective, but would not be compliant with 5.3.6.2. And just adding lots of electrodes wouldn't improve the performance of any thing.

The primary purpose of the MEC & electrode is to tie the distribution N to ground, as part of the "MEN" system of distribution. As far as the ability to carry fault current goes; it's never very good - fault current flows through the MEN link to distribution N.

The secondary duty of the MEC & electrode is to carry load current in event of a fault on supply N, worst case an open-circuit supply N. Assuming these installations are all off a common 3-phase + N supply; the situation WRT the electrode under this scenario is no different than for a single installation of the same rating (or any rating). One electrode, but in this case with several smaller MECs instead of one big one. The combined CCC is likely to be greater than for a single installation. The Ns of each installation are all connected together, so worst-case loading for collection of MECs is the out-of-balance current of the combined installations.

The fact of the Ns all being common explains the current you found, because yes of course there's a parallel path between every MEN point and every other MEN point. With separate electrodes it would be reduced, because you'd have a moderately high impedance between the electrodes. The level of this current will vary according to the out-of-balance (neutral) current for each of the installations, and the relative impedances of the N paths vs the E paths. But - as above - it's never going to be such that the MECs can't safely carry it.

The Exceptions to 5.5.1.1 - like those for 5.3.5.1 - are directly related to the alternative arrangement - illustrated in Fig 5.2 - where the installation has no electrode and no MEN of it's own. A network distribution box is a long way from being the "substation" needed for this configuration.

It's perfectly Ok - in fact quite common - to have multiple installations within one structure[1.4.49]; and each installation is required to have its own main switch(es), its own MEN link, and its own MEC. It will also obviously have its own mains, and each will have a "point of supply" as the starting point for those mains.
Those are the matters that comprise "mains work", and each one need to be inspected, and recorded - separately - onto the high risk database.

There may well be conductors and other fittings between the connection to network and the POS for each of these installations. such fittings are private works. The most obvious example of this is a high-rise with a "rising main" - which, by definition, is generally not consumer mains at all but works.

The alternative configuration with a common MSB and only a DB for each tenancy; is a single installation.


   

Nickg
Jul 31 2019 09:41

Thanks a lot for the help guys. Yeah, making no changes would be ideal. Just not something I've previously encountered, so thought it best to look into it and ask some questions.

I'll just have to pull my head in, and tell the sparky he's right after all.

Thanks again.
   

AlecK
Jul 31 2019 10:29

No problem. We all get situations that are a bit different from what we may be used to, or what we expect.

The good thing is you're thinking about it, looking at the "good book". Sometimes what it doesn't say is just as important as what it does say. And one of the things it doesn't say is that every installation needs a separate electrode. Just as well when you think about a 30-floor office block with maybe several installations per floor.

And your customer will probably respect you more for discussing the issue than he will for just digging your heels in and insisting.

Thing to remember, especially when it comes to inspection, is that if you can't cite the rule that makes it non-compliant, then you shouldn't have an issue with it.

Similar for failing to reach "electrically safe"; have to be able to identify a "significant" risk of injury or property damage.


   

mrsparky
Jul 31 2019 21:11

Had a read and can't tell from your description but I wouldn't allow them all to be connected to the electrode through the same stud, IE all individually connected.

Just so if for whatever reason one needs removal the others aren't disturbed. Who knows probably even a rule about that somewhere.
   

AlecK
Aug 01 2019 08:20

Yes, individual connection of each MEC to the electrode is required
   

Kingswell
Aug 01 2019 20:16

I'm involved with a multi-tenancy block with 3 shops on the ground floor, and professional offices upstairs. Inside every tenancy is a combined meter/MEN switch bd; all ground flr shops have their own driven earth electrode, and bond to common cu water pipe. The upstairs offices never having been electrically upgraded retain the 1960's main earth onto the cu pipe. Some tenancies are 3phase, some single phase. Leakage currents into the earth electrodes is only milliamps, but the cu pipe seems to act like a star point. At the largest 1 phase shop, approx 65% of the phase current returns into the neutral, and about 35% flows into the cu pipe in the bldg. Vector have checked the earth loop impedance at the POS, and it is OK. There were many faults in the buildg which had to be fixed, but nothing has reduced the leakage.
   

DougP
Aug 01 2019 21:55

Has the loop impedance been checked at the switchboard for that shop, with the main earth and the bond for that shop disconnected?
Checking only at the POS wouldn't test that shop's neutral.

It sounds like that shop's neutral is poor, and the 35% is going back by a different installation's neutral.
   

Kingswell
Aug 02 2019 20:47

On this particular shop, as the old board had been fire damaged, and the old mains were too small for the load, we installed a new board plus new mains back to the network distribution box. None of this made any difference.
   

monty
Aug 05 2019 23:18

Just thinking out loud but in this situation could you install an Earth bar like they have in most substations to connect all your Earths to then wire from your Earth bar to the Earth electrode
   

nobody
Oct 07 2019 20:16

hi how do you define an outbuilding?/ installation

this is listed in 3000

There shall be not more than one MEN connection in any one
outbuilding.

im intrested in what kingswell found that the conductive parts of given building within the installation could act as a star point or conductor, then there is this also maybe reading in isolation but


No short circuit exists, because a short-circuit current flowing between
live conductors and through part of the earthing system can cause
considerable fire damage or personal injury, particularly in high current
locations.

not trying to reinvent to wheel but has any one looked at 3000.2018 about this

thanks
   

AlecK
Oct 08 2019 08:42

None of this is at all uncommon; it's just that as requirements change they only apply to work done from that time on.
So we end up with legacy installations that complied when installed, and still comply with original rules, but would not comply if installed that way today.

ESR 113 says they are allowed to continue in service, as long as they are not electrically unsafe. Which is simply the specific electrical rule that expresses a significant and fundamental principle of NZ law: no mandatory upgrading is required.

However just because upgrading is not required, doesn't mean it wouldn't be a good idea.
   

nobody
Oct 10 2019 20:48

Agreeded about legacy installations, so ur saying under 3000 this is compliant today as per Nickg original post dated this year at the top?