Can someone actually detail the actual clauses that relate to to this not being allowed? I can't find the information I'm looking for to back up why I believe it's not allowed
220.127.116.11 Earthing requirements would be a limiting factor
18.104.22.168 (B) (iv)
If you have a DB with a link supplied by a submain including A(s), N, & E.
The submain N & E conductors are effectively in parallel, so the PEC will carry a proportion of load current.
This set-up used to be permitted by NZ Regs, and was called a "linked busbar switchboard" .
There were restrictions on using this arrangement.
Until 2018 edition there was no clearly-stated up-front rule in "3000" about this.
It was governed by 22.214.171.124 (a); but no-one looks "testing" requirements until after completion.
2018 edition has added appropriate words to 126.96.36.199 ; so that there's an up-front rule stating the design requirement.
It was also (sort of) covered by 5.5.2, which sets the rules for PECs.
All the possible / permitted configurations are shown in Fig 5.3 (2007); and none pof them shows a DB with a N-E link.
From which we can conclude that it is not permitted to have a DB with a link that also has a PEC
That leaves the possibility of not having a PEC (in which case 5.5.2 would not apply).
That would include using a submain with a PEN instead of separate E & N conductors (same as we generally use for mains).
However the ONLY clause that permits that configuration is 188.8.131.52;
and it only allows it as an option for outbuildings; NOT for DBs within same structure. Further, if you adopt this option; several conditions apply.
Another (possible) configuration would be if the DB was supplied by a submain with no PEC, and had an electrode but no link.
All earth fault currents would have to travel back to source via ground; and due to the high impedance of the current path not enough current would flow to operate overcurrent devices within time required. Therefore not an acceptable configuration.
And of course (just for completeness) if the electrode was also removed, the items required to be earthed would not be.
The general rule for reading Wiring Rules is that if something isn't prohibited, it's permitted.
However the prohibition isn't always directly stated; sometimes it's simply a result of some other requirement.
In this case - and several others - the basic requirements around fault protection [2.4] carry over into the detailed requirements for earthing arrangements.
As long as we're using "automatic disconnection" as the means of fault protection; we have to do it i.a.w. 184.108.40.206 - and not all possible arrangements of earthing can comply.
You may set up a separate MEN installation in an outbuilding only, as you say, in accordance with 220.127.116.11
I always thought a separate MEN installation had to also be supplied from a MEN switchboard but I can't find anything to back this up
For some reason I thought once the PEN became split into a N and PE you couldn't feed a PEN from that N?
So as an example, MSB with PEC submain to DB in the same building, can this DB supply a separate MEN switchboard in an outbuilding or must this be fed from the MSB?
That is, the distribution system is TN-C; with the "multiple" points of tying the distribution N to earth being 1 at the transformer and one in each installation - known as the "MEN link". So the mains are also TN-C.
Generally everything in the installation downstream of the MEN link is TN-S; but with some exceptions.
Currently the only exception for new work is a PEN submain to an outbuilding.
But over the years there have been others.
Always the MSB has been an "MEN swbd", ie it has had a connection between incoming mains N and mass of earth; however the exact methodology for this has varied quite a bit, and the connection itself hasn't always had to be / include a "removable link".
For instance you'll find some older installations that still have a single bar for N's & E's, and/or with the MEC soldered into the mains N lug.
1976 Regs required 2 bars, and removable link; but still had the MEC on the same stud as the mains N (reflecting the fact that the primary (if not sole) purpose of the MEC +electrode was - and still is - to earth the distribution system.
In some parts of the country, the local ESA required that all DBs had to be MEN. This was largely due to local conditions; since they were under obligation to keep their distribution N down at or close to earth potential, and it helped to get as many electrodes as possible within installations.
Other ESAs would be happy as long as the installation was down to some arbitrary reading.
But always it's about the fact that the "MEN" variant uses connections-to-earth within installations, whereas other variants of TN-C use connections-to-earth on the distribution system instead.
Under 1993 Regs [70 to 73]; there was a 3-tier classification of switchboards, with "MEN switchboards", "linked busbar switchboards" (with N-E link similar to an MEN, but without any direct connection to an earth electrode), & "distribution switchboards" (with No N-E link) .
An "MEN" could only be fed only from another "MEN" (except the MSB which of course was fed from point of supply and not from another switchboard) ; and a PEC in a submain feeding an MEN was optional.
A "linked" could be fed from either an MEN or another linked; and a PEC in a submain feeding an "linked" was mandatory
A DB could be fed from either an "MEN" or from another DB , but not from a "linked"; and a PEC in a submain feeding an "linked" was mandatory.
This same system was replicated in 1997 Regs 80 - 83
I believe that system is where the idea came from that once an installation's earthing system has been split to have separate N & E conductors (TN-S); it's not OK to have the earthing of any part of the installation downstream of the split as TN-C.
True; under that system, it wasn't OK to feed a PEN (TNC) submain from anything but an MEN swbd .... but those Regs were revoked with introduction of ESRs 2010.
ESRs don't continue this hierarchical system; and "3000" prohibits some configurations that it allowed (eg having submains with N & E connected in parallel). So these days we really only have 2 types of swbd: main swbds & DBs.
MSBs are always MEN; and DBs mostly aren't: the exception being for (some) outbuildings - which are treated, electrically specking, as being the MSB for that structure.
We can even supply a whole string of outbuildings using a common PEN submain; provided we take mandatory precautions to avoid the PEN to the last one in the string being open-circuited by work done at an intermediate swbd.
Since a PEN submain doesn't have a PEC; there's no logical reason to require it to be fed from an MEN swbd. The important thing is the integrity of the PEN - exactly the same as it is for mains. And that integrity does not depend in any way on whether or not the swbd it's fed from has an MEN link or not.
It helps understanding if we reverse our normal way of thinking about installation design, where we take load current from source to load and back to source. Earthing isn't about where the load current flows; it's about where the (earth) fault current flows. So our thinking has to be in terms of
providing a suitable fault current path ; in order for fault protection to operate and to ensure the high currents involved don't result in hazardous voltages between "earthed" items and other, simultaneously accessible, items that are connected to mass of earth.
As far as fault current is concerned, it doesn't matter whether it has a single path to follow (the PEN conductor of the second submain) or two parallel paths (the N & PEC of an intermediate submain); back to a single path (mains N).
Which is why there's no longer a rule to prohibit taking a PEN submain from a DB that's fed with a PEC.
Mostly whatever "rules" have been in place have been based on sound safety/ engineering principles (sometimes "simplified" to avoid having to allow for Exceptions or variations).
But often the reason for a requirement isn't explained properly; if at all. And where there's not a proper / official explanation, people invent their own "reasons"; and those get spread around the trade and passed down as gospel to trainees.
We can end up dealing with someone's half-remembered "requirement" that's either never really was or no longer is; but not necessarily equipped to work it through.
Your approach is correct: What's not expressly forbidden is allowed, and where permitted options are limited that's also expressly stated.
If after a thorough search you can't find an actual current rule for a "must do" or a "must not do", and you also can't see any sound reason for the alleged rule; then generally safe to assume that there probably isn't any such rule.
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