MEN vs TNS

[TPower]

Had a discussion with an electrician that’s proposing to install a different type of earthing system at his main switchboard than the standard MEN/TNCS. It’s a larger type install, supplied from a transformer.

His intention is to install it as TNS. It’s not really something I’m familiar with, but understand a separate protective earthing conductor would be required to run back to the supply transformer. He advised me there was an article regarding this in ‘Electrolink Issue 136’, unfortunately I don’t have a copy, so not quite sure of all the details.

I’m just trying to understand whether or not this could be a compliant install in NZ? Does clause 5.1.4 of 3000 make allowance for this? Would it become a Part 1 solution, therefore requiring a certified design? I imagine if this were possible, the Network company would need to have some input, as you would need to connect the main earthing conductor from the installation to the neutral within the transformer?

My own belief is it should be installed as MEN and comply with Part 2, as Part 1 is reserved for non-typical installations that (for whatever reason) can’t comply with Part 2. You couldn’t just chose to install as TNS if MEN were an option? Thanks.

[TPower]

I done this before, the transformer has to be for exclusive use of the installation, so no other customers off the transformer. You could install as MEN but keep in mind the transformer LV will have quite a large earth mat, way better than you could hope to achieve by putting in a earth pin.
Keep this in mind, what is the main earth there for?
1) provide an alternative return to the star point of the transformer (Neutral) in the event of the neutral failing (although not very good due to soil resistance)
2) keeping the ground at a same/similar potential to any earthed metal work.

[AlecK]

All our so-called "MEN" installations are TN-S.
The "S" stands for "separate"; ie the neutral and earthing functions are performed by separate conductors.
One is a neutral conductor (N), and the other is a protective earthing conductor (PE).
By contrast, the distribution system for "MEN" is TN-C, where a common conductor is used for both these functions.
When on conductor performs these two functions, it's a PEN conductor (for protective earthing & neutral)
The combination of these systems is classified as TN-C-S; and MEN is one of several possible variants under this classification.
Both TN-C & TN-S are under the general heading TN; which covers all systems that have an earthed neutral.

In NZ, under "3000", we use TN-C for distribution, and (generally) TN-S within the installation.
The usual point for the change from TN-C distribution to TN-S installation happens within the installation, at the MSB, specifically at the MEN link. That's the configuration illustrated in Fig 5.1.
We also sometimes use TN-C for submains to outbuildings - also shown in Fig 5.1.

An alternative configuration is shown in Fig 5.2; with the change from TN-C distribution to TN-S installation happening at the distribution substation instead of at the MSB.

As per clause 5.1.3; both these configurations are "MEN"; and are available under Part 2.
Using the configuration in Fig 5.2 is subject to satisfying several conditions; as per the particular clauses for each part of the earthing system.
WRT the main earthing conductor; 5.5.1.1 has the normal(Fig 5.1) arrangement in the clause; and fig 5.2 is available via the Exceptions.
WRT the MEN connection; again normal (Fig 5.1) is in clause 5.3.5.1, with Fig 5.2 arrangement available as an Exception.

So there's no need to be thinking about a Part 1 solution; both these are "MEN"; just one is bog-standard and the other less common.
And while "Electrolink" is a useful industry magazine; it's not the appropriate source for advice on how to set up the earthing.
All you need to do is read the relevant section of "3000".

In fact the primary function of the installation earth electrode is not providing a path for return current; and it's not keeping ground at same potential as earthed metalwork either. Those are (very) secondary functions.
It's a feature of TN-C systems that the N is earthed at origin and also (generally) at intervals along its length.
The term "MEN" specifically refers to this fact that the N is earthed at multiple places.
In our normal arrangement, the earthing-along-the-length of the distribution N is actually inside the installation.
The alternative arrangement in Fig 5.2 simply moves it back to outside the installation
Regardless of where it is, the primary function of the electrode (and the MEN & MEN link) is to maintain the distribution N at close to earth potential.

For a bunch of small installations, having an electrode at each one means the combined effect keeps the distribution N at close to earth potential/
None of them are very good on their own, but they are effectively all resistances-in-parallel so the overall impedance to ground is low.
But for a large installation adding a single - and generally ineffective - electrode within the installation doesn't help much in keeping the N - E potential low. So can be better to just use the good earth that's been established for the substation.


Schneider produce an excellent handbook that covers all this (and much more); based on the IEC 60364 series of Standards.
Since 60364 is also what our Standards are largely based on; much of it is directly applicable here.
So well worth some study - and it's free:
www.se.com/nz/en/download/document/EIGED306001EN/

[TPower]

Thanks a lot guys.

Figure 5.2 depicts an ‘HV customer supply’. I’m assuming this same arrangement could be applied to a ‘LV customer supply’?

Reading through the exceptions you’ve pointed out, leads me to believe the main earthing conductor would be connected to an earthing connection within the supply transformer? The example in question is a supplied from a ‘distributor’ transformer so exception 2 of 5.5.1.1 would apply. Is there any requirement for a ‘removable/MEN link’ between neutral and earth bars at the supply transformer? Perhaps this is something Network companies provide as a matter of course?

So 3000 allows for the MEN connection to be made elsewhere (rather than the MSB), but is ESR27(4) a problem here? Is there not a requirement for a MEN switchboard to be located within the installation itself?

Thanks again!

[AlecK]

As with most Figs, and as per para 3 of 5.3.1, Fig 5.2 illustrates the rile rather than being the rule.
The rule itself is the words of the relevant clause(s).
So the reference to HV within the Fig doesn't restrict application of the arrangement.

Yes, as per (both) Exceptions to 5.5.1.1 , the MEC goes to the substation instead of to an electrode.

Yes there still has to be an MEN connection; but as per Exceptions to 5.3.5.1 it is within the substation instead of at MSB.

The MSB itself still meets the definition of "MEN switchboard" in ESR 5; so ESR 27 does not prevent this arrangement from being adopted.

The wording of the definition for "MEN switchboard" could be interpreted as requiring the link to be within the switchboard;
but Note that it doesn't actually say that. It doesn't say "contain" a link; it says "have" a link .
So I don't believe the wording does, let alone was intended to, prohibit this arrangement.
After all; it's been in use for many years, and is not uncommon.

The point of ESR 27 is to prohibit arrangements that don't have
a) a connection to an earth electrode via the MEC; and
b) an effective - but removable - connection between N & E.
Under Part 1 of "3000", such arrangements could be used; eg the TT system or an IT system
ESR 27 currently prohibits such arrangements at installation level (still permitted for part-installations downstream of MSB)

[TPower]

Right, thanks for clearing this up Alec. So the first switchboard is still a MEN switchboard, it just happens to have the MEN connection located elsewhere, as per the exceptions to clause 5.3.5.1.