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Posted By Topic: submain MEN link

Timesup
Sep 08 2019 18:49

Ive been asked up grade the submains to an out building and need to install a new earth to a driven stake at the out building
Im getting confusing answers about the inspection of the earth from different inspectors.
One say it needs an inspection because it new and is connected to a new neutral and earth bar with MEN link and is a main earth for the out building
another say no its not the main earth for the property.
whos right.
   

gregmcc
Sep 08 2019 19:14

it's a bit of a curly one, the way I look at it is by reading the definition of "mains work" in the ESR's.
(a)(ii) work on main earthing systems (including connecting the conductors of main earthing systems at a MEN switchboard)

you have a sub board (the definition of mains work says "...a MEN switchboard") that is going to have an earthing system connected to it including a MEN link, I would say yes it needs an inspection as it becomes mains works, if you had run a PEC with the sub-main then it wouldn't be mains work.
   

Haydos123
Sep 08 2019 20:34

If the DB board in the outbuilding is supplied by an earth from the main switchboard then no MEN link should be installed and no inspection is required.
If there is no earth supplied by the main switchboard and you drive an earth stake for the outbuilding, it is now classed as a main switchboard, it needs a MEN link and 100% needs an inspection. Any time you drive an earth and create a MEN point requires an inspection.
I had an argument about 3months ago about this same subject. 3 inspectors with and 50years experience between them all said no inspection is required. It’s written quite clearly in the standards.
   

evanh
Sep 08 2019 20:35

NZS3000:2007 5.5.3.1(b)(iii) and NZS3000:2018 5.5.3.1(c)(iii) The earthing conductor between the distribution board in the outbuilding and the earth electrode shall be regarded as a main earthing conductor for the purposes of earthing of the electrical installation in the outbuilding.

   

DougP
Sep 08 2019 21:46

The definition of "mains work", "main earthing system" in the ESRs, and 6A(2)(b) is all you need to look at.

But half the inspectors will tell you that it doesn't need to be inspected.

If you're upgrading the submain, running a PEC with the submain is a better and safer option anyway.
   

broadsword
Sep 08 2019 21:56

I'd argue that 'a better and safer option' is a somewhat sweeping statement with not a lot of basis ;)



I would concur that if the electrode, conductor and their connection at the MEN board at the end of the submain ARE NOT low risk work then they absolutely are high risk and inspectable for sure

Also concur that many many many don't agree but I'd like to see the argument!
   

DougP
Sep 08 2019 23:03

Not a lot of basis? If you don't think it's better or safer, maybe you should explain why you don't agree? I think there will be some members here that will back me up ;)

Just a couple of simple facts. Firstly, under normal circumstances, the earth stake does nothing. Under fault conditions like an A-E short in the outbuilding, it still does nothing. The fault current is carried by the lower resistance PEN conductor.

Under fault conditions such as a bad neutral in the PEN submain, the earth stake struggles to hold the earthed equipment in the outbuilding to the same potential as the local earth, reducing touch potential, but not eliminating it. And also these days, of course it will provide a reference for operation of RCDs, which improves the safety considerably.

But at an even more basic level, with a PEN submain, you are using a current carrying conductor to provide the protective earthing, which means that any voltage drop in that current carrying conductor translates to voltage rise above actual earth potential on any earthed items in the outbuilding. Once again, the earth stake and slab bond if present will try and hold the earth close to the local earth potential, but it's not perfect.

On the other hand, a PEC with the submain does not carry current and it's directly connected to the main installation PEN earthing (the main incoming neutral), so all earthed equipment in the outbuilding is always as close as possible to actual earth potential, all the time. Some people might say that "the smaller PEC is a higher impedance than the larger neutral", so they think the larger neutral is better. But in reality, for normal distances, the difference in impedance is in the hundredths of an ohm, and makes no difference under fault conditions if correctly sized.

Lastly, if you've ever dealt with any faulty main neutrals or service neutrals on installations, you would probably agree that it's preferable to have the additional PEC for submains if possible.
   

broadsword
Sep 09 2019 06:56

hey Doug

not looking for an argument at all :) perhaps I should simply have asked that you explained what you meant by your statement

interested in your commments thank you
I agree with them theyre all valid for sure

peronally though I would argue that the circumstances dictate the 'best' solution and that 'best' solution is not always 'safest' and 'safest' isnt always 'best'.

my preference is to run a PEC in a domestic or residential situation for many of the reasons you have explained

where the outbuilding is a considerable distance away I would be inclined not to if the impedance in the PEC is likely to be markedly greater than the correctly sized submain neutral

at the end of the day an MEN outbuilding is not really any different to the first MEN board from a point of view of safety risk due to failure of main supply neutral and main earthing system

THAT is why the main earthing system in a MEN outbuilding absolutely requires inspection as high risk work

TNCS is a pretty darn safe system of supply but it does (as youve touched upon) allow for faults to be present often unnoticed for some time until things go south.

It relies upon us to be diligent and know our stuff and do our testing

the sad thing is that there are far to many cowboy dont give a stuff 'sparks' about who can't even follow what we are talking about even if they cared
   

broadsword
Sep 09 2019 07:00

and before anyone jumps on the 'safe' and 'best' bit... im just suggesting that every situation is different... eg fitting an RCD is 'safer' until its tripping creates a different problem thats less safe
   

AlecK
Sep 09 2019 10:00

Which is where we will end up once TT earthing is permitted, even if it's only for some installations.

A while back this issue was raised in an NZEIA meeting; where ES's senior technical advisor was present. At first he said "no inspection" but on having the words mentioned drawn to his attention (after the meeting) he agreed the current ESRs DO require an inspection for the earthing system of a PEN-fed outbuilding.

Those arguing the other way are clearly NOT basing their view on current ESRs; but on older lists of"what work requires inspection" in older - now revoked - Regs.

Personally I believe that not only is the earthing system classified as high-risk PEW, but the installation of any PEN should also be. Too many use it solely to save cost on the submains cable, and don't bother to check for factors that render it unsafe such as parallel paths; or other requirements that are come in when PEN is used.

There are a number of situations where PEN is not appropriate, as is reflected in IEC Standards; and these flow through to AS/NZS.
Eg caravan park pillars, and EV charging. however that flow-through has not always been accurate, leading to more restrictions in AS/NZS than in IEC.