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Posted By Topic: MEN link in distribution board

OctaneOutlaw
Oct 26 2019 07:39

I have seen MEN links in distribution boards that are not set up as separate MEN switchboards

These are old boards and I have read this used to be common and a requirement

At what point, if any, do I have to remedy this? What are my obligations working on either the main board inside and finding something like this when testing or whilst working on the distribution board?

Do I leave them as is? Or pull them?
   

Aydin
Oct 26 2019 08:53

If there is an MEN board before that you have to pull the MEN link from the DB out.
But, if the DB has its own main earth conductor with earth electrode then you can leave the MEN link as it is.
   

AlecK
Oct 26 2019 09:45

read this in conjunction with my answer to your related question "Main earth & maisn N on same stud".

Yes this was a legitimate method for many years; what 19973 Regs called a "linked busbar swbd" which had a link but no MEC. So it was kosher (if done right) up until 1/4/2010.

There is NO requirement to alter them; ESR 113 says they can remain in service.
And even though generally it may seem like a good idea to remove the link, in some cases could have unfortunate consequences - this would mainly apply in three-phase commercial / industrial installations where the submains N may have been designed small.

   

pluto
Oct 26 2019 09:45

Before removing the MEN link in a distrigbution switchbaord, and there is a MAIN switchboard upstream of the distribution swittchboard, you need to check that the submain earthing conductor is large enough (or low impedance) to carry any earth fault currents and modifed submain arrangement meets the current AS/NZS 3000 rules, EFLI, etc.

My ususal recommendation is, unless there are very good technical reasons to do so, the distribution switchboard MEN (or Neutral to earth link) should NOT be removed. Typical technical reasons are high currents in the submain earth conductor and the need to separate protective and equipotential earth systems with electronic control or measurement systems.
   

OctaneOutlaw
Oct 27 2019 09:30

Interesting

If you did a switchboard upgrade on such a board I'm assuming you'd replicate it like it was with the MEN connection in place due to the same reasons you'd leave the link in the old board in the first place

Is this likely to cause confusion to the next person seeing a new board and an MEN and come through and rip the MEN out thinking it's non-compliant? Or is it just another case of it's not up to us to worry about what those do after us and as long as our work is done to standard and clearly specified on our certification. I suppose the other issue you'd run into is even though you'd done the right thing you'd run the risk of being reported for non-compliant work because people have no idea what's actually gone on
   

AlecK
Oct 27 2019 12:02

and could even be found "guilty"; 'cos the Board - and it's lawyers - sometimes seem to be in a world of their own when it comes to applying the rules.
   

OctaneOutlaw
Oct 27 2019 12:24

Typical haha you try do the right thing and get condemned for doing so

So reading above seems people don't recommend removing it and the main reason for this is due to the submain cable?

I guess if you did the upgrade and pushed a new submain cable then you'd be fine to have the new board with no MEN like a usual distribution board

Only issue with that is "usually" submains really aren't easy to replace

What would you do in such a situation? Try and find a safe way to have the new board with no link, or leave the link in place in the new board

Or would it be better to try disconnect the incoming earth and give the board its own electrode and have it set up as a separate MEN board if the subboard was in an outbuilding?



Another question, is the MEN link in a "linked busbar switchboard" considered part of a main earthing system such that changing it should be inspected?
   

DougP
Oct 27 2019 15:05

Some of the submains would have been PEN but didn't have an earth stake - so the N&E link is necessary.

Other submains might have had a PEC and nuetral, but also have a MEN link. On those ones, the MEN link could be removed as long as the PEC is correctly sized and tests ok.
   

pluto
Oct 27 2019 22:56

If replacing the distribution switchboard, or or wanting to remove the MEN link and the existing submain active and neutral conductors are protected by a overcurrent device of the correct rating. If the protective earth core was too small in accordance with AS/NZS 3000, running of a new correctly sized protective earth conductor is satisfactory, just follow the existing submain route beck to the main switchboard and connect both enda of the new cable to the earth bars of both switchboards and don't fit the MEN link in the distribution switchboard.
   

OctaneOutlaw
Oct 28 2019 07:12

Understood, so it's just that in some cases the PEC is too small and that needs to be accessed before making any changes
   

pluto
Oct 28 2019 09:14

Correct