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Posted By Topic: switch board upgrading

bluesky
Nov 12 2019 17:04

Hi all.
I m upgrading old style rewirable fused s/b .

There is one meter box with 4 meters that supply 4 unit houses 40 years over
Sb in a unit has men linked.
mec from meter box to sw is 4mmm2.

Which earthing arrangement need to be?

See a photo

Thanks.



.


   

DougP
Nov 12 2019 18:21

The simple answer is, that if the "meter box" is the main switchboard, then the wiring to each unit is a submain and the boards shouldn't have a MEN link. They would have had one originally of course, as that was how it was done back then.

For the meter box to be the main switchboard, the (only 1) main earth would have to connect at that main switchboard. There would also have to be a main switch, and submain protective devices, and main neutral bar & MEN link of course.

If the submain size is 10mm2, then the 4mm2 earth is ok. As per table 5.1.

Can't really read your picture. You could use a different image site like below.


http://www.iforce.co.nz/
   

DougP
Nov 12 2019 22:36

As I said above. If you're replacing the swbd in D, then you won't fit a MEN link in that board.

Obviously you need to check that there is a good main earth, and the mains cable size is suitable for the 4mm2 earth to that board (it's a PEC, not MEC). And check the submain protection etc.

And of course carry out all the required testing.
   

bluesky
Nov 12 2019 23:10

Doug,
Thank you for your answer.
Any problem with #3 arrangement ?
   

DougP
Nov 12 2019 23:54

If what you are calling the "meter box" is the main switchboard (I suspect it is because you're showing an earth electrode there), then you wouldn't have another earth electrode and MEN link at a distribution board unless it was a PEN submain to a separate building.

And you certainly wouldn't do that in addition to the 4mm2 earth, because that earth conductor would be a parallel path with the neutral, and therefor carrying current.