Armoured Cable Query

[ban-all-sheds]

 

[PBC_1966]

From what we've been told, the Americans appear to have worked this out already - by demanding (rather than 'forbidding') an earth electrode to be connected to the incoming TN-C-S earth

And in requiring that the TN-C-S earth be "exported" to an outbuilding and an additional earth electrode provided at said outbuilding. There just isn't the same range of earthing options for the main building and auxiliary buildings as exists in the U.K., although until very recently there was one variation for feeding an outbuilding which would not be acceptable in Britain.

 

[EFLImpudence]

I'm only keeping on about that because you, John, are saying that connecting the electrode to TN-C-S earth makes no difference.

It could.


If all outbuildings were wired in large csa earthing conductors (as it seems in the US if I have read correctly) then it would be alright - but they aren't.

 

[JohnW2]

<a diagram>

That diagram is misleading, since no-one has suggested that the TN-C-S earth should be literally connected to the earth electrode, so that it's path to the (other) extraneous-c-ps went though the electrode's conductor. The actual situation (and 'choice/decision') I'm talking about is:

The conductor from the TN-C-S earth to the 'real' extraneous-c-pc in the outbuilding either is, or is not, of adequate CSA as a boinding conductor for those parts. Either way, whether one also connects the local earth electrode to the 'local MET' is, AFAICS, neither here nor there.

Kind Regards, John

 

[ban-all-sheds]

until very recently there was one variation for feeding an outbuilding which would not be acceptable in Britain.

CNE on the consumer's side?

 

[EFLImpudence]

I do not see the difference in the diagrams.

The conductor from the TN-C-S earth to the 'real' extraneous-c-pc in the outbuilding either is, or is not, of adequate CSA as a boinding conductor for those parts.

The situation I have been taking about is when the csa is inadequate - the reason for TTing it.

Either way, whether one also connects the local earth electrode to the 'local MET' is, AFAICS, neither here nor there.

I'm lost.

 

[JohnW2]

I do not see the difference in the diagrams.

There is a significant difference since, as I said, your diagram had path from the incoming TN-C-S earth to the (other) parts it was meant to be bonding went via the earth rods conductor (which, as I said, electrically speaking could be as small as 1mm²).

The conductor from the TN-C-S earth to the 'real' extraneous-c-pc in the outbuilding either is, or is not, of adequate CSA as a boinding conductor for those parts.

The situation I have been taking about is when the csa is inadequate - the reason for TTing it.

If the CSA of the connection to the (very low impedance) extraneous CPs in the outbuilding were inadequate, one would have to abandon it and TT the outbuilding instead of using the TN-C-S earth at all, since there would otherwise be a theoretical risk of that cable becoming seriously overloaded as a a result of (extremely rare) very high fault currents. Adding a TT electrode as well as retaining use of of the TN-C-S would not significantly (virtually not at all) reduce that risk.

Either way, whether one also connects the local earth electrode to the 'local MET' is, AFAICS, neither here nor there.

I'm lost.

As above, to use an exported TN-C-S earth with an inadequate CSA conductor to bond 'very low impedance' extraneous-c-ps is theoretically 'not safe', and adding a connection to a local earth rod would make that situation neither more safe nor less safe (aka "neither here nor there").

Kind Regards, John

 

[PBC_1966]

CNE on the consumer's side?

Yes, combined on the feeder to the outbuilding, where the neutral and ground busbars at the outbuilding's distribution panel are bonded together and to extraneous parts as usual, and connected to the required local earth rod.

The arrangement was permitted only if no other metallic path existed between the buildings, and only where the feeder at the main building originated from the main distribution panel, not from a sub-panel (since the neutral & ground at the main panel are bonded together, but not at a sub-panel).
I'm not certain of the exact date this arrangement was removed from the NEC, but it's quite recent, perhaps 2008 or even later.

The other combined neutral/ground arrangement which used to be permitted was specifically for an electric range or clothes dryer, where the frame could be grounded to the neutral. The appliances generally have a jumper provided at the terminal block to permit the appropriate arrangement for connecting a 3- or 4-wire cord. Again, the 3-wire hookup was permitted only where the range or dryer circuit originated from the main panel, not from a sub-panel. It was removed as a permissible option from the NEC in 1996.