Must the armoured outer sheath of SWA be earthed?

[AJB Temple]

I have a metal Hager sub CU, 10 way RCBO, installed in 2018 in an accessible attic space, this sub-CU being fed from another unit close to the meter. The two are connected with 10mm 3 core SWA: line, neutral and CPC. At the end close to the meter, the armoured cable metal sheathing is earthed as usual, by being attached with a brass interior use gland to the metal box CU plus a wire going from the gland bolt to the earth bar. The earth wire in the SWA is properly connected at both ends.

At the sub-CU in the loft, the brass gland is bolted to thick plastic trunking and the internal wires go through this, via holes top and bottom, to the CU. However, the brass gland itself only makes contact with the top section of plastic trunking, there is NO connection between the protective wire sheath and either the metal CU or the earth bar inside it.

I would expect the wire sheathing to be attached to earth at both ends? Obviously it is easy to remedy but I would like the opinion please of qualified domestic installation electricians as to whether it is correct NOT to earth both ends of the sheathing or whether this is most likely an oversight or error by the installing electrician. I am not seeking to blame him for anything - I just want to understand.

 

[loz2754]

If not being used as the circuit protective conductor, the armour of a swa cable need only be earthed at one end.
Many would choose to earth the swa armour at both ends though, if practicable.

 

[AJB Temple]

Thanks. Further reading agrees with you. Incidentally, this is not an outbuilding and not a TT system (I know you did not suggest that but I understand why a TT arrangement would be different). The armoured sheathing is not being used as the CPC: the black wire is properly connected to the CU earth blocks at both ends and taped as earth.

It is perfectly practicable to earth the glands at both ends rather than just one in this application.

 

[ImpededLoop]

It is perfectly practicable to earth the glands at both ends rather than just one in this application.

That's correct, but it's also acceptable in this instance to only connect the armouring at the supply end as you have a cpc in the cable. The armour should be earthed, but there's no requirement for it to be a cpc.

 

[AJB Temple]

Thanks Imp. That has put my mind at rest re the installer (whose work looks very good to me). For my own peace of mind I isolated the CU and ran in an earth link from the gland to the CPC bar in the CU. The reason why I queried it was because this CU was always intended to supply some additional (but hitherto uncommissioned circuits) including four or five Quinetic receivers for some LEDs, two loft LED spot lights and some external lighting off of the available but unused RCBOs. The LED units have a CPC but the quinetic units do not, just LN in and LN out, Hence the need to link the CPCs in anyway.

 

[mainline]

I have a metal Hager sub CU, 10 way RCBO, installed in 2018 in an accessible attic space, this sub-CU being fed from another unit close to the meter. The two are connected with 10mm 3 core SWA: line, neutral and CPC. At the end close to the meter, the armoured cable metal sheathing is earthed as usual, by being attached with a brass interior use gland to the metal box CU plus a wire going from the gland bolt to the earth bar. The earth wire in the SWA is properly connected at both ends.

At the sub-CU in the loft, the brass gland is bolted to thick plastic trunking and the internal wires go through this, via holes top and bottom, to the CU. However, the brass gland itself only makes contact with the top section of plastic trunking, there is NO connection between the protective wire sheath and either the metal CU or the earth bar inside it.

I would expect the wire sheathing to be attached to earth at both ends? Obviously it is easy to remedy but I would like the opinion please of qualified domestic installation electricians as to whether it is correct NOT to earth both ends of the sheathing or whether this is most likely an oversight or error by the installing electrician. I am not seeking to blame him for anything - I just want to understand.

Why is it glanded into trunking instead of the CU

 

[mainline]

If not being used as the circuit protective conductor, the armour of a swa cable need only be earthed at one end.
Many would choose to earth the swa armour at both ends though, if practicable.

As long as it's the source end

 

[loz2754]

As long as it's the source end

If not being used as the protective conductor, it only needs earthing as a conductive part. It doesn't matter which end.....

 

[mainline]

If not being used as the protective conductor, it only needs earthing as a conductive part. It doesn't matter which end.....

I do agree, but personally I would earth at source rather than the other way around, difference being you have less chance of losing the earth on the armour, especially if earthed both ends.

 

[loz2754]

I do agree, but personally I would earth at source rather than the other way around, difference being you have less chance of losing the earth on the armour, especially if earthed both ends.

I always do both ends as a matter of course, even if not strictly necessary.
Except for last Friday of course....
I was fitting an EV charge point, and had to separate the two earthing systems.

 

[pc1966]

Friday earthing is always tricky...

 

[loz2754]

Friday earthing is always tricky...

Especially as Friday is normally dedicated to liquid.