S.W.A. Armour Earthing.

mapj1:
... and potentially put electricians at risk?


I'm now wondering in what way I've put an electrician at risk, and of what?  (any more than say having live terminals like line taps  in a plastic box would do.) What failure mode am I missing?

M.

Two simultaneously-accessible "earth" potentials.

"just not exposed to touch when the box is shut."

Is the armour inside the box that's exposed to the poor electrician IP2X, for example? What happens when the cable moves a little? There's no "restraint" from a stuffing gland as we all know from experience. And then of course, we're not just talking about the electrician either.

The simultaneously-accessible exposed-conductive-parts of different earthing systems is prohibited by BS 7671 ... as I said covered by practices on UK railway and similar infrastructure ...

Zoomup:

Which regulations(s) require the steel wire armouring of a S.W.A. cable to be earthed if it is NOT used as a circuit protective conductor?


Case 1. Cable buried underground.


Case 2. Cable NOT buried underground.


Z.

Case 1: covered by a specific Reg 522.8.10 as we've seen, so always required.


Case 2: if the cable carries anything other than SELV or PELV, then always required because it's an extraneous-conductive-part, and double insulation is not "assumed to exist". So take your pick but 411.3.1.1 pretty much, but the conditions for "double insulation" don't exist if you thought you had that.

Zoomup:

Which regulations(s) require the steel wire armouring of a S.W.A. cable to be earthed if it is NOT used as a circuit protective conductor?


Case 1. Cable buried underground.


Case 2. Cable NOT buried underground.


Z.

Case 1: covered by a specific Reg 522.8.10  so always required if buried direct without any mechanical protection 


Case 2: if the cable carries anything other than SELV or PELV, then always required because it's a reasonable assumption that the wire armour could become live under fault conditions since double insulation is not "assumed to exist". Unfortunately I can’t find a regulation to support that other than 411.3.1.1 if the armour can be touched in some way, otherwise it’s not an exposed conductive part in terms of the strict definition in Part 2. However, for what it is worth,  reference regulation or not, I wouldn’t dream of not earthing the wire armour in a low voltage cable!

Two simultaneously-accessible "earth" potentials. "just not exposed to touch when the box is shut."

AJJewsbury:

Two simultaneously-accessible "earth" potentials. "just not exposed to touch when the box is shut."

Inside an enclosure is perfectly OK for bare hazardous live conductors - are you really suggesting we need to go further for differing Earthed conductors?

  - Andy.

Yes it is, from an "ordinary persons" perspective.


However, I believe you pointed out that the armour was required to be accessible for testing purposes. So, what's your CDM risk assessment for a safe environment for maintenance (including inspection and testing) and decommissioning? BS 7671 says IP2X/IPXX/D ... are you suggesting a lesser requirement is acceptable?


Since it's reasonably practicable to take this into account, why put those carrying out inspection and testing at unnecessary risk?

AJJewsbury:

Nice diagram! Screw connection between G/Y and earthing nut doesn't seem to be accessible for inspection & maintenance though (526.3).


I can't help thinking it would be simpler just to gland into an insulating box - boot prevents the gland being accessible outside (and a cable tie to prevent the boot being removed without a tool if you prefer) - gland accessible for testing from inside the box after lid removed (using a tool).


I see we're relying on the sheath of the SWA to provide insulation of the TN earthed armour within the TT environment....


  - Andy.

Well, this is the debate ... which is correct??


If we accept that the armour is not accessible because of the sheath, there is no issue with 411.3.1.1.


I can verify the screw connection by testing continuity, so that seems like the lesser of the two evils ... certainly, I'd back that in CDM risk assessment, arguing it's available for inspection and testing if you really need it to be (but you may need to replace the cold-shrink sleeving)?

BS 7671 says IP2X/IPXX/D