S.W.A. Armour Earthing.

Does plastic conduit provide insulation ... or is it there for mechanical protection?

Does the sheath of a cable provide insulation ... or is it there for mechanical protection?

I think of it this way - if the plastic conduit (or sheath) was there for purely mechanical protection, then I could replace it with anything thing else that was mechanically equivalent or superior with no additional precautions being necessary. Thus (in a situation where PVC conduit was suitable) I could directly replace PVC conduit with steel conduit - without having to Earth it - since the mechanical protection to the basic insulation would be just as good, if not better. Of course we don't accept that - thus there must be some claim on the insulating properties of the plastic conduit.


(If you want a theoretically difficult one, imagine a semiconducting conduit  (perhaps some new carbon nanotube kind think with impressive physical properties) but its semiconducting nature meant that while a potentially fatal current could flow through the small wall thickness, the resistance of a long length would make ADS impractical if earthed at the end.)

 

The situation still remains that, if you don't insulate the armour in the enclosure, it's not "finger-proof" from the perspective of accidental contact by a skilled or instructed person on opening the enclosure.

As is often the case with live conductors within enclosures - which sometimes have to be exposed when live for testing purposes by skilled persons. If we have a problem with a PME'd gland within a robust enclosure within a TT environment, what on earth should we be suggesting for a PME'd Class I item outdoors (e.g. outside light) where it's likely to be within reach of all sorts of bits of metal stuck into the ground (fences, gates, decorative ironwork) not to mention the ground itself.


Also looking at Graham's diagram again, I suspect it's not going to work physically - the thread length on most SWA glands is only a little longer than the depth of a typical earthing nut - enough to go through the thin wall of a plastic enclosure but not much more (similarly I've sometimes had problems with some glands not properly reaching into the brass earthing bar on Wiska boxes due to the limited amount of thread on the brass gland) - with the earthing nut on the end of the gland there's only be a few mm of thread left - not enough to make a physically robust connection to the 'gapping adaptor' (which I guess is something like a length of PVC conduit with a female adaptor on the end).


  - Andy.

Does plastic conduit provide insulation ... or is it there for mechanical protection?

Does the sheath of a cable provide insulation ... or is it there for mechanical protection?

I think of it this way - if the plastic conduit (or sheath) was there for purely mechanical protection, then I could replace it with anything thing else that was mechanically equivalent or superior with no additional precautions being necessary. Thus (in a situation where PVC conduit was suitable) I could directly replace PVC conduit with steel conduit - without having to Earth it - since the mechanical protection to the basic insulation would be just as good, if not better. Of course we don't accept that - thus there must be some claim on the insulating properties of the plastic conduit.


(If you want a theoretically difficult one, imagine a semiconducting conduit  (perhaps some new carbon nanotube kind think with impressive physical properties) but its semiconducting nature meant that while a potentially fatal current could flow through the small wall thickness, the resistance of a long length would make ADS impractical if earthed at the end.)

 

The situation still remains that, if you don't insulate the armour in the enclosure, it's not "finger-proof" from the perspective of accidental contact by a skilled or instructed person on opening the enclosure.

As is often the case with live conductors within enclosures - which sometimes have to be exposed when live for testing purposes by skilled persons. If we have a problem with a PME'd gland within a robust enclosure within a TT environment, what on earth should we be suggesting for a PME'd Class I item outdoors (e.g. outside light) where it's likely to be within reach of all sorts of bits of metal stuck into the ground (fences, gates, decorative ironwork) not to mention the ground itself.


Also looking at Graham's diagram again, I suspect it's not going to work physically - the thread length on most SWA glands is only a little longer than the depth of a typical earthing nut - enough to go through the thin wall of a plastic enclosure but not much more (similarly I've sometimes had problems with some glands not properly reaching into the brass earthing bar on Wiska boxes due to the limited amount of thread on the brass gland) - with the earthing nut on the end of the gland there's only be a few mm of thread left - not enough to make a physically robust connection to the 'gapping adaptor' (which I guess is something like a length of PVC conduit with a female adaptor on the end).


  - Andy.