S.W.A. Armour Earthing.

Not disagreeing with that argument, just whether you'd be introducing unnecessary risk in this instance, where access is required for testing, and 411.3.1.1 applies regardless of whether you've isolated a supply?

Hmm. some folk clearly worry more than I do, perhaps I'm a bit too gung ho. ?


However,  if the armour is at a dangerous potential relative to the terra-firma earth or the metalwork of the building to which the cable is attached, then surely the whole of the SWA needs over-sleeving or putting into plastic conduit ?

We have just agreed, I think, that the outer plastic is not intended to serve as insulation and should not be relied upon. I'd never propose we use the armour for the 3rd core in a 3 phase system or anything like that, but if we are to treat a CPC as it as if it is live , then perhaps we should pretend  it is the same as if we had.


It is perfectly possible to fit a stuffing gland to an SWA cable in a way that it does not pull out without destroying the gland  and tearing it put  from the box first. Like any gland you do need to select the right sort of gland for the cable, and not some cheap knock off,  and use the right size spanners to the correct torque ( you need a few ft-lbs for the ones that fit a 25mm hole) to do it well.

It is also fair to say that on installations I have seen, there appear to be  plenty of SWA glands fitted in a way that do not fare as well as I'd like in a decent pull test. That may be partly a lack of technique or maybe some makes of glands are better at gripping the cable than others. Certainly some of the SWA designs that only grip the armour and have no rubber gland compression part are pretty poor.


In summary, I agree the GK solution is perfectly fine, if erring on the over engineered, but am not really convinced my approach is especially unsafe.

Mike

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 we all agreed earlier in the thread, that if you can't touch the armour it was OK (inside sheath), but where you can, it should be either enclosed or insulated.


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.


I'd argue that any "incident" that may occur with accidental contact with the armour under certain conditions is reasonably foreseeable, and is reasonably practicable to prevent.


As a designer under CDM risk assessment, the decision is, of course, yours.

• Long runs of cable down railway or highway infrastructure probably present a greater risk, and definitely should be addressed in the way I illustrated (or similar)

• If we had a situation where, for example, connection to PME was seen as a risk, then that risk must still be present for the electrician working in the enclosure, however small - I guess it's up to the designer to determine the likelihood?

I am amazed we still have people arguing that the armour should not be Earthed. No one has made any case that it should not be in terms which I can understand. The special case of an SWA which is TN changing to a TT location just needs some care at installation time and should be immediately obvious to any instructed or competent person. Persons trying to do electrical installation work who are not competent are accepting the risk themselves, it is not your job to try to make their actions safe. Underlying this is the measure of what is "competent"? The likelihood of an incompetent person trying to do inspection and test should be zero, the fact that it is not, is not  my or your problem, the need for competence is clear in the Electricity at work regulations and BS7671.


Earthing exposed conductive parts is inherent in BS7671, and the idea of conductive containment which is somehow insulated (perhaps by paint) has never crossed my mind as satisfactory. There is no requirement that armoured cable is actually insulated at all, so we must assume that it is not. Clearly SWA glands are not, and boots are not fixed, and mechanical damage to the sheath is quite likely (that's why we use SWA) so we must assume it is not insulated. If pressed in Court, I am sure my statement would overcome the rest of the excuses which are being made. Lack of Earth to the Armour must be a code C2. It seems simple enough to me. It will always be Earthed with a 4 core 3 phase cable, as the CPC, why should a single-phase domestic be different?

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.

There seems to be no opposition to the competence argument then? During my life I have been exposed to many potentially fatal electrical and RF situations. So far they have failed to kill me in any meaningful way. Thus I think it fair to suggest that competence is a solution to all of the "problems" being discussed above. Simply make sure a tool is required to get at the potential danger. Job done.


A slight aside. Bristol is one City where the "electric scooters for hire" are being tried out on an experimental basis. If you want an accident waiting to happen (and quite a few have) this is the place to look. I was under the impression that a driving license was required to use one, but judging by the age of many users it is being ignored. The standard of roadcraft demonstrated is exceptionally bad, and driving a car or van is now a constant danger of watching out for even more idiots than before. The roads no longer have room for multiple wheeled vehicles, in many places, the lanes are too narrow having been taken for bus lanes, cycle lanes, bus stops,  and any other prevention of driving known to man. Do not come here, much of the traffic is stationary, the latest "anti-pollution" measure! They have also invented something called a "Bus Gate", which is unknown elsewhere but distributes fines at a very high rate. It is intended to be a bus-only entrance to parts of the City centre, (bus being: taxi, bicycle, motor bike, electric scooter etc). The is nothing except a road marking yards into the area to warn you of this wonderful new feature! They just want your money of course.

"Bus Gate", which is unknown elsewhere

Not so - they are quite common in several of the city centres around here and have been for several decades now.

  - Andy.

It seems the best way for towns and cities to combat traffic congestion and pollution is to make it as difficult and expensive as they can for the motorist thus dissuading many to make the attempt. With the rising costs of entry zone permits, parking, wasted time sitting in traffic and locating said parking space, risk of theft from and of vehicle it's a wonder why anyone does it. For a tradesman it's a total nightmare! 

We should boycott those towns and cities until we can get workers parking permits and clean zone dispensations (not many electric vans available!).