S.W.A. Armour Earthing.

gkenyon:

Zoomup:

 

In my case 2, the high level run S.W.A. entered an all insulated enclosure unglanded and not connected to earth at both ends. The three core S.W.A. was connected as L., N. and C.P.C. 


Z.

In this case, an undetected fault to armour might leave the armour "live" and wouldn't necessarily be detected by the tests prescribed in BS 7671 (insulation resistance test to cpc or earth won't pick it up) - you'd specifically need to test to armour. What if the outer sheath becomes damaged at some point?


Thinking about the permutations logically, I'm tending towards it being negligent, in most cases, to avoid earthing armour of a cable operating at low voltage, unless some other precautions are taken.

Indeed, I noted the "deficiency" and am not entirely happy with the situation. It was installed on the cheap by a sports' club member. I was keen to learn of regulations that apply if I have to defend my position of criticising the installation.


Z.

Zoomup:

Indeed, I noted the "deficiency" and am not entirely happy with the situation. It was installed on the cheap by a sports' club member. I was keen to learn of regulations that apply if I have to defend my position of criticising the installation.

Naturally, we know better, but I have to say that it is not obvious to a lay person that the armour should be earthed, or indeed how to do it. The word, "armoured" simply suggests a means of mechanical protection for the inner cable.

lyledunn:

It could be argued that the special case alluded to in 522.8.10 implies that for other situations earthing the wire armour may not be necessary.


 

I think that's a long stretch of the imagination. The special case alluded to in 522.8.10 is a means of burying non-armoured cables, although in that case, the duct or similar has to offer equivalent mechanical protection to the armour. The only argument about that requirement, is whether the duct still has to be metallic and capable of acting as a cpc.


I don't think you need to earth armour all the time where 522.8.10 doesn't apply ... but to leave it without either earth or permanent insulation where the conductors are at hazardous voltages is negligent.


There are cases where you don't have to earth an armour of a cable for it to be safe ... but if you take it to the extreme of a cable with no other electrical conductors - such as a fibre cable with steel armour and internal steel catenary - do you need to earth that? Well, perhaps not on short runs, but if the cable travels a long distance, you may want to because otherwise it will be floating all over the place - induced voltage, static, etc., all provide unknowns.


We often have posts about voltages on unused cores in cables ... common practice is to connect to earth at least at one end. What's different with armour?


BS 7671 doesn't say that metallic containment must be earthed in explicit terms ... it's just very easy to see that it's an exposed-conductive-part and Regulation 411.3.1.1 applies . I don't see armour of an LV power cable as being any different.


Overall, I think this is one for good practice and guidance.

Chris Pearson:

Zoomup:

Indeed, I noted the "deficiency" and am not entirely happy with the situation. It was installed on the cheap by a sports' club member. I was keen to learn of regulations that apply if I have to defend my position of criticising the installation.

Naturally, we know better, but I have to say that it is not obvious to a lay person that the armour should be earthed, or indeed how to do it. The word, "armoured" simply suggests a means of mechanical protection for the inner cable.

Page 39 of latest (2018) GN3, item (o) of the "non-flexible cables" list refers the reader to Regulation 411.3.1.1

(o) Metal sheaths and armour earthed (411.3.1.1)

Zoomup:

Indeed, I noted the "deficiency" and am not entirely happy with the situation. It was installed on the cheap by a sports' club member. I was keen to learn of regulations that apply if I have to defend my position of criticising the installation.


Z.

As previous post - page 39 of GN3

gkenyon:

Zoomup:

Indeed, I noted the "deficiency" and am not entirely happy with the situation. It was installed on the cheap by a sports' club member. I was keen to learn of regulations that apply if I have to defend my position of criticising the installation.


Z.

As previous post - page 39 of GN3

With respect, that's not the point. Z was looking for a reference in BS7671 2018 itself. To invoke 411.3.1.1 one would need to accept that the armour is an exposed conductive part and the definition of exposed conductive part in part 2 would not support that determination. 

To avoid imagination stretch the normal practice would be to employ explicitness, which is clearly absent in BS7671 with respect to the issue of earthing the armour, other than for certain underground applications. 

Interestingly, in 522.8.10 of IS10101:2020 there is no explicit mention for the need to have the wire armour earthed in underground applications. However, 526.5.1 states "Adequate electrical conductance shall be provided between metal sheaths or armouring of cables and the earthing terminals of equipment. This requires proper design and a proprietary method"........."A cable gland is a proprietary method"

lyledunn:

gkenyon:

Zoomup:

Indeed, I noted the "deficiency" and am not entirely happy with the situation. It was installed on the cheap by a sports' club member. I was keen to learn of regulations that apply if I have to defend my position of criticising the installation.


Z.

As previous post - page 39 of GN3

With respect, that's not the point. Z was looking for a reference in BS7671 2018 itself. To invoke 411.3.1.1 one would need to accept that the armour is an exposed conductive part and the definition of exposed conductive part in part 2 would not support that determination. 

To avoid imagination stretch the normal practice would be to employ explicitness, which is clearly absent in BS7671 with respect to the issue of earthing the armour, other than for certain underground applications. 

Interestingly, in 522.8.10 of IS10101:2020 there is no explicit mention for the need to have the wire armour earthed in underground applications. However, 526.5.1 states "Adequate electrical conductance shall be provided between metal sheaths or armouring of cables and the earthing terminals of equipment. This requires proper design and a proprietary method"........."A cable gland is a proprietary method"

With respect also, I don't agree with you that the definition of exposed-conductive-part in BS 7671 'doesn't support that determination'

Exposed-conductive-part. Conductive part of equipment which can be touched and which is not normally live, but which can become live under fault conditions.

A cable is (electrical) equipment as defined in BS 7671.


The armour is a conductive part. It is not normally live.


As I've said, unless it is covered by insulation that can only be removed by destruction, it can be touched. The circumstances may change as to when it can be touched, but an electrician would not expect armour to be a live part ... CDM designer's duties come in here, for all installations including domestic.


BS 7671 isn't a manual, and it's been in the published guidance for some time.

 

gkenyon:

lyledunn:

gkenyon:

Zoomup:

Indeed, I noted the "deficiency" and am not entirely happy with the situation. It was installed on the cheap by a sports' club member. I was keen to learn of regulations that apply if I have to defend my position of criticising the installation.


Z.

As previous post - page 39 of GN3

With respect, that's not the point. Z was looking for a reference in BS7671 2018 itself. To invoke 411.3.1.1 one would need to accept that the armour is an exposed conductive part and the definition of exposed conductive part in part 2 would not support that determination. 

To avoid imagination stretch the normal practice would be to employ explicitness, which is clearly absent in BS7671 with respect to the issue of earthing the armour, other than for certain underground applications. 

Interestingly, in 522.8.10 of IS10101:2020 there is no explicit mention for the need to have the wire armour earthed in underground applications. However, 526.5.1 states "Adequate electrical conductance shall be provided between metal sheaths or armouring of cables and the earthing terminals of equipment. This requires proper design and a proprietary method"........."A cable gland is a proprietary method"

With respect also, I don't agree with you that the definition of exposed-conductive-part in BS 7671 'doesn't support that determination'

Exposed-conductive-part. Conductive part of equipment which can be touched and which is not normally live, but which can become live under fault conditions.

A cable is (electrical) equipment as defined in BS 7671.


The armour is a conductive part. It is not normally live.


As I've said, unless it is covered by insulation that can only be removed by destruction, it can be touched. The circumstances may change as to when it can be touched, but an electrician would not expect armour to be a live part ... CDM designer's duties come in here, for all installations including domestic.


BS 7671 isn't a manual, and it's been in the published guidance for some time.

I disagree. A steel wire armouring which is part of a S.W.A. cable can NOT be touched as it is protected by a heavy duty P.V.C. oversheath. Therefore it is NOT an exposed conductive part. It simply is NOT exposed.


If the cable in my case 2 has a copper core used as a C.P.C., then the regs. appear not to require the armouring to be earthed.



Z.

I disagree. A steel wire armouring which is part of a S.W.A. cable can NOT be touched as it is protected by a heavy duty P.V.C. oversheath. Therefore it is NOT an exposed conductive part. It simply is NOT exposed.


If the cable in my case 2 has a copper core used as a C.P.C., then the regs. appear not to require the armouring to be earthed.



Z.

Well I guess that would come down to how the SWA was terminated - if the armour is cut back to the same line as the sheath, and oversleeved  with insulating sleeving that cannot be removed except by destruction at the end of the sheath, and the cable terminated on a simple plastic compression gland - the same at both ends then I would agree it can not be touched.  


Jason.