S.W.A. Armour Earthing.

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.

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.