Armoured cables

[madregulator]

I have come across, on more than one occasion, where thesteel wire armour of an armoured cable is not correctly glanded so that thearmour is not earthed. This is where there is an earth core within the cable.
I have recently spoke to an electrician who maintains the armour is notrequired to be earthed if there is an earth core within, because the armouronly serves as mechanical protection.
However, I maintain that regardless of an earth conductor contained within, thearmour should be earthed because it is used for mechanical protection, andtherefore if the cable sustains mechanical damage, and after all that’s why isinstalled. If the mechanical impact forced the steel wire into the cable, andthey made contact with a live conductor (this could be a three phase cable),the armouring throughout would become live. And if the armouring was earthedthe circuit protection would operate on short circuit. If the armour is notearthed no protection would operate.
Further if the damage to the cable was someone with a metal hacksaw in hishand, well we can only guess on the outcome.

My problem is I am not familiar with a specific Reg that states the armour mustbe earthed, only where it refers where the armour is the main cpc. I don’tthink it would come under extraneous metal! The redundant Mines & Quarries Regs did have a specific Reg which stated all metal outer protective covering of cables must be earthed.
Is anyone more familiar with such a reg, I would be grateful.

 

[snowhead]

Yes the Armour should be earthered at least at one end for your given reasons, don't ask me for the reg tho'

 

[derek preece]

So would he suggest that trunking and metal conduit doesn't need earthing? He is talking crap- I'm not sure of the reg(s) but it requires correctly terminating, preferably at both ends and at the very least at the supply/protective device side.

 

[Silly Sausage]

I consider the armour of a SWA to be an 'Exposed conductive part' (see definition), so should be earthed, see 411.3.1.1

 

[Deleted member 26818]

510.3 "Every item of equipment shall be selected and erected so as to allow compliance with the reulations stated in this chapter and the relevant regulations in other parts of BS7671 and shall take account of the manufacturers' instructions."

 

[madregulator]

Many thanks for your contribution gents. Thank goodness the vast majority of us got some common sense. My default setting when I am not able to recollect a specific regulation is to carry out good, sound engineering standards, and that should automatically comply with some regulation somewhere !

 

[Dean Williams]

i found the same thing last night oddly enough. SWAs go into DB at top with no gland whatsoever, no contionuity to earth on armour but there was a phantom induced voltage.

 

[telectrix]

the armour is an exposed conductive part of the installation, the same as a metal faceplate of a socket or switch and therefore must be earthed.

 

[nickblake]

So realistically if you install an armoured cable not bother with earthing the armouring because one of the cores is the earth , then why use armour , 3 core flex would do , this is incorrect external influences comes into consideration the armour is there fo a reason that is protection of the cores so always earth the armour

 

[Darkwood]

If an armoured has a seperate independant earth core then and it meets requirements for the earthing needs then the Armour is not required to be earthed at both ends but must be still earthed as it is a conductive mechanical protection, if one end is earthed in this situation then it should be the supply end and not the load end, having said this i cant see any reason why the cable if using a seperate earth core cant be glanded correctly at both ends anyway, its a lazy attitude and approach to implement this in design when there is nothing stopping the correct glanding of both ends.

Ive done this when wiring radial spurs or sockets in swa etc where the design of the spur/ socket dosn't allow room for the banjo and the earthed/locknuts are too bulky to allow you to fit the socket or spur to the backbox, so spare core as earth and swa correctly terminated and earth at supply end.

 

[Farmelectrics]

The 5w wonder is talking rubbish as said above the armour requires earthing in all cases maybe he can't make a gland off

 

[lauriehurman]

As I understand it (I may be wrong) there are circumstances in which earthing (actually bonding) both ends of the armour would create a loop if there is a CPC in the cable and is not recommended.

Laurie

In fact Fig 44.9 in 444.4.6 appears to show this.

 

[Engineer54]

As I understand it (I may be wrong) there are circumstances in which earthing (actually bonding) both ends of the armour would create a loop if there is a CPC in the cable and is not recommended.

Laurie

In fact Fig 44.9 in 444.4.6 appears to show this.

Think your talking about single core cables here with a metallic sheath, yes??

 

[Darkwood]

Think he's confusing some situations where screening is only connected at one end to stop circulating eddie current that can be generated which can interfere with the operation of the circuits, usually a requirement on some control systems especially delicate measuring devices. This is not an issue with the content of this thread.

 

[loop3]

Earth at the supply end at least. Because it would be considered an exposed conductive part. Imagine if the plastic or shroud can off? Ok induced voltage might not be enough to sit you on your ---. Madreg, did you calculate to see if the steel is good enough to use as an earth?