Earth rings on SWA cables

[chocca]

Hope this is in the right place...

Our firm recently tested a 20 year old office installation and one of the flagged items is the lack of earth rings on the swa cables feeding the under floor tracks. The cables are glanded into the metal D/B. Earth for the sockets is provided by a third core in the armoured so the glanding provides the bond for the armouring only.

Are earth rings required?

 

[somersetsparks]

Not really I would say providing locknuts are tight and the paint beneath them scratched off, Ideallythey would of used serrated washers. At worst, if there was good continuity from the armour you could give it a c3,
If the paint is scratched off and the locknuts tight, then NO code, think conduit and MI.

 

[Plonker 3]

Personally I would say as long as one end of each SWA is earthed then it is fine just to guarantee that the armourings are earthed, so fitting "Banjo's" should be done weather it is a C2 or C3 would depend on several factors.

As Somerset says if they are tightly terminated and no paint causing a continuity problem then C3 otherwise C2.

 

[somersetsparks]

I don't think there is any specific requirement for banjos to be fitted in bs7671, many large jobs specify serrated washers, no banjos, Therefore I would not code unless there was an earth continuity problem. Nobody ever banjos conduit or MI.

 

[Plonker 3]

I don't think there is any specific requirement for banjos to be fitted in bs7671, many large jobs specify serrated washers, no banjos, Therefore I would not code unless there was an earth continuity problem. Nobody ever banjos conduit or MI.

MI is rarely used these days unfortunately.

On tray, conduit etc it should all be bonded and again continuity too the MET should be proved.

Maybe I was a little harsh on my initial assessment, but in certain circumstances it should be i.e. buried underground on walls etc even if the armor isn't used as a C.P.C.

 

[somersetsparks]

I think the key is the soundness of the connection, really I would like to see either a banjo or a serrated washer. I would much rather see a scratchy washer and paint removed then the usual 'banjo on top of paint!' that is pretty common!

 

[Guitarist]

As you point out, the earthing is done via a cable core, so the swa glanding is to earth the steel only. Have to agree with what the others have said, not ideal but as long as there is a good connection then no problem.

 

[chocca]

Thanks for confirming my thoughts. There are times when its easy to talk yourself into doing more than necessary just cos you overthink the job...
Once again, thanks for the replies.

 

[madseb]

im a bit confused about the inital question?
i have to ask, as you have pointed out the earth return path is supplied by the third core?

so are the earthing rings only providing protection should the steel armour become live? (obviously and absence of earthing banjos)
in which case as a precaution i would check as above, that serated rings are used. and provide a suitable sized fly lead from the steel box.
this would insure a fault free earth path.
and should be easily changeable from the existing installation.

 

[Guest55]

Although not strictly required i'd consider it good practice to banjo & earth lead at least at the DB end of the swa.
But at worst its only a code 3 defect.

 

[madseb]

have to agree with the above. ive allways, used banjo rings regardless. as a precaution. be sure to sand/file back the paint on the boxes aswell. for a good connection.

 

[jamie-spark]

MI is rarely used these days unfortunately.

On tray,( conduit etc it should all be bonded and again continuity too the MET should be proved.

Maybe I was a little harsh on my initial assessment, but in certain circumstances it should be i.e. buried underground on walls etc even if the armor isn't used as a C.P.C.

Surely Tray only requires bonding if its extraneous and earthing only if it falls under exposed conuctive part which it generally is not

 

[Plonker 3]

Well seeing as most tray work will be carrying cables and could become live, I would bond all exposed electrical containment together, otherwise you wouldn't need to bother getting couplers tight on it along with trunking or bother threading galv conduit properly would you?

 

[jamie-spark]

There is only a requirment if it falls into the 2 i have listed if not then does not reqire to be bonded. look at gn8 earthing and bonding it states this too. Earthing stuff that does not require it can infact make it more dangerous.

 

[Plonker 3]

So you are happy leaving metal work that is carrying cables not to be bonded back to the MET?

Explain how that is dangerous and not a safety connection then please?