SWA cables and banjos!

[ewatty]

Hi all, quick question on people's thoughts....

Someones installed a swa for a lift/ramp in a garage but not used any banjos either end.

its a 5 core cable (brown black gray g+y blue) it's glanded to a metal db on end, and a plastic isolator the other, now should it have banjos and fly leads both ends or just the plastic end or not at all as the sheath is connected via the gland at the metal db? Also as its 3-phase should it be phased marked as in brown marking on each live wire? Or just left brown black gray?

bit of a gray area in my head...

Any help? Cheers

 

[pete.willerton]

Sounds like the armour is only for mech protection so no need for banjo's and regarding the cable colour it should be brown-black-grey not all brown

 

[Pete999]

Surely the armour must be earthed correctly so a banjo at the metal DB end is essential or am I being to pedantic??
Pete999

 

[Darkwood]

You should ensure the swa is correctly glanded and earthed at the supply end... as you have a separate earth core their is no need to banjo the load end but its considered good practice as it helps reduce Zs values.

 

[pete.willerton]

I only would if I was using the armour as the earth but as its not I don't see the point .
It's the same as if the electrician used metal conduit . You wouldn't put a banjo around the bush?

 

[Guest111]

Surely the armour must be earthed correctly so a banjo at the metal DB end is essential or am I being to pedantic??
Pete999

Agreed,in the event of the cable becoming damaged the armour could become live so should be earthed,however if they have cleaned the metal below the locknut they may have a good contact,I personally prefer a banjo though.

 

[Darkwood]

I only would if I was using the armour as the earth but as its not I don't see the point .
It's the same as if the electrician used metal conduit . You wouldn't put a banjo around the bush?

Older regulations did not require earth tailing of the swa gland in metal-clad boards and enclosures but for various reasons this is now a requirement in the BS7671.... regardless of whether the OP has a separate earth core or not the armour still provides a protective function and must be able to carry the full fault current if say the cable is damaged, il re-iterate then what i said earlier - The OP will need to gland and earth tail the supply end of the swa regardless of the facts of separate earth core or metal enclosure or not... its only the load end that he doesn't need an earth tail but as ive already said its good practice to do so but not essential when using separate earth core.

 

[pete.willerton]

Fair enuf. I can understand you point

 

[Darkwood]

Fair enuf. I can understand you point

You mentioned conduit too... to understand if required then you need to know why the regulation exists... most boards have a glanding plate along with Mechanically folded construct usually screwed/nut&bolt or spot welded together... as all these components can lead to resistive points if loose or damaged its thus now required to earth tail direct from the outgoing mechanical protected trunking/swa/conduit....

If you bolt a trunking onto the metal clad DB then an earth tail will still be required to link the fixing point or separate earth stud on the trunking.

If you have a conduit then the same applies and yes a banjo may be necessary if you dont use a Earthing Nut... ill agree its not often seen on conduit but in a new installation it should be although alternatively if separate earthing is run in the conduit and earth down to a conduit box this will suffice as long as it can handle the expected fault current of the largest circuit in the conduit.

If all the above is earthed correctly then this leaves only 1 possible weakness which is the tightness of the gland or bush itself as oppose to several areas where a issue could arise... If you run trunking where it provides the sole earth path for any circuits within its the case that all the joints within the trunking run will require earth straps across for the very same reason we have to at the dist' board, conduit however is mechanically screwed together giving a good secure and structurally sound joint so strapping across joints not required.

 

[ewatty]

thank you all for clearing trhis up!

 

[cobrauk]

Older regulations did not require earth tailing of the swa gland in metal-clad boards and enclosures but for various reasons this is now a requirement in the BS7671.... regardless of whether the OP has a separate earth core or not the armour still provides a protective function and must be able to carry the full fault current if say the cable is damaged, il re-iterate then what i said earlier - The OP will need to gland and earth tail the supply end of the swa regardless of the facts of separate earth core or metal enclosure or not... its only the load end that he doesn't need an earth tail but as ive already said its good practice to do so but not essential when using separate earth core.

Please post where it says we have to use the banjo as I do not remember seeing it anywhere ta


I always make sure that the paint is removed before glanding an swa or installing conduit as most boards are painted using an non conductive paint so it has to be done.

 

[Darkwood]

Please post where it says we have to use the banjo as I do not remember seeing it anywhere ta


I always make sure that the paint is removed before glanding an swa or installing conduit as most boards are painted using an non conductive paint so it has to be done.

Reg' 543.2.7 is a good start but it does actually depend on your interpretation of it...


"Where the protective conductor is formed by conduit, trunking, ducting or the metal sheath and/or armour of a cable, the earthing terminal of each accessory shall be connected by a separate protective conductor to an earthing terminal incorporated in the associated box or other enclosure"

Now if it has to be applied where you have the accessories connected it would imply the same rules would also apply where the metallic sheath/conduit/trunking terminates at its source.

Reg 543.2.4 (i)

"Where the metal enclosure or frame of a low voltage switchgear or control-gear assembly or busbar trunking system is used as a protective conductor, it shall satisfy the following three requirements"
(i) - Its electrical continuity shall be assured, either by construction or suitable connection, in such a way as to be protected against mechanical, chemical or electrochemical deterioration.
(ii) & (iii) N/A to this post


If you are in the habit of glanding/fixing to gland plates then these are usually fitted with several self tap screw through which you will be relying on a good earth, also removing paint to ensure gland contact to metal should be done but this exposes the steel of the enclosure to the atmosphere and as the paint was protecting it before you cannot guarantee corrosion wont set unless you take extra measures like repainting or spraying after with a seal.

If you can ensure your set-up meet clause (i) above then a earth tail won't be needed but in general a simple 5min job will take all the guess work out of the regulation and satisfy it.

E.G. - Glanding into a solid 1 piece enclosure that is made of stainless steel will not require a earth tail as long as the gland has good solid contact direct to the metal, next time you go through an old factory install look at the condition of the glands ...its surprising how many have rusted or corroded in some way yet are still tight.. thus electrical continuity cannot be guaranteed under fault conditions ...now if a separate earth lead was fitted it should still give good electrical continuity.


There are other BS standards that go deeper into this subject and give more details as to when and where but to simplify it and keep it to BS7671 for this thread its simpler to just fit the earth tail than trying to ensure your set-up wont be subject to mechanical damage, chemical and electrochemical deterioration.

 

[Brightspark2]

Also it doesn't need brown on each of the cores as brown, black and grey are the correct colours for 3 phase.

 

[cobrauk]

Thanks Darkwood for that info.

Reg' 543.2.7 "The earthing terminal of each accessory shall be connected by a separate protective conductor to an earthing terminal incorporated in the associated box or other enclosure"
I do always connect a earth/cpu from the back of the metal flush/surface box to the accessory, I was taught to do that from day one.

Reg 543.2.4 (i) I would have to agree "protected against mechanical, chemical or electrochemical deterioration." might be a problem if my cleaning of the paint would not guarantee corrosion would not set in, and hand on my heart I can't guarantee it wouldn't happen.

I would not think however that any manu would design a DB that design is not making sure that all removable parts are correctly bonded to each other to guarantee it has a good earth connection throughout the DB.

But having said that I will agree that after reading that i would need to start fitting banjo's or earth nuts in the future.

 

[Darkwood]

543.2.7 states where the protective conductor is formed blah blah the earthing terminal of each accessory shall be connected by a separate protective conductor to an earthing terminal incorporated in the associated box or other enclosure...

If you then look up the interpretation of what concludes as an accessory in part 2 this would suggest that a DB is under the accessory definition.

Now here is the grey part... the term used as a protective conductor comes up a few times and it could be argues that if you have a separate earth core as in the case of the OP then the armour isn't the protective conductor but as it is deemed exposed metal work it will be required to be earthed thus by this very fact can be an earth path for a fault current so the above reg' is thus applicable.

BS 6121 has a better wording of this set-up and explains when you need to and when you don't but as i said before im just trying to stick to BS7671 as this is the guidelines the forum revolves around and at the end of the day its good practice regardless to have a earth fly lead without going indepth to when you need to and when you dont ... its cheap, its a 5-minute job so to argue when its not needed is really a pointless exercise.