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.

 

[nickblake]

easiest option would be to install an earthing nut on it job done

 

[Aindowsimon]

Wot I like doing if I have a metallic db is i allways if possible have peace of trunking above dis board gland and banjo all swa's to the trunking ( no earth tail to each banjo in the instance below if I were to gland swa's to a db solely probably would earth tail each ) and usually earth the trunking via the banjo of the supply swa . I always remove paint as tidy as possible at db side when glanded directly .



I trust the trunking to to supply a reliable sound interconnection of earth between glands but yes have a earth tail to trunking usaly same csa as MEC .

This last pic is a mystery had uploaded it as a mistake and deleted it but its still there ????? No relevance

 

[Darkwood]

I see a need for a quick of galv' spray paint may be i order to cover any revealed steel judging by the damp on them walls

 

[Aindowsimon]

Just been roofed . Ye damp wall's why arnt I allowed to remove the photo ?

 

[Darkwood]

Talk to admin .... ive noticed the same thing too when i want to take a photo down it wont let me ...

 

[44ALLAN]

"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".

Yup.

At both ends mate with a 6mm brass nut and bolt either tapped or drilled through the banjo and box(paint removed with file and contact grease applied of course lol).

You then take a fly lead from the 6mm bolt and armouring to the internal earthing terminal of the board...........sized accordingly.
You then have your internal earth as your cpc and your swa as a cpc(if required) and they can then both be a bond and vice versa.............complimenting each other as it were
Then you have the parallel paths and disconnection times in a nano second.......................................................

Or you can just use the Swa as your cpc of course

 

[tannerselectric]

Surely you should know how to do this, if you are a qualified electrician

 

[nickblake]

had to replace an isolator in a wash up area of a hot it had a TPN switch right next to a dishwasher water every where the armour was the CPC , I replaced the isolator with a rotary IP rated switch used the earthing nut tightened the grub crew earth tail to earth block and tadarr job done spot on continuity readings and Zs happy days

 

[SirKit Breaker]

Un earthed SWA armouring to the main earthing terminal in the plastic DB is a non conformance on the inspection assessment for the inspection and test course, if you dont pick it up you will fail, so read into that what you will.

Cheers..........Howard

 

[ewatty]

So bottom line is its best to be earth tagged at least one end if not both!

 

[telectrix]

if non-metallic boxes, tag at one end if you just want to earth it. tag both ends if it's being used as cpc.

 

[44ALLAN]

had to replace an isolator in a wash up area of a hot it had a TPN switch right next to a dishwasher water every where the armour was the CPC , I replaced the isolator with a rotary IP rated switch used the earthing nut tightened the grub crew earth tail to earth block and tadarr job done spot on continuity readings and Zs happy days

So what your saying is the IP rating wasn't correct from the initial install but it is now
Shrouds and outside IP rated glands to IP56(ships deck) if there was water everywhere? :thinking2:

 

[44ALLAN]

So bottom line is its best to be earth tagged at least one end if not both!

depends what you mean by tagged? earthed? or bonded?


Why would it need any from one end or both ends?

 

[ewatty]

:juggle2:

 

[44ALLAN]

:juggle2:

thought so lol

 

[Engineer54]

"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".

Yup.

At both ends mate with a 6mm brass nut and bolt either tapped or drilled through the banjo and box(paint removed with file and contact grease applied of course lol).

You then take a fly lead from the 6mm bolt and armouring to the internal earthing terminal of the board...........sized accordingly.
You then have your internal earth as your cpc and your swa as a cpc(if required) and they can then both be a bond and vice versa.............complimenting each other as it were
Then you have the parallel paths and disconnection times in a nano second.......................................................

Or you can just use the Swa as your cpc of course

That statement isn't actually correct. If there is a 3rd core being used as the circuits CPC, then the SWA armouring isn't classed or considered as a CPC at all, it'll just be an earthed exposed conductor, that only needs to be earthed at one end of the cable. You should never rely on two different metals being used as a single or combined CPC, as any fault current will NOT be shared equally between the two differing metal conductors!!

 

[Darkwood]

Hypothetical set up .....

You have a large SWA where the charted armour does not meet the required size to use it as the CPC, you use a spare core as your CPC. Obviously we would need to earth the SWA as you do so here's my query!

The cable is accidentally damaged (pierced) that avoids the earth core but creates a dead short down to the armour.... because the armour isn't large enough to meet the regulations for using it as a cpc will it still be able to carry full fault current safely and trip the protective device within requirements?

 

[44ALLAN]

That statement isn't actually correct. If there is a 3rd core being used as the circuits CPC, then the SWA armouring isn't classed or considered as a CPC at all, it'll just be an earthed exposed conductor, that only needs to be earthed at one end of the cable. You should never rely on two different metals being used as a single or combined CPC, as any fault current will NOT be shared equally between the two differing metal conductors!!

Totally agree and I take your point.
I've already stated several times that I use an integral earth core in the cable and earth the armouring at both ends...........it's the way I have always done it (apart from hazardous area where sometimes they ask for the armouring to be earthed in hazardous area only and a sign put on the other end stating that fact).

 

[44ALLAN]

Hypothetical set up .....

You have a large SWA where the charted armour does not meet the required size to use it as the CPC, you use a spare core as your CPC. Obviously we would need to earth the SWA as you do so here's my query!

The cable is accidentally damaged (pierced) that avoids the earth core but creates a dead short down to the armour.... because the armour isn't large enough to meet the regulations for using it as a cpc will it still be able to carry full fault current safely and trip the protective device within requirements?

That would depend on the fault current at that point and the rating of the OCPD !
Or are you asking something else?

 

[Darkwood]

That would depend on the fault current at that point and the rating of the OCPD !
Or are you asking something else?

What im suggesting is that in certain set-ups where you require separate earth conductor because the CSA of the armour is below requirements then a situation can arise where the separate correctly sized earth conductor would not be part of the fault path but instead a large fault current could flow down the armour alone and because its below csa requirements the fault could exist longer than BS 7671 would permit.

Im trying not to state size of fault or OCPD size and type its just for a hypothetical query ...

 

[44ALLAN]

What im suggesting is that in certain set-ups where you require separate earth conductor because the CSA of the armour is below requirements then a situation can arise where the separate correctly sized earth conductor would not be part of the fault path but instead a large fault current could flow down the armour alone and because its below csa requirements the fault could exist longer than BS 7671 would permit.

Im trying not to state size of fault or OCPD size and type its just for a hypothetical query ...

Well if there was an internal core being used as the cpc the armouring would then be classed as an exposed conductive part the same as all the other metalwork which could become live under fault conditions and would need to be connected to earth.................I can see you point though

 

[Darkwood]

Kinda my point it word play ... yes the SWA is an exposed conductive part not the CPC in this case yet as its function is to protect the internal conductors it has a increased chance when damaged of been the sole fault path back to earth but in certain cases the undersized csa of the armour can lead to the OCPD not tripping within permitted limits.... this could be said od any exposed conductive part that could become the fault path but this is one of them cases where the probability is higher due to it been wrapped around the cable as a protection.

 

[benji]

Not forgeting that earth tags can only take about 10ka

 

[Engineer54]

Well as the SWA is no longer being considered as a CPC, it doesn't have to take out the OCPD within permitted limits, that's now the job of the full sized separate G/Y conductor (or additional cable core)!!

 

[Darkwood]

Yes agreed E54 im not debating that but its only word play as the armour can be the sole path so hypothetically just for debate the fault can return down 2 paths 1 which will take out the OCPD within permitted limits and the SWA which can under certain set-ups be undersized and exceed the OCPD permitted tripping times... just because its lost its identity as a CPC doesn't solve the fact that a fault can occur that won't comply for tripping times.