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

 

[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.

 

[44ALLAN]

Which means its just a part that needs equipotential bonding then for touch voltages and not 'earthing' which allows fault currents to take out the ocpd

lol

 

[Darkwood]

You get my drift now .... rename it and the give it the same job it meets BS7671 where as ID it as a CPC it may fail.... a flaw IMHO which can only be solved with making sure the CSA of all SWA is in compliance ... how can its function under fault conditions be in compliance in one set-up and not in the other when at the end of the day it may have to carry the full fault on its own.

 

[Engineer54]

The SWA on it's own may not be suitable without calculation, but will in most circumstances be of a sufficient value, to enable taking out the OCPD, if not within permitted times, then very close to them!! Something that i wouldn't spend too much time worrying about anyway!! lol!!

 

[Darkwood]

Soz E54 it was just one of those things where in reality the regs don't consider every scenario and its practicably impossible to comply in such cases so rather than the IET draft in complex solutions they brush it under the carpet and hope it isnt noticed ..... only problem i have is the 2 responders to my query are well educated/experienced and realise its really not an issue but for the lesser knowledged and experienced its a good thinking point ....

 

[44ALLAN]

Soz E54 it was just one of those things where in reality the regs don't consider every scenario and its practicably impossible to comply in such cases so rather than the IET draft in complex solutions they brush it under the carpet and hope it isnt noticed ..... only problem i have is the 2 responders to my query are well educated/experienced and realise its really not an issue but for the lesser knowledged and experienced its a good thinking point ....

Well thanks for those kind words..................who was the other one?:smilielol5: