Using SWA armour for earth on a sub-mains supply

[keen-apprentice]

Hi all, long time reader first time poster. I am an apprentice that has mainly done domestic work, but more recently we have been doing commercial jobs (which i much prefer lol), i do really enjoy electrical work and I'm keen to learn more, which means i ask a lot of questions.

The last couple of weeks has been working with SWA's, re-routeing a 70mm 4 core (as in 3 phase conductors and neutral conductor - apologies in advance if my terminology is wrong here) for a sub-mains to a 3 phase distribution board. Also pulling in SWA 10mm 3 core cables (as in phase/neutral/CPC conductors) for the individual circuits that will be fed from said distribution board.

When it came to glanding off the SWA's the electrician i was working with wasn't really interested in answering any questions about using the 70mm 4 core's armour for earth, or what bonding was required for the banjo's on all the smaller SWA's.

So my questions are:

1. The 70mm 4 core SWA sub-mains runs from the plant room main distribution board, to the new TP distribution board. The SWA armour is used as earth for the sub-mains, but what size cable do use as for the fly lead from the gland's banjo to the main earth bar in the new distribution board?

I think the spark i was working with used 10mm, is that large enough given the fact that it is such a short run?

I have had a look through my books, i did a bit of googling, and did a couple of calcs, and after a lot of head scratching using the adiabatic equation i calculated you would need a 13mm cable, which surely would mean you would need a 16mm earth cable not 10mm?

2. He didnt do any bonding for the 10mm SWA's. He just glanded the cables to the distribution board and bolted the banjos down. I can understand his logic that its a metal clad enclosure, so the banjo and main earth bar will have continuity without a fly lead, but I thought all the glands needed to be linked together via their banjo's and then a fly lead up to the main earth bar. Is that correct? If so what size conductor would you use? 10mm for bonding?

3. On a side note he was really struggling to terminate the 70mm phase conductors into the main switch terminals, surely there must be an easier way lol any tips for how it can be done easier? Or do you just have to crimp a lug to the conductors, to reduce the size of the conductors so they can then terminate easily into the main switch terminals?

Sorry for all the questions, but i am finding it hard to get my head round this, i think i have started to confuse myself a bit lol, so it was time to seek advice. Any help/advice to help me understand a bit better would be awesome.

Cheers, KP

 

[LankyWill]

id be worried if he didnt pull in a separate cpc with that 70mm 4 core

 

[Ian1981]

The flying lead can be determined with the Adiabatic equation or by the use of the tables in bs7671 table 54.7 same as any cpc.
If the swa wasn't being used a cpc then yeah I'd supplementary bond it with 10mm if you like but as it's a cpc it needs to carry the fault current as well as the armour

 

[Andy-1960]

id be worried if he didnt pull in a separate cpc with that 70mm 4 core

Why? Using the armour is perfectly acceptable providing it is calculated correctly. Supplementary CPC is only required if Zs is too high.

 

[Leesparkykent]

From memory I think 70mm 4core in either PVC or XLPE has a steel armour that is of an adequate CSA to act as or form part of a circuit protective conductor.

 

[Ian1981]

Most swa running at operating temp of 70 degrees has an expectable armour CSA to act as a cpc an exception being 300mm.
Guidance notes 1&8 have excellent tables giving actual size of the armour and the size needed to comply with table 54.7 and the formula as given in bs 7671
K1/k2 x s/2 for sizes >35mm
Without looking at guidance notes or cable manufacturers data I don't know the actual CSA of a 70mm 4 core as it's steel CSA off the top of my head

 

[Doomed]

138mm2 if I can read correctly without my glassess

 

[westward10]

Fly lead should not be less than 16.0 which is the rough copper equivalent of the armour off the top of my head.

 

[Ian1981]

It's usually a copper to steel ratio of 8.5
So 85mm steel / 8.5 is the equivalent of 10mm copper.

 

[westward10]

It's usually a copper to steel ratio of 8.5
So 85mm steel / 8.5 is the equivalent of 10mm copper.

10.0 it is then my top of my head isn't what it was.

 

[Toneyz]

When you install distribution boards (assuming this is a T.P. mcb board) with a large swa or any cable size consideration needs to be given to the terminal size of any connection especially if it is a tunnel connection also a spreader box may be required to accommodate the physical size of cables. I always install a earth between the banjo and corresponding earth terminal due to the poor contact due to paint.

 

[davesparks]

It's usually a copper to steel ratio of 8.5
So 85mm steel / 8.5 is the equivalent of 10mm copper.

That is the ratio of conductivity, this is not the same thing as the current carrying capacity so it cannot be used for working out the size of a CPC.

 

[davesparks]

Most swa running at operating temp of 70 degrees has an expectable armour CSA to act as a cpc an exception being 300mm.
Guidance notes 1&8 have excellent tables giving actual size of the armour and the size needed to comply with table 54.7 and the formula as given in bs 7671
K1/k2 x s/2 for sizes >35mm
Without looking at guidance notes or cable manufacturers data I don't know the actual CSA of a 70mm 4 core as it's steel CSA off the top of my head

The adiabatic equation can also be used which will give a smaller but accurate size for the CPC in whatever conductor material. The table 54.7 method gives a one size fits all answer which results in a CPC larger than it actually needs to be. For smaller cable sizes the difference makes very little difference but for larger cables the cost saving made by using thenaccurate calculation can be appreciable.

 

[davesparks]

id be worried if he didnt pull in a separate cpc with that 70mm 4 core

Why?

 

[davesparks]

2. He didnt do any bonding for the 10mm SWA's. He just glanded the cables to the distribution board and bolted the banjos down. I can understand his logic that its a metal clad enclosure, so the banjo and main earth bar will have continuity without a fly lead, but I thought all the glands needed to be linked together via their banjo's and then a fly lead up to the main earth bar. Is that correct? If so what size conductor would you use? 10mm for bonding?

3. On a side note he was really struggling to terminate the 70mm phase conductors into the main switch terminals, surely there must be an easier way lol any tips for how it can be done easier? Or do you just have to crimp a lug to the conductors, to reduce the size of the conductors so they can then terminate easily into the main switch terminals?

Most DBs have a removable gland plate which is held on to the main enclosure by a few small screws. If the banjos are bolted to the gland plate only then the fault current will only have those tiny little screw connections to pass through, so a suitably sized flylead should be connected to the gland plate so that you aren't relying on this little screws. And if you are doing that then you might as well connect the flylead to the banjo if it's just the one or two SWAs.
The other problem is that the metal enclosure is painted, so the paint needs to be cleaned back to bare metal where the banjo and bolt are in contact with it otherwise you will not be making a good connection.
It is quite common to fit galvanised trunking above and below the DB, one of the advantages of this is that SWA glands will be attached to the bare metal of the trunking and make a good connection.

Off the top of my head most DB main switch terminals are designed for a maximum conductor size of 50mm, larger cables will need reducing lugs (aka reducing pins or pin lugs) in order to connect them to the terminals.
Alternatively some manufacturers make adaptor kits to convert the main switch to take normal bolted lug connections.

 

[Wilko]

Taking on board DaveSparks #13, maybe confirm fly lead size against adiabatic (543.1.1) where 10mm is ok for 4.5kA for 100mS (k=143) ?
Just noticed the time - so I hope this isn't rubbish

 

[Ian1981]

That is the ratio of conductivity, this is not the same thing as the current carrying capacity so it cannot be used for working out the size of a CPC.

I know I was referring to the conductivity
I think you've misinterpreted my comment
My example was if you bonded an extraneous part equivalent to 10 mm copper , the steel would have to be 85mm if using the armour for the 10mm equivalent

 

[Ian1981]

The adiabatic equation can also be used which will give a smaller but accurate size for the CPC in whatever conductor material. The table 54.7 method gives a one size fits all answer which results in a CPC larger than it actually needs to be. For smaller cable sizes the difference makes very little difference but for larger cables the cost saving made by using thenaccurate calculation can be appreciable.

The adiabatic equation can also be used which will give a smaller but accurate size for the CPC in whatever conductor material. The table 54.7 method gives a one size fits all answer which results in a CPC larger than it actually needs to be. For smaller cable sizes the difference makes very little difference but for larger cables the cost saving made by using thenaccurate calculation can be appreciable.

See comment 3

 

[LankyWill]

Why? Using the armour is perfectly acceptable providing it is calculated correctly. Supplementary CPC is only required if Zs is too high.

My bad i guess i am still learning too, ive always done installs that involve dis boards full of D-Type Breakers. I assume that the supplementary bonding was installed to help the Zs of these circuits. Time to get my design book out and study...

 

[mrloy99]

Is it not as a rule of thumb half the size of the phase conductor ie 35mm.
And for the 10mm cables,they already have a cpc included as a core.So provided a sound connection is made to gland plate it may be that bolted banjos (with brass nut and bolt not 6mm roofing bolts)are adequate..possibly only one fly lead from gland plate to earthing bar if tiny gland plate bolts are in use