SWA banjo fly lead size

[Alan Marriott]

Hi all.

Could I ask some advice please regarding SWA banjo fly lead sizing.
Can anyone explain what sizing or how you size the required earth size when:

A) The armourings are not used as cpc

B) When armourings are used as CPC?

I’m still learning and can’t find anything explain I guess this so was hoping for some advice please.

Many thanks

 

[James]

Honestly in the real world, most of the time you will use a piece of the cable you trim off the end of the SWA.
it is over and above the requirements for earthing conductor size and is freely available right where you need it.
just remember to add some stripy green/yellow tape and all is good.

 

[westward10]

It isn't important whether it is being used as the primary cpc or not, under fault conditions it must be able to carry the fault current regardless.

 

[Alan Marriott]

So if that is the case it needs to be same size as CPC for that circuit?

 

[Lucien Nunes]

Over on this thread here, a couple of us agreed that it's usually sensible to make it the same size as you would make a copper CPC for the same circuit.

Fly Lead Size to SWA - in - UK Electrical Forum

, Fly Lead Size to SWA, UK Electrical Forum, ElectriciansForums.net Est.2006 | Free Electrical Advice Forum and page_number.

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Because it's usually short compared to the run of SWA it tends to be a small fraction of the total resistance, so it is sometimes possible to use a smaller conductor provided it is adequate adiabatically. But unless there is a serious shortage of space in an accessory, what's the point? It would just end up looking flimsy and cheapskate.

Obviously if the SWA has been sized to minimise voltage drop and is much larger than needed to achieve the required Iz, it's not necessary to match that CSA with the flylead.

 

[davesparks]

Honestly in the real world, most of the time you will use a piece of the cable you trim off the end of the SWA.

Yes, most of the time you would, but only when the cable is relatively small. Once you get in to the medium/large sizes you are more likely going to want to be using something smaller and not deprive the Xmas party fund of the scrap value.

I've got a bunch of remedials to do next week which involves adding a flylead to an SWA gland which the original installers didn't seem to think was necessary. Zs is a bit too high on it and the armour is the only CPC, thankfully they put the banjo on the gland as its 400mm 4 core and I wouldn't want to be trying to take that apart to add one.

 

[pc1966]

Generally the simple & safe option is you make the banjo lead the same as the phase size. That is always OK (assuming the cable itself is OK for fault protection) and typically makes it larger than the equivalent* of the armour on bigger cable.

Next option is you look at having the CPC lead as half the phase size, providing the CPC is not less than 16mm copper, and that satisfies Table 54.7 as your simple guide to reduced size CPC design.

Last option is you look carefully at the adiabatic limits of the cable. This option really should be you last resort as you need to establish the worst-case Zs and so the resulting OCPD let-through I2t value, and from that and the CPC construction (martial and insulation start/max temperatures, hence 'k', and also CSA) you can decide if it would survive a fault-clearing incident.

[*] Comparing steel armour to copper or aluminium conductors is not so simple. In terms of resistivity Fe is about 8 * Cu so your PME-case bonding is typically 80mm of armour, met by 70mm 2C, almost-ish by 50mm 3C or 50mm 4C. But your adiabatic 'k' is around SQRT(R.Fe/R.Cu) as more material is needed to drop R but it has the side-effect of increased thermal capacity.