Fly Lead Size to SWA

[lolbot235]

Hi there,

If I’ve got a SWA, what size should my fly lead cable be from banjo to earth bar ? The wire armouring is only being used for mechanical protection and not by means of supplying a earth.

If I am using the SWA as means of a earth connection how do I size the fly lead then?

 

[davesparks]

In my opinion the flylead should always be at least the size required for a CPC for the circuit whether the armour is the CPC or just needing to be earthed.

 

[Lucien Nunes]

I would also tend to make it CPC-size, whether the only CPC or in parallel with a copper CPC. One could argue that if the armour is only serving to protect the cable and is not connected to anything downstream (e.g. the far end is TT) it only needs to provide for the short-circuit requirements and could be sized adiabatically. However it most situations the saving would be trivial so I would tend to ignore this option.

 

[Markyd]

On the subject of earthing the swa been to a job today where they have extended a ring to a small outhouse using a wiska box that is too small to use the banjo and locking nuts inside.....any tips on earthing the armoured without having to take it all apart and replace the box

 

[westward10]

On the subject of earthing the swa been to a job today where they have extended a ring to a small outhouse using a wiska box that is too small to use the banjo and locking nuts inside.....any tips on earthing the armoured without having to take it all apart and replace the box

If it is a Wiska 308 fit one of these.

Wiska Earth Clamp Bar for SWA Glands for WK308 | Wiska (TLC308)

 

[lolbot235]

Thanks guys that cleared it up for me.

 

[pc1966]

Agree with the above: the flying lead should be able to take out the supplying OCPD in the event of an internal cable fault so probably no less than half the phase conductor CSA as a rule-of-thumb.

 

[Dustydazzler]

As above

what ever the cpc is the fly lead should be the same or bigger

 

[Vortigern]

Thinking about the above, the steel wire has a given factor which can be read on a chart I have seen here several times. I understand the SWA of the cable has a factor of 8 times to consider when using as an earth. If you match the CPC of the SWA surely the flylead would have a higher fault current tolerance than the SWA of the the cable?

 

[pc1966]

Thinking about the above, the steel wire has a given factor which can be read on a chart I have seen here several times. I understand the SWA of the cable has a factor of 8 times to consider when using as an earth. If you match the CPC of the SWA surely the flylead would have a higher fault current tolerance than the SWA of the the cable?

I suspect @Dustydazzler was referring to the CPC size of the supplying circuit (e.g. if fed from from 4mm T&E then 2.5mm CPC, etc)

Generally for smaller SWA sizes the CPC is fine, but there comes a point when the armour is no longer able to cope with the adiabatic stress due to the higher resistance of the steel.

 

[timbobelfast]

If in doubt use the adiabatic equation to work out the required cpc size. Depending on size if a 3C or 4C is used you will need a nominal sized green & yellow to tie the armour down to earth!

 

[davesparks]

Thinking about the above, the steel wire has a given factor which can be read on a chart I have seen here several times. I understand the SWA of the cable has a factor of 8 times to consider when using as an earth. If you match the CPC of the SWA surely the flylead would have a higher fault current tolerance than the SWA of the the cable?

The 8 times conductivity is used as a factor when assessing the Armour as a bonding conductor rather than its use as a cpc.

 

[pc1966]

Prysmian has a useful table at the back of the data sheet listing resistance and CSA of the armour (for adiabatic test), but I suspect these are the DC terms as it is slightly less than the AC volt drop equivalent:

https://www.whyprysmian.co.uk/BS5467.pdf

Somewhere I remember seeing a table that was a quick yes/no to armour as CPC limits

As @timbobelfast has already said you can run a copper CPC external to the SWA cable, subject to it being mechanically safe (e.g. in a duct, etc) if you can't get, or afford, SWA with an internal copper conductor used as CPC. After all 5-core is well over 20% more expensive than 4-core, and often the supplementary CPC need only be half the phase size.