Suitability of SWA armouring as CPC

[IAmSparkytus!]

Use SWA as CPC, a guide to the acceptability of steel wire armour use as CPC.

SWA as the CPC? In most case it is fine to use the SWA as a CPC.

gadsolutions.biz

Anyone know how reliable the tables on this website are for determining whether the armouring of a cable can be used as it's earth?

Not too sure how old the page of the website is so could have changed since it was made or could have been wrong to start with I suppose

TIA

 

[westward10]

My tablet flags that up as a dodgy website.

 

[pc1966]

I don't get it flagged as dodgy, but maybe my standards are lower...

It reminds me of something I read a while back, probably an IET article or publication. the page says last updated 2012 so not quite current, even though most of the factors remain the same.

 

[IAmSparkytus!]

My tablet flags that up as a dodgy website.

Definitely not X-rated so you're not missing out I wouldn't worry!

 

[davesparks]

Use SWA as CPC, a guide to the acceptability of steel wire armour use as CPC.

SWA as the CPC? In most case it is fine to use the SWA as a CPC.

gadsolutions.biz

Anyone know how reliable the tables on this website are for determining whether the armouring of a cable can be used as it's earth?

Not too sure how old the page of the website is so could have changed since it was made or could have been wrong to start with I suppose

TIA

That table has been knocking around in various forms for years and is based on the simplified calculation of CPC size. As a result it will be correct where it says that the armour is a suitable size, but if you calculate using the adiabatic equation you will find that where the table says the armour is unsuitable it is in actual fact almost always suitable.

This all assumes the armour is only being used as a CPC and is not required to also be a main bond, in which case it will almost certainly be unsuitable.

 

[pc1966]

Nope, checked again and for 90C thermosetting k1 is 100 from Table 43.1 and k2 is 46 from Table 54.4 so here are the values I get, assuming those k values and assuming Fe is 8 times poorer than Cu for conductivity:

Live Cu​	Min CPC Cu Table 54.7​	SWA adiabatic eq 54.7​	CPC PME Table 54.8​	PME SWA​
1.5​	1.5​	3.3​	10​	80​
2.5​	2.5​	5.4​	10​	80​
4​	4​	8.7​	10​	80​
6​	6​	13.0​	10​	80​
10​	10​	21.7​	10​	80​
16​	16​	34.8​	10​	80​
25​	16​	34.8​	10​	80​
35​	16​	34.8​	10​	80​
50​	25​	54.3​	16​	128​
70​	35​	76.1​	25​	200​
95​	47.5​	103.3​	25​	200​
120​	60​	130.4​	35​	280​
150​	75​	163.0​	35​	280​
185​	92.5​	201.1​	50​	400​

 

[pc1966]

What you see is the PME main bonding cases is far more demanding, basically as it might be carrying tens or hundreds of amps for hours under open-PEN fault conditions and is not met by any of the SWA cable construction. So it is really a case of external CPC in parallel, or use an extra core in the SWA if that is the means by which any bonding is returned to the DNO connection point.

EDIT: Also note the extra note in 544.1.1 with AM2 about cables to other building not having to be fully to this in some cases.

 

[pc1966]

deleted deleted deleted

 

[cliffed]

Nope, checked again and for 90C thermosetting k1 is 100 from Table 43.1 and k2 is 46 from Table 54.4 so here are the values I get, assuming those k values and assuming Fe is 8 times poorer than Cu for conductivity:

Live Cu​	Min CPC Cu Table 54.7​	SWA adiabatic eq 54.7​	CPC PME Table 54.8​	PME SWA​
1.5​	1.5​	3.3​	10​	80​
2.5​	2.5​	5.4​	10​	80​
4​	4​	8.7​	10​	80​
6​	6​	13.0​	10​	80​
10​	10​	21.7​	10​	80​
16​	16​	34.8​	10​	80​
25​	16​	34.8​	10​	80​
35​	16​	34.8​	10​	80​
50​	25​	54.3​	16​	128​
70​	35​	76.1​	25​	200​
95​	47.5​	103.3​	25​	200​
120​	60​	130.4​	35​	280​
150​	75​	163.0​	35​	280​
185​	92.5​	201.1​	50​	400​

The end list… is it for the swa used for that circuit..16mm being 80 mm & confirms that the armouring is compliant

 

[pc1966]

The end list… is it for the swa used for that circuit..16mm being 80 mm & confirms that the armouring is compliant

No, it means if the SWA armour is your main bond for PME situations it needs to be 80mm section to match 10mm copper as specified in Table 54.8. That is only met for big cables such as (I think) 35mm 4C, 50mm 3C, and 70mm 2C or larger. That 10mm PME requirement is why often folks use 10mm 3C for an out-building with extraneous parts as then the copper CPC meets it.

The PME size I used is based on the incoming cable with Table 54.8, for a sub-main you might well have much larger conductors if the length is significant just to meet VD.

But under AM2 there is some relaxing of that if its a sub-main to other buildings in a group but would need to read the details of it.