Another Armoured Cable Qestion

[Jim2287]

If i have 16mm2 two core SWA ... how do i determine the CSA of the steel armouring ?

a)
would it have the equilvalent current carrying capacity as the P+N conductors ? ie 60amp copper - 60 amp steel armour

b)
would it have the same CSA ie 16mm2 copper therefore 16mm steel wire

c)
or do you need to look it up in a table ?


any help would be good

 

[Spark123]

You can look it up in a table for compliance with table 54G. Some info here //www.diynot.com/wiki/electrics:swa-earthing
Steel has a higher resistance than copper hence the ratio of the K factors needs to be taken into account when selecting steel as a CPC. The armour should only be used as a CPC.

 

[Jim2287]

i found the K value and formulae to work it out ... just needed to know, how i know the CSA of my armouring, to ensure it complies


thats great ...

is that in the regs any where you know of ?

 

[Spark123]

You can either select it in line with table 54G using the copper equivalent of the steel armour, (see the table I linked above) or select it using the adiabatic equation where you need to know the fault current, time and the K value of the conductor, in this case steel to the temperature rating of the cable.

You won't find that particular info in the regs, they are calculated from the K values which are in the regs. They do however appear in GN1.

 

[Jim2287]

when you say SWA can only be used as CPC ...
does this mean i can or can not use it to supply another fuse board, as a supply earth

atm i have ...

TN-S 63 amp supply to out-building using 2 core SWA


there out building has no water/gas services therefore the supply earth doesnt need to be a main equipotential bond conductor as well (?)

 

[Spark123]

Yeah, you can use it as a CPC to supply the fuse board providing it passes the foresaid. The point I was putting across is it must not be used as a live conductor.

 

[Jim2287]

yeah its definately not live conductor

 

[Jim2287]

using adiabatic eq.

BS 88 63 amp
0.4ohm earth loop

=0.2 secs disconnection

600 x 600 = 360,000 x 0.2

= 72000

sqrt 72000 = 268.3

SWA K = 51

268.3 / 51 = 6 mm2 CSA

16mm2 2 core SWA = 46mm2


so therefore its fine as a supply to a fuse board ?



and the table shows it conforms too ( required = 36.1 < 46 )



The table you supplied a link to comes from Guidance Note 1 ?

 

[Spark123]

Just to confirm you are using PVC?

 

[Jim2287]

no idea what XPLE is

i havent installed the cable... which is the most common SWA installed ?


dunno i will have to check what it says

 

[Spark123]

XPLE is generally 90ºC cable
PVC is generally 70ºC cable
What does it say on the cable? BS nos?

 

[Adam_151]

You got PVC or XPLE cable? you are using the numbers for PVC, but I thought just about all the SWA you can buy today is XLPE?

Also you need to check the calc at both ends and take the worst case, although I can tell you that with a fuse the worst case will be far end where the Zs are highest and the fault current is lowest (with a breaker its the other way around)

and sure its a BS88? and not a 1361, not seen many single phase BS88 switchfuses

 

[Jim2287]

no im not sure, but used that as an example ... will check which type of cable and fuses


if i plug in the right info then that is the correct way to do it, yes ?

Does the table you linked come from guidance note 1 or similar table ?

 

[Jim2287]

sorry to bring this up again...

100 amp supply, 16mm main earth so 10mm main equipotential bonds

10mm copper required, but using steel .. k1/k2 x 10 would give me the equivalent CSA for steel ?

115 / 51 = 2.25 x 10 = 22.6 mm Steel

=>22.6 steel required to equal 10mm copper

so i would need 22.6mm steel plus, what ever i need for the CPC

correct ?

 

[Spark123]

Is it going to be used for a MEB?
I don't think you need to add the CPC size on, it is either the minimum CPC size required, or in the event of it being required for use for a main equipotential bonding conductor, minimum of the MEB size (or the CPC size if this is larger). Of course, as you point out the sizes are for copper hence you need to correct for this. You should try to keep your MEBs to a maximum of about 0.05 ohms too.