This has probably been asked a thousand times but I haven't been able to find the info I need by searching the topics.

Therefore, would anybody be kind enough to guide me to where I would find the size of the steel armour around a 25mm 4 core SWA. The cable has been used to supply a Remote DB, and the amour is the CPC.

Any help would be greatly appreciated.

Thanks

Steve

Guidance Note 1 has 25mm 4 core as 76mm2 armour CSA.

Steves'choo' Remember that the armour is made of steel so if you are looking for the R2 the size isn't going to help you against the usual charts for copper conductors.

I've a handy book at the office with an SWA R2 chart in it, and I'm there tomorrow so I will look it up for you unless someone on here has one to hand sooner.

Zs

Divide by 8.8?

This is a useful table for armour sizes:
http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf
Regards
Bruce

--while 25mmsq 4 core SWA has an armour of 76mmsq
the csa equiv is only 6mmsq.

if its a PME situation you will need a 10sqmm run alongside.

Regards.

In a PME situation, we would always run a 16mm alongside rather than a 10mm. (assuming 100amp supply of course)

It's just something we've always done as good practice, even if it costs a bit more.


Stu

GN8 indicates a ratio of 8:1.
Therefore steel armour with a CSA of 76mm², would have the equivalent conductance of a copper conductor with a CSA of 9.5mm².

--9.5? i will stick with 6.

Regards.

I don't buy that, where would you draw the line?

Regards

Any more bids?

Resistance of the armour of 25mm four core 70 degree PVC insulated with copper conductors @ 20 degrees 2.1 milli ohms per meter

If it is XLPE insulation then 2.3 milliohms per meter.

Much better to look it up don't you think?

Zs

If the armour is only being used as a c.p.c. (and not as PME bonding conductor) then you're not really interested in its actual resistance (either absolute or copper equivalence) - just its ability to survive an earth fault current for the required time - i.e. it's "k" value (tables 54.2 and following). E.g. if the cable runs at max 70 degrees in normal service with a maximum ambient of 30 degrees, then k for copper is 143 and steel 52 - making copper just 2.75 times "better" than steel for the same c.s.a.

So if the cable had 76mm2 of steel that would be the equivalent of 27.6mm2 of copper - easily satisfying table 54.7.

- Andy.

Yes but...

i) personal view, (I buy cable), table 54.7 is for those who can afford expensive cable for use as a CPC.

Without knowing the relevant R2 it would not be possible to calculate the circuit Zs and hence the level of the fault thereafter.

As such, the relationship between the k values is only really relevant if you are selecting cable and not doing a proper job of the calculation

Zs

meter sic, is the clocker! Why the difference in ohms/metre?

Regards

point taken!


But in this case we already have the cable - we only need to decide whether it's adequate or not. Why use an "expensive calculation" when a "cheap selection" will do?

- Andy.

Because I'm an expensive kind of a girl Andy.

Nor really because a cheap selection is a bit ham-fisted. Steve has already told us that the armour only is going to be used as CPC. I reckon it would be good to run the entire gumut of calcs just to be sure that in the event of a long term fault the cores are not going to be gently frying inside their armour.

but not, of course being semantic.

Zs

If you buy the cable based on the CCC, and the CSA of the armour meets table 54.7 which in most instances it will, it complies.why worry about the calculations.

Edited: 18 July 2012 at 12:43 PM by Chris123

Well I think you answered that yourself - "most cases". For sure if you rocked up at my desk with some some vague idea that the armouring is probably going to be fine guv, I might just raise an eybrow.

Equally, how do you know what fault level (or effective Zs) exists at poits along the cable - if you buy it on CCC, yoiu are going to look a bit wet when the measured zs at the load end is gretare than your protective device requires for disconnection !!

Zs is quite correct - if you are going to do it, do it properly

Regards

OMS

Edit, spelling - because we are having a pedantic day

-------------------------
Failure is always an option

Edited: 18 July 2012 at 11:25 AM by OMS

Sorry about poor speeling. I'll get you back for that when you least expect it.

Because there is a different overall CSA of the cable, hence a difference in the CSA of the armour. But you knew that really Jaymack.

Zs

Based on a assumption that disconnection times were met