Help with calculating the armour of SWA as a CPC

[LearnerDave]

Hiya guys.

I am learning about earthing, bonding and cpc’s at the moment, and I am getting a bit stuck with using the armour of SWA as a CPC. Iwonder if anybody would mind helping me by reading this, and correct what I have got wrong so I can make sure I understand it properly.

If I had a 2 core 16mm2 SWA cable. I read that this has a armour CSA of 46mm2. From reading threads in this forum I understand that steel has a stronger resistance compared to copper by a factor of approx. 9. Am I right in dividing 46 by 9 this would give a comparison to copper of 5.1mm2?

My confusion comes when I look at 543.1.4 and table 54.7
If I look at the table for 16mm2 line conductor, I would need a 16mm2 copper CPC. Allowing for the difference in resistance as above I would: 16 x 9 = 144mm2 for a steel CSA equivalent.

If I look at the table 54.7 for protective conductor not the same material as the line conductor and follow the calculation (k1/k2) X S

I make k1 to be 115 Using table 43.1 for copper line conductor in thermoplastic at 70’
I make k2 to be 51 Using table 54.4 for steel in 70’C thermoplastic

(115/51) x 16 = 36mm2

My confusion is that calculating the x9 method means that using the armour of the SWA would be massively undersized, yet using table 54.7 the armour would be acceptable.

I know I am making some stupid mistake but just can’t see the wood for all the trees at the moment. Any help would be really appreciated.
Dave

 

[Knobhead]

Have a look at http://www.electriciansforums.net/e...ulations/23288-can-we-use-armour-swa-cpc.html

 

[Deleted member 26818]

Where did you get that figure of 9 from?

Do you understand the adiabatic equation (i.e. what it does, not just how to crank the handle and get a result)?

The figure 9 is the relationship between the conductivity of copper and steel, about 9:1.
i.e. copper is about 9 x more conductive than steel.

To answer the OP's question:
Sizing a conductor to be a CPC, to be able to withstand high currents for a short duration is different to sizing a conductor to be a protective bonding conductor where the earthing arrangements would entail that the conductor has to be capable of withstanding various currents for a long duration.
When siziing a conductor to be a CPC, the disconnection times are from instantaneous to 5s, hopefully not long enough for the conductor to reach a temperature so that it is damaged.
Where PME conditions apply, which is where the conductace of a protective bonding conductor is required to be equivalent to that of a copper conductor, the conductor will be expected to withstand diverted neutral current at unknown voltages and amperages for unknown durations, perhaps years.
Table 54.7 is for use when you don't want to use the adiabatic equation when sizing a CPC or earth conductor.
Regulation 544.1 and Table 54.8 is used for sizing a bonding conductor.

 

[LearnerDave]

Tony: Thank you very much,

Ban-all-sheds: I'm just really the tea boy at the moment, and if I ask too many questions, all I get is that you must be bored get the kettle on!!! I am however trying to but theory to what we do in practise.

Spinlondon: Thank you very much, it was your post in 'Useof SWA armour as protective bonding conductor' that made me realise that the CSA of the steel wasn't a direct match for copper which got me thinking.

Just to recap:
If sizing a armour SWA as a cpc then table 54.7 can be used due to the short duration of the fault.

If however the armour is being used for protective bonding then the x9 would need to be used to be comparable to copper.

Thank you for all your replies and help, much appreciated.
Dave

 

[SPARTYKUS]

GN8 page 103

 

[Silly Sausage]

The figure 9 is the relationship between the conductivity of copper and steel, about 9:1.
i.e. copper is about 9 x more conductive than steel.

This may well be a really dumb question but I'm going to ask anyway.

Where does this 9:1 ratio come from?
From conductivity tables I'm getting copper:steel ratios of about 5.8:1 for iron up to around 40:1 for stainless steel. Is it just the particular value for the metal in SWA, or just some 'standard' figure?
(I've got a lot of time on my hands at the moment )

 

[Knobhead]

LearnerDave, keep on asking questions. There’s two sides to every coin. It keeps us on our toes as well as helping you out.

Also have a look at http://www.wiley.com/legacy/wileychi/eca_wiringregulations/supp/Appendix_16.pdf

 

[Deleted member 26818]

This may well be a really dumb question but I'm going to ask anyway.

Where does this 9:1 ratio come from?
From conductivity tables I'm getting copper:steel ratios of about 5.8:1 for iron up to around 40:1 for stainless steel. Is it just the particular value for the metal in SWA, or just some 'standard' figure?
(I've got a lot of time on my hands at the moment )

There are many tables which show the ratio of conductivity between copper and steel.
There are differences in the conductivity of various steels, 9 is the value that appears to be on the safe side of average, could possibly go as low as 8.
Are you sure that the tables you are using show the electrical conductivity, or the thermal conductivity?

 

[Knobhead]

Based on copper being 100%, steel can be anywhere between 3 – 15%.
I can’t find tables relating to conductivity steel as used in railways.

I think we have to rely on the published tables unless you fancy doing advanced metallurgy in your spare time.

Just to chuck a rock in the water it’s possible to order high conductivity armouring for cable. These have 1 in 4 or 5 armours made of tinned copper. I used them a lot under M&Q regs.