Discuss How to calculate the resistance of steel armour under fault conditions 70 oC for CSA 95mm2 in the Commercial Electrical Advice area at ElectriciansForums.net

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Hi All, hope your all well.

I was hoping if I can please get some advice in regards to SWA. In Particular using SWA as CPC. Cable size 95mm2

I have a table for SWA which shows various information, in the last two columns it shows the "resistance of steel armour under fault conditions at 70 degree" and "resistance of copper core at 20 degree"

it then shows different value of resistance from both columns as the cable of CSA changes. In the first table it stops at 35mm2 CSA. I have looked at other manufacture cable sizing and they give further cable sizing, it also gives me the "resistance of one copper core at 20 degree" for 95mm2 which is 0.193

Now my question
Is there a way to identify or to calculate what the "resistance of steel armour under fault conditions at 70 degree" would be for 95mm2?

Thank you
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Thank you
 

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If you do a quick search online you'll find quite a few calculators for this - eq. CSA for given sizes of SWA. As a general rule of thumb, once your cable is larger than around 25mm / 5c then you're OK under normal applications although there are often installation benefits to running a single CPC alongside a 2c / 4c armoured.
 
Thank you @ Rockingit for guidance, the cable size is 95mm2 4 core.
I'm trying to determine what my R1 and R2 will be for this particular size cable when checking if it complies with the Zs.

As you can see in the image below it shows an example of 16mm2 SWA being used as CPC and the caculation carried out for R1 and R2.

The calculation is not the problem, I can carry that out, however the difficulty I'm having is how to identify the "Resistance of steel armour under fault condition for 70 degree" when it comes to 95mm2 SW?

Thank you
 

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Hi - here’s the page from GN8 for some types of SWA. It’s got 95mm 4 core as having 147 mm of steel. The factor for fault disconnection (k1/k2) is shown as 115/51. So for CPC fault disconnection it’s assumed to behave like 65mm of copper.

To find out what the armour resistance is at 20 degrees C (say) we need to use the ratio of resistivity for the two. I don’t have it to hand, but I recall steel as about 8 times copper. So the 147mm of steel should have the resistance equivalent of 18.4 mm copper. Perhaps for ease try 16mm which is 1 mOhm/m (OSG Appendix I).

How to calculate the resistance of steel armour under fault conditions 70 oC for CSA 95mm2 FA4AFEF1-4210-4D13-B1F5-4BA6B8429975 - EletriciansForums.net
 
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As I said, there's several online calculators that will give you both the actual and equivalent Cu CSA, thereafter just do the maths in the normal way. Though, I'm now intrigued as to why you want a 70deg R1R2 value - are you designing a circuit to 'max out' right from the start? If so, or if you're that close to your max Zs for the install, then you may wish to consider a larger cable anyway?
 
So 95mm 4 core SWA over 30m - just for fun - I think your continuity tester at 20 degrees C would report :
R1 as 0.8x30x0.47/2 = 5.64 mOhms
R2 (steel) as 0.8x30x1.15(sorry, said it was 1) = 27.6 mOhms
R1+R2 about 33 mOhms
 
So 95mm 4 core SWA over 30m - just for fun - I think your continuity tester at 20 degrees C would report :
R1 as 0.8x30x0.47/2 = 5.64 mOhms
R2 (steel) as 0.8x30x1.15(sorry, said it was 1) = 27.6 mOhms
R1+R2 about 33 mOhms
(And here was me thinking we don't do people's sums for them... ;))
 
Hi - here’s the page from GN8 for some types of SWA. It’s got 95mm 4 core as having 147 mm of steel. The factor for fault disconnection (k1/k2) is shown as 115/51. So for CPC fault disconnection it’s assumed to behave like 65mm of copper.

To find out what the armour resistance is at 20 degrees C (say) we need to use the ratio of resistivity for the two. I don’t have it to hand, but I recall steel as about 8 times copper. So the 147mm of steel should have the resistance equivalent of 18.4 mm copper. Perhaps for ease try 16mm which is 1 mOhm/m (OSG Appendix I).

View attachment 60447

Thanks alot @ Wilko

Sorry for the short coming at the moment my knowledge of electrical is basic. This is one of the question I have received for my electrical course, and have been trying to understand for quite sometime but not been able to comprehend it.

Sadly I haven't got GN8 at my disposal, but that page seems helpful. Furthermore mentioning resistivity of steel being 8 times that of copper.

Would that mean using the value resistivity of steel or in particular SWA would give you the resistance?

This is what I have learned previously in my electrical course on Level 2 and applied the formula which I previously learnt covered to this current situation. Correct me where I have gone wrong please

The resistivity of SWA I have got it from this website

myCableEngineering.com > Elctrical Resistivities & Temperature Coefficients - https://mycableengineering.com/knowledge-base/elctrical-resistivities-temperature-coefficients
 

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As I said, there's several online calculators that will give you both the actual and equivalent Cu CSA, thereafter just do the maths in the normal way. Though, I'm now intrigued as to why you want a 70deg R1R2 value - are you designing a circuit to 'max out' right from the start? If so, or if you're that close to your max Zs for the install, then you may wish to consider a larger cable anyway?

I have been trying to look for online calculation but due to my limited knowledge I haven't been able to find the right source of information.

I'm carrying out this question out towards my design project for level 3. This is the last question I have left wherein I have to calculate the main incoming cable size.

Main incoming Cable size = 95mm2
Load Per Phase = 144A
BS88-2 Fuse = 160A
Max Zs = 0.27ohms
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So 95mm 4 core SWA over 30m - just for fun - I think your continuity tester at 20 degrees C would report :
R1 as 0.8x30x0.47/2 = 5.64 mOhms
R2 (steel) as 0.8x30x1.15(sorry, said it was 1) = 27.6 mOhms
R1+R2 about 33 mOhms

When you mentioned.
R1 as 0.8x30x0.47/2 = 5.64 mOhms

I understood where you got 0.8 as being the correction factor, and 30M being the length, however as for the value of 0.47 and 2

is that fixed value for when it comes to Copper?

and as for the R2,how comes you did not divided it by 2 like that of R1 or is it only specif to R1?
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As you can see in this last question I have answered 10.A and 10.B.

So I have been trying to search the correct calculation to get R1 and R2 for the SWA specifically 95mm2 as this table only shows up to 35mm2 afterwards which I can then answer 10.C and 10.D
 

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Looking at the manufacture cable sizing, for 95mm2 they state the resistance of copper at 20C being 0.193 which this is for the R1

So to find out what SWA resistance would be when using it as CPC. I have to identify what the resistivity of Steel Wire amour is, then from that I can get my R2 for SWA

Is it possible if I can use this value (0.193) as being the R1 and R2 when carrying out the calculation to work the out the Zs value?
 
Hi - yes that could work.

Just checking :
147mm steel / 8 (my estimate) = 18.375mm copper equivalent
95mm copper x 0.193 (manufacturers data) = 18.335mm copper

Scary closeo_O
 

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