Discuss Can I use the sheath of the SWA as the CPC in this example? in the Electrical Forum area at ElectriciansForums.net

happyhippydad

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Morning..

I have a garage fed by 4mm, 2 core SWA. It is a 25m run from the CU. Can I use the armour as the CPC? The Main house is TT.

I have always used 3 core so have not come across this before. i am about to check my notes as I know I have information on this and will find the formula to work it out but I wanted to start a thread as well in case I miss anything.

Cheers.
 

Pete999

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Morning..

I have a garage fed by 4mm, 2 core SWA. It is a 25m run from the CU. Can I use the armour as the CPC? The Main house is TT.

I have always used 3 core so have not come across this before. i am about to check my notes as I know I have information on this and will find the formula to work it out but I wanted to start a thread as well in case I miss anything.

Cheers.
 

Lucien Nunes

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Re. thread title, no, you cannot use the sheath as a CPC, it's plastic.;)You also ask about the armour, which might be possible. Does anything in the garage require bonding?
 
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happyhippydad

happyhippydad

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Re. thread title, no, you cannot use the sheath as a CPC, it's plastic.;)You also ask about the armour, which might be possible. Does anything in the garage require bonding?
Your first line worried me a little then :D

Nothing requires bonding.

I have now had a quick read and it looks like I need to use the adiabatic to find S. I then use Table 54.7 as the protective conductor is not the same as the line conductor.

That makes sense but do I use 0.1s for t as this is a TT and the RCD will be used for fault protection?
 
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happyhippydad

happyhippydad

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I have also read this article.
Using the armour of SWA[1].pdf
The table on page 4 suggests it will be fine to use the armour for a 4mm cable. However, how do they arrive at there figure of 9.0 in the column for min CSA of SWA as they do not have a Ze or t?
 

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Pete999

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There is a table in either the ODG or BS7671 relating to the size of steel armouring to CU CONDUCTORS
 

westward10

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Manufacturers produce two core cable so you would have to assume in most if not all circumstances that the armour is adequate for use as a cpc. You have a TT system so the chances of it not being adequate is slim to none. Of course you should prove this and disconnection times are 1s for distribution circuits and 0.2s for all other.
 

davesparks

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I have also read this article.
Using the armour of SWA[1].pdf
The table on page 4 suggests it will be fine to use the armour for a 4mm cable. However, how do they arrive at there figure of 9.0 in the column for min CSA of SWA as they do not have a Ze or t?
That table has been doing the rounds for years, and is only partially correct at best.
It uses the table 54.7 method to determine the required size of cpc.
 
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happyhippydad

happyhippydad

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With regards that table... How can he just give a fixed 'yes' or 'no' dependant solely on the CSA of the cable (and the amount of cores)?

The table he bases it on 54G is now table 54.7 on page 198 of the BBB. That table uses the value of K1, K2 and S (assuming the CSA of the line conductor is <16mm as in my example). All of these are variables not constants so you cant give fixed values based solely on CSA of the conductors of the SWA.

For example... K1 and K2 vary depending on the cable being thermoplastic or theromosetting and even more of a variable is S as this depends on I and t??

Edit... I've read the tables a little more now and see there are different tables for thermoplastic/thermosetting etc so that just leaves S?
 

Baddegg

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Apologies if I’m hijacking @happyhippydad, isn’t there a certain swa cable size that most all cables under that size would comply for use of swa armour as cpc?....
 

westward10

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Apologies if I’m hijacking @happyhippydad, isn’t there a certain swa cable size that most all cables under that size would comply for use of swa armour as cpc?....
Two core, 120mm starts to become the size where Table 54G may not be complied with. The three and four cores are larger than that but without looking can't remember.
 

davesparks

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With regards that table... How can he just give a fixed 'yes' or 'no' dependant solely on the CSA of the cable (and the amount of cores)?

The table he bases it on 54G is now table 54.7 on page 198 of the BBB. That table uses the value of K1, K2 and S (assuming the CSA of the line conductor is <16mm as in my example). All of these are variables not constants so you cant give fixed values based solely on CSA of the conductors of the SWA.

For example... K1 and K2 vary depending on the cable being thermoplastic or theromosetting and even more of a variable is S as this depends on I and t??

Edit... I've read the tables a little more now and see there are different tables for thermoplastic/thermosetting etc so that just leaves S?
S is the CSA of the line conductor so is the value you are using as the basis for the selection of the CPC.
 

Bellendian

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None of my wholesalers stock 2core SWA. Seems to suggest the lack of understanding of the use of armour as cpc is widespread...I got funny looks asking for 2 core, turned into a sheep and use 3 core with sleeving!
 

Dustydazzler

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I think originally 3c swa was really only meant for 3 phase machines etc

If you just required line and neutral you used 2c swa

I think once swa started being used more domestically it all changed and home electricians and DIYer s would use 3c because they could connect line neutral and earth using cores and not having to use the armour as an earth
 

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