2 core armored cables

[kiggie89]

i came a cross a 16mm two core cable today and they want an other small supply taking off if. can i still use it? as i got told, you can only use the armor for an earth if the cable size is no bigger than a 6mm. i was thinking can i install an earth rod and RCD's to get round this problem
thanks​

 

[Sintra]

i came a cross a 16mm two core cable today and they want an other small supply taking off if. can i still use it? as i got told, you can only use the armor for an earth if the cable size is no bigger than a 6mm. i was thinking can i install an earth rod and RCD's to get round this problem
thanks​

I think you have been misinformed. The SWA of a 16mm 2 core should be fine to use as a CPC.


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[Canoeboy]

+1 See the pdf in this post

 

[mechelec]

For a 16mm^2 armoured cable the minimum required CSA of armour for use as a circuit protective conductor is 49.74mm^2

A bs 5467 or bs 6724 (XLPE) 2 core 16mm^2 SWA cable has 42mm^2 of armour so would be unsuitable for use as a circuit protective conductor

A BS 6346 2 (Low Smoke Zero Halogen) core 16mm^2 SWA cable has 46mm^2 of armour so would also be unsuitable as a circuit protective conductor

 

[yellowvanman]

CSA also depends on protective device and disconnection time - see Adiabatic Equation section 543.1 of 17th Edition, that will give you a more accurate calculation of what the minimum CSA of the armour needs to be.

 

[IQ Electrical]

CSA also depends on protective device and disconnection time - see Adiabatic Equation section 543.1 of 17th Edition, that will give you a more accurate calculation of what the minimum CSA of the armour needs to be.

Good man!

There's often a bit more to it than 'it doesn't comply'

 

[Canoeboy]

For a 16mm^2 armoured cable the minimum required CSA of armour for use as a circuit protective conductor is 49.74mm^2

A bs 5467 or bs 6724 (XLPE) 2 core 16mm^2 SWA cable has 42mm^2 of armour so would be unsuitable for use as a circuit protective conductor

A BS 6346 2 (Low Smoke Zero Halogen) core 16mm^2 SWA cable has 46mm^2 of armour so would also be unsuitable as a circuit protective conductor

Where did you get 49.74 from ?

 

[Electrifying1]

I've never heard what mechelec said either, some companies ask for a separate cpc with the larger armoureds.

 

[Guest123]

The PDF in post #3 is copied from Guidance Note 1 by The IEE, and supports the use of the armour as a CPC where it determines it to be large enough.

Of the 2 types of SWA (PVC & XLPE), only one of the 16mm 2 core cables is suitable for use as a CPC so you need to be careful if you do decide to use the armour about which type you have.

 

[yellowvanman]

If you read the ECA book - Guide To the Wiring Regulations, they basically say that most SWA cables don't need an external cpc, and say that the SWA is sufficient. If I remember correctly anything up to 400mm would be Ok and the 120mm size is marginal and should be OK unless its used at full current carrying capacity.

Again if you use the Adiabatic equation that should prove it!

 

[Marvo]

If you have an SWA cable where the armour falls short of requirements to be used as CPC can you just make up the difference by having the armour combined with a small external earth wire terminated on an earth ring with the SWA gland or must you use either the armour or an extraneous earth wire alone.

 

[IQ Electrical]

If you have an SWA cable where the armour falls short of requirements to be used as CPC can you just make up the difference by having the armour combined with a small external earth wire terminated on an earth ring with the SWA gland or must you use either the armour or an extraneous earth wire alone.

The additional cpc must be sized as if it alone was the cpc. Guidance Note 8

 

[Marvo]

Thanks. I don't have a copy of the UK regs to refer to I'm afraid.

 

[mechelec]

Where did you get 49.74 from ?

from the nic technical manual and if someone can tell me how to insert a PDF file I will attach it

 

[Canoeboy]

from the nic technical manual and if someone can tell me how to insert a PDF file I will attach it

Did you read the correct column as the figure I have is different ?

To post a pdf when you reply to a post, scroll down to MANAGE ATTACHMENTS and click this then browse on your hard drive to the PDF file then click UPLOAD

 

[mechelec]

QUOTE=Canoeboy;208748]Did you read the correct column as the figure I have is different ?

I may have missed something but I hope the attached is correct and answers your question.


View attachment armour.pdf

 

[IQ Electrical]

QUOTE=Canoeboy;208748]Did you read the correct column as the figure I have is different ?

I may have missed something but I hope the attached is correct and answers your question.


View attachment 3307

You haven't missed anything as such but the table only indicates a non-compliance with table 54.7 whereby you can then use the adiabatic equation to size your cpc which invariably results in a lower required CSA by calculation.

 

[malcolmsanford]

View attachment SWA-pvc-xple cpc comparision.pdf

The table attached shows the same but on this one as it mentions that you can use the SWA as a CPC providing the adibiatic equations is proved and in most cases should be as IQ as pointed out

 

[mechelec]

You haven't missed anything as such but the table only indicates a non-compliance with table 54.7 whereby you can then use the adiabatic equation to size your cpc which invariably results in a lower required CSA by calculation.

Yes you can use the adiabatic equation but I was asked where my figure of 49.74 came from. The table will not give the most economical solution in all cases but it is an easy reference chart and always works on the side of caution. This means that at least your CPC will not be undersized. You would still be advised to use the equation to check though.