Can we use the armour of an SWA as the CPC???

[Tester Steve]

Just to add to the debate!

Steve.

 

[malcolmsanford]

To me providing you calculate using the adiabatic equation as in reg 543.1.3 then using the armour is an acceptable CPC and it even gives it in the reg 543.2.2 (v).

This seems to be a modern development same as not using conduit or trunking as a CPC, it is regarded as safer to run a seperate CPC. I have to admit that in most cases now I do in fact run a seperate CPC, I think most design engineers, unless there is a costing reason, design containemnt systems and SWA with the seperate CPC.

I think even most schemes advise that a seperate CPC is used. It is going with the flow I suppose, and certainly the sparks who trained me would be spinning in their graves to hear that seperate CPC's are run, but sometimes I think you have to go with that flow, rather than fight against it.

 

[davelerave]

i've been trying to clear this up(for myself)

there is no reason not to parallel the armour and separate cpc is there?-for SP and 3P SWA on PME

thanks

 

[IQ Electrical]

i've been trying to clear this up(for myself)

there is no reason not to parallel the armour and separate cpc is there?-for SP and 3P SWA on PME

thanks

The separate cpc must be sized as if it alone were to take the earth fault current, it is not permissible to add the CSA's of the two conductors together

 

[davelerave]

The separate cpc must be sized as if it alone were to take the earth fault current, it is not permissible to add the CSA's of the two conductors together

yes-i know,but otherwise there is no reason why they can't be paralleled is there? on pme

 

[IQ Electrical]

yes-i know,but otherwise there is no reason why they can't be paralleled is there? on pme

No reason.

 

[Guest123]

If you are intending to export then no.

 

[davelerave]

If you are intending to export then no.

explain that thanks

 

[Guest123]

If you are intending to export the PME earthing system out to an outbuilding for instance, then no, using both is not an issue.

 

[Steve Richards]

If you are exporting PME earthing out to an outbuilding that HAS extraneous metalwork (=<22000ohms between metalwork and main earth terminal) and you are intending to use the armouring only as an earth conductor then for a:
TNCS system you will need 8 x the required equivalent in copper e.g 10.00 copper required = 80.00mm armouring.
TNS system you will need 4 x the required equivalent in copper.

 

[davelerave]

If you are exporting PME earthing out to an outbuilding that HAS extraneous metalwork (=<22000ohms between metalwork and main earth terminal) and you are intending to use the armouring only as an earth conductor then for a:
TNCS system you will need 8 x the required equivalent in copper e.g 10.00 copper required = 80.00mm armouring.
TNS system you will need 4 x the required equivalent in copper.

thanks

it's been all separate cpc's here for years anyhow

 

[Marty48]

I always thought that the armouring was sufficient to be used as cpc. The issue with the steel rusting on some external cables would suggest incorrect glands used or improperly applied.

 

[sparx24]

The ECA commisioned a study by ERA on this and the answer was you should always use the armour as CPC unless on the very odd occasion the EFLI might be exceeded or there is concern over loss of connection of SWA due to poor maintenance.
If a wired CPC is to be used it should be an additional core within the cable NOT an external run single core.
One reason being under earth fault conditions an external CPC may upset the balance impedance of the swa and can in fact INCREASE the effective impedance of the cable.
If after installation it is found to require external CPC it should be a minimum of 25% csa of phase conductor.
"ECA guide to the wiring regs."

 

[IQ Electrical]

If after installation it is found to require external CPC it should be a minimum of 25% csa of phase conductor.
"ECA guide to the wiring regs."

Which is a contradiction of IEE Guidance Note 8 which states that 'the additional core or separate cpc is sized as if it alone were to take the fault current'

 

[Dave_]

Hello everyone just been reading this thread and i find it very interesting......

I must admit i´m a little surprised that only one person has mentioned using the adiabitc equation for calculating the size of a cpc... It doesnt take long and i have found i have never needed to run a seperate cpc when making calculations for distribution cables. As a general rule though, if the cable is to be run outside or where other external influences are present then i always run a copper earth (not a seperate single core cable, an extra internal core) due to corrosion.

I undertook the 2391-20 design course where a lot of cable calculations were made and i always used the adiabatic, as by using the table at the start of this thread you will generally find that you run in an earth that frankly is oversized. If you want to price a job competitively (im talking larger commercial, industrial jobs) then using the adiabatic is alot more accurate, if were talking about large SWA cables from say 95mm upwards you will find a seperate earth costs a fair bit... 50mm BS6491x say 150m.... NOT CHEAP! a simple calc.. oh its not required so why bother....?

If anyone wants a bit more clarity on this like exactly how the calculation is made (can show examples) then let me know as i would be more than happy to help....!

cheers Dave