Seperate / Integral earth CPC on 400A supply

[Andy-1960]

This one might get the old grey matter going!

400A supply 260m run on ladder fed from BS88 fuses to sub main distribution board.

Did a cable calc using our Trimble software (was Amtech) and looked at the option of using an integral earth or a separate earth so I could use a conventional 4c armoured instead of a 5c. The programme was happy with an integral earth but threw the calc out when I specified a separate earth even though the conductor size was the same.

I raised this with Trimble / Amtech and below is the response I got back.. prepare yourselves....


For guidance please review the detail below as to why the option to suppliment with a sep. CPC becomes an issue.

When consider using both the circuit armour that is also supplemented with an external\separate CPC.

Generic information:

The issue you raised is likely to relates to the changes in both IET Amd 3 (2015 July) Regs which as well as including introduction of ‘Voltage Derating Cmin factor’ of 0.95 for MaxZs calculation & the changes to the calculation for cable Armouring, parallel armours & supplemented by separate CPC conductor as CENELEC requirements TR50480 & IEC60909 having being incorporated for 2015 (See details below).

………………………………………………

Amendment 3 change includes: Typically *CPC in parallel with swa



The armour resistance is multiplied by 1.1, paralleled with the separate cpc resistance and a reactive impedance of 0.4 milliohms/m added.

In all calculations for the separate cpc with SWA cables, the cpc has the reactive impedance value of 0.4 mΩ/m added.

………………………………………………….



Below is a generic calculation the conclusion is that the introduction of IET Amendment 3 Regulations with its changes previously detail means

that introduction of the reactive impedance for separate CPC will mean that in certain design case this is no longer a solution to aid earth fault disconnection.

Example Calculation

Cable: 90oC, 4 core, SWA, 150mm2, 160m, 3 in parallel.

CPC: 300mm2 90oC single core unarmoured.

Armour Only as CPC

Tabulated armour impedance at 90oC = 0.8636 mΩ/m

GN6 and TR50480 state a factor of 1.1 to be applied to the armour resistance

Factored Armour resistance = 1.1 x 0.8636 = 0.94996 mΩ/m

GN6 and TR50480 also state a reactance of 0.3 mΩ/m to be added to the armour resistance.

Armour impedance = [0.94996 + j0.3] mΩ/m

For a circuit length of 160m impedance = 160 x [0.94996 + j0.3] = [0.1519936 + j0.048] Ω

For 3 in parallel = [0.1519936 + j0.048]/3 = [0.0506645 + j0.016] Ω = 0.05313 Ω

Armour + Separate CPC

Separate CPC impedance = [0.0775 + j0.125] mΩ/m

For a circuit length of 160m impedance = 160 x [0.0775 + j0.125] = [0.0124 + j0.02] Ω

This now has to be paralleled with the armour impedance (3 armours in parallel)

[0.0506645 + j0.016] in parallel with [0.0124 + j0.02] = [0.0119587 + j0.0123869] Ω

TR50480 now states that a reactive impedance of 0.4 mΩ/m is used. this replaces the thus far calculated

reactance

For a circuit length of 160m, reactance = 160 x 0.0004 = 0.064 Ω

Total Z2 = [0.0119587 + j0.064] Ω = 0.065108 Ω

 

[Pete999]

Not sure if you realise Andy. Running a supply to a new building (could be as small as a detached shed) requires the Supply earth you are supplying need sto be large enough to cater for any bonding requirements in the building you are supplying

 

[Andy-1960]

Not sure if you realise Andy. Running a supply to a new building (could be as small as a detached shed) requires the Supply earth you are supplying need sto be large enough to cater for any bonding requirements in the building you are supplying

Hi Pete, Yes, appreciate that, what I could not get my head around was the difference between an integral and supplementary CPC

 

[Pete999]

Hi Pete, Yes, appreciate that, what I could not get my head around was the difference between an integral and supplementary CPC

No I can't get my head around that either, maybe they are thinking you are NOT going to use the armouring as any form of earthing/bonding, which seems silly for a supply of that size, there is a chart somewhere that gives you the equivalent size armour to copper, got it somewhere but not sure where I 'll have a root around.

 

[PEG]

Forgive the numbness,but this is a 3 phase supply,i take it.

 

[Andy-1960]

Forgive the numbness,but this is a 3 phase supply,i take it.

Yes TP&N

 

[Ian1981]

If it's TN-c-s I'd consider running a separate cpc in parallel rather than using an internal core depending if the cpc is to act as a combined cpc/bonding conductor as you would need to consider any effects of diverted neutral currents in the cpc/bonding conductor which may produce heat in the core due to network conditions.
Take a look at GN8 page 58

 

[Ian1981]

• For 70 degree operating temperature

 

[buzzlightyear]

would you not use 4core &separate earth, having that you could
rely on the swa has a cpc.

 

[Andy-1960]

I think there is a bit of confusion here, the thing I was trying to highlight was that the software said it was ok to use a 5c cable with an integral earth conductor, but not ok to use a 4c cable with a supplementary earth of the same size.

 

[Ian1981]

I think there is a bit of confusion here, the thing I was trying to highlight was that the software said it was ok to use a 5c cable with an integral earth conductor, but not ok to use a 4c cable with a supplementary earth of the same size.

Did you try using a half sized cpc in the software in parallel with a 4 core swa?

 

[Andy-1960]

Did you try using a half sized cpc in the software in parallel with a 4 core swa?

Tried full size cpc in parallel with a 4 core SWA and it just would not have it. I think the software was suggesting either one 300mm 5c SWA or 2 x 150mm 5c SWA, but if I tried 4c SWA + supplementary it threw it out on earth loop value.

 

[Ian1981]

Tried full size cpc in parallel with a 4 core SWA and it just would not have it. I think the software was suggesting either one 300mm 5c SWA or 2 x 150mm 5c SWA, but if I tried 4c SWA + supplementary it threw it out on earth loop value.

Why not just use table 54.7 and use a half size cpc rather than a cpc the same size as the live conductors?
That's if you want to install the supplementary cpc

 

[Andy-1960]

Why not just use table 54.7 and use a half size cpc rather than a cpc the same size as the live conductors?
That's if you want to install the supplementary cpc

Because it will not pass - the earth loop value is too high

 

[Ian1981]

Because it will not pass - the earth loop value is too high

Sorry Andy understood
I think I'd go with the 150 x2 rather than pulling a 300mm swa in.

Maybe cable manufacturer can help with regard to cable calcs.