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Topic Title: Combined cpc and bonding conductor
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Created On: 20 April 2017 08:27 PM
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 20 April 2017 08:27 PM
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Ian1981

Posts: 8
Joined: 28 January 2017

I have a 25mm 4 core swa 3 phase supplying building 2 fed from the main building 1.
The swa Is to be the cpc and bonding conductor as building 2 has structural steel that needs bonding back to the MET.
Guidance note 8 suggests a 4 core swa at 70 degree swa CSA is 70mm steel.
This comes out at 8.23 copper equivalent.
The earthing arrangement is tn-s I have a 16mm earthing conductor at the origin.
Normally I would then select 10mm as my bonding conductor but as the swa is less than 10mm it's not an option.
When carrying out the Adiabatic equation I get a size of 5mm required for the earthing conductor ze was 0.14 ohms.
As it's tn-s can I then base my bonding conductor as 6mm minimum as the earthing conductor need only be 5mm even tho I have 16mm in place for compliance to bs 7671 then the armour CSA will comply on the swa or must I keep it as 10mm minimum
 20 April 2017 10:42 PM
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mapj1

Posts: 9383
Joined: 22 July 2004

Well checking table I often use here
gives rather different numbers, or I have misread your data.

Note the steel to copper equivalent ratio is not a constant between PVC and XPLE jacketed cables as the numbers used in the adiabatic equation are different, and neither number is anything like the same as simply the resistance ratio you may imagine.

For 4 core XPLE of copper core 25mmsq the jacket is 50mmsq of steel - which in terms of adiabatic performance is electrically equivalent to 16mm sq copper.

It may be worth a peruse of John Peckham's note on SWA CPC ratings copied here, oddly enough

and then in the regs
TABLE 54.7
Minimum cross-sectional area of protective conductor
in relation to the cross-sectional area of associated line conductor


and

TABLE 54.4
Values of k for protective conductor as a sheath or armour of a cable

I think it would be OK.

-------------------------
regards Mike


Edited: 20 April 2017 at 11:04 PM by mapj1
 20 April 2017 10:54 PM
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AJJewsbury

Posts: 15855
Joined: 13 August 2003

Guidance note 8 suggests a 4 core swa at 70 degree swa CSA is 70mm steel.
This comes out at 8.23 copper equivalent.

That sounds like equivalent as in having the same resistance - by the letter of the regs that's only required for bonding in PME systems (or when applying the 25mm2 upper limit). If your system is really a guaranteed TN-S system (e.g. private transformer) then the requirement is just half the c.s.a. of what's required for the earthing conductor (which in turn can usually be calculated down using the adiabatic) regardless of the material.
- Andy.
 20 April 2017 11:14 PM
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mapj1

Posts: 9383
Joined: 22 July 2004

Actually you only use the unmodified resistance ratio when considering CPC sizes for bonding on TNC-s, although it comes into the value of Zs of course.


Instead for sizing bonding TN-S and TT it is K1/K2 times the physical steel cross-section that you need to consider- the K terms include both the heat capacity
(temperature rise per watt-second if you like) and the resistance, not just resistance alone.
The heat capacity of steel is about 500 joules per degree rise per kilo, as opposed to 350-400 or so for near pure copper.

Edited in the light of error realised during lightbulb moment further down thread.

-------------------------
regards Mike


Edited: 20 April 2017 at 11:50 PM by mapj1
 20 April 2017 11:17 PM
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AJJewsbury

Posts: 15855
Joined: 13 August 2003

Actually you don't use the unmodified resistance ratio when considering CPC sizes even on TNC-s

But you would for main bonding conductors under PME - which I think is where the OP started from.
- Andy.
 20 April 2017 11:46 PM
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mapj1

Posts: 9383
Joined: 22 July 2004

Ah yes you are quite right, as diverted neutral current is a steady state thing , not fast and adiabatic.
Its those wretched parallel universe regs.
apologies.
you do need to verify the resistances for PME.

TABLE 54.8
Minimum cross-sectional area of the main protective bonding conductor in relation to the neutral of the supply NOTE: Local distributor's network conditions may require a larger conductor.
....
The minimum copper equivalent cross-sectional area is given by a copper bonding conductor of the tabulated cross-sectional area or a bonding conductor of another metal affording equivalent conductance.


-------------------------
regards Mike
 21 April 2017 06:47 AM
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Ian1981

Posts: 8
Joined: 28 January 2017

Earthing system is 100% tn-s as originally the building had lost its earthing connection
The DNO where called out and repaired.
As building has exposed structural steel they will not provide tn-cs earthing.
I've just used guidance note 8 which suggests the armour swa is 70 mm steel.
Using the ratio 8.5 from gn8 it comes out at 8.23 copper equivalent.
Here lies my problem. The armour is ok as a stand alone cpc but I want it to be a bonding conductor also.
Earthing conductor at the origin only needs to be 5 mm using Adiabatic equation.
So can I size bonding conductor to half the size required of the earthing conductor subject to the
Minimum of 6mm?
I'm aware I can if needed I can tt the other building but would rather not
I've read John Peckham data but only backs up what I already have found i.e. The copper equivalent of the swa armour is not enough
Niceic technical suggests I needed 10mm for bonding and at least 5mm for earth which means I required 15mm which I think they are getting the wrong end of the stick.
Thanks.

Edited: 21 April 2017 at 07:28 AM by Ian1981
 21 April 2017 10:57 AM
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AJJewsbury

Posts: 15855
Joined: 13 August 2003

Earthing conductor at the origin only needs to be 5 mm using Adiabatic equation.
So can I size bonding conductor to half the size required of the earthing conductor subject to the
Minimum of 6mm?

For a true TN-S system I believe that's correct.
Niceic technical suggests I needed 10mm for bonding and at least 5mm for earth which means I required 15mm which I think they are getting the wrong end of the stick.

Your protective conductor needs to satisfy the requirements for both a c.p.c. and a bonding conductor simultaneously - not their sum total. E.g. a 10mm2 Cu conductor would satisfy both the 5mm2 and 10mm2 requirement.

10mm2 Cu bond is probably based on the assumption that the supply is PME.

Earthing system is 100% tn-s as originally the building had lost its earthing connection
The DNO where called out and repaired.
As building has exposed structural steel they will not provide tn-cs earthing.

Are you absolutely sure about that? Almost all mainland public supplies to multiple premises these days are PME. Even if the cable feeding your particular installation has a separate earth conductor the overall system is still PME as there will be N-PE connections in the street (almost all repairs and alterations use CNE techniques). What most DNOs do for multiple supplies to steel buildings is to have one point where all the supplies derive from and run, have one N-PE link there, and run cables with separate earth wires from that point to each consumer - that avoid problems with diverted N currents running between consumers in the same building via the steelwork, but the overall system is still PME. E.g. see - http://library.ukpowernetworks...on+Earthing+Design.pdf (your DNO may vary of course, but they all tend to be based on the same underlying principles).

- Andy.
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