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pete89987

An outbuilding is fed from a separate single way board in the house and has a 2c 4mm SWA to the building of 7m. Its a TNS. This outbuilding has its own water main (albeit in plastic) which comes directly from the water meter not through the house. Testing between copper pipes below the sink and the SWA earth gives 400Ohms

Given that Main protective bonding should be continuous and 6mm, am I correct in thinking that strictly speaking I should make this TT, or would anyone bond the pipes to the local CU earth terminal?
 
If the metallic water pipe is an extraneous conductive part, as it appears to be, then it should be bonded back to the MET at the origin, which would require a suitably sized conductor back to the MET. Bonding the pipes to the local CU would be a form of supplementary bonding and would not meet the requirements for main bonding.
Separating the earthing systems would permit the local bonding arrangement.
 
The main bond doesn't have to be continuous and

the armour of 2core 4mm is greater than 6mm Cu equivalent or

fit more plastic pipe.
 
The main bond doesn't have to be continuous and

the armour of 2core 4mm is greater than 6mm Cu equivalent or

fit more plastic pipe.

I think you'll find that 19 or 20mm (csa) of steel, is nowhere near the equivalent of 6mm of copper!!
It's more than adequate for CPC purposes, but NOT for main bonding....
 

The calculation for a bonding conductor is based solely on resistivity, whereas cpc's take into account temperature coefficients and specific heat capacity as they are intended to carry fault current.
This means that a steel bonding conductor would need to have 8.5 times the csa of a copper bonding conductor, since the resistivity of steel is about 8.5 times that of copper. The 19mm of steel in the armour would therefore be equivalent to about 2.2mm² copper for a bonding conductor.
 
I am a bit confused.

If the swa is capable of handling fault current and so suitable as a CPC, why is it not similarly suitable for a bonding conductor?


Using Richard's 2.2 the run of 7m in this thread would mean that the resistance of the swa is less than 0.06ohms.
 
I am a bit confused.

If the swa is capable of handling fault current and so suitable as a CPC, why is it not similarly suitable for a bonding conductor?


Using Richard's 2.2 the run of 7m in this thread would mean that the resistance of the swa is less than 0.06ohms.
Bonding conductors and earthing conductors are being used for entirely different purposes.
Earthing reduces the duration of touch voltages, bonding reduces the value of touch voltages.
A conductor designed to carry a large current for a short time will be differently specified than a conductor designed to carry minimal current and simply provide a low resistance path to the MET (unless carrying load current in a neutral disconnected PME supply which is why the bonding conductors tend to be larger csa in that situation).
 
Bonding conductors and earthing conductors are being used for entirely different purposes.
Earthing reduces the duration of touch voltages, bonding reduces the value of touch voltages.

That sum's up the reasoning perfectly!! lol!!

Also seen at the bottom of all Wigglers Posts
 
Bonding conductors and earthing conductors are being used for entirely different purposes.
Earthing reduces the duration of touch voltages, bonding reduces the value of touch voltages.
Yes, I know that but I do not see the significance in this respect.


A conductor designed to carry a large current for a short time will be differently specified than a conductor designed to carry minimal current and simply provide a low resistance path to the MET
Precisely, so, if the swa is suitable for the fault current as CPC, why is it not suitable for a bonding conductor of half the c.s.a?

No one (including me) has asked what the PFC is nor what is the actual c.s.a. required for the EC and therefore MPB of the installation.
(I realise 6 is the minimum allowed)

Surely you are not saying that for the swa to be used as bonding it has to be 51mm (6 x 8.5).
Why is k1/k2 not applicable?

(unless carrying load current in a neutral disconnected PME supply which is why the bonding conductors tend to be larger csa in that situation).
It is TN-S.
 
Reg 544.1.1 which covers TN-S states main protective bonding conductor if not copper must have a cross-sectional area affording equivalent conductance as Richard mentioned with his 8.5 figure for steel .
 
Yes, I know that but I do not see the significance in this respect.

If you were to say this length of fence wire is suitable for holding up a fence, why is it not suitable for using as a bonding conductor, you would be asking a similar question.
The current handling capabilities of a cable are irrelevant when considering bonding as bonding is not intended to carry current.



Precisely, so, if the swa is suitable for the fault current as CPC, why is it not suitable for a bonding conductor of half the c.s.a?

No one (including me) has asked what the PFC is nor what is the actual c.s.a. required for the EC and therefore MPB of the installation.
(I realise 6 is the minimum allowed)

Surely you are not saying that for the swa to be used as bonding it has to be 51mm (6 x 8.5).

To use a steel conductor as a bonding cable as silverfox1 says it must have equivalent conductance so yes 51mm² would be suitable in this case.
Obviously it is much easier to use a 6mm² copper conductor.


It is TN-S.
Why is k1/k2 not applicable?
Cpc's take into account temperature coefficients and specific heat capacity of the material of the conductor to assess how hot the cable can get without being a danger, as they are intended to carry fault current. This is the origin of the k values given in the regulations.

The calculation for a bonding conductor is based solely on the conductance of the material this is where the 8.5 for steel and 1.68 for aluminium come in.
I do not know why the bonding is sized in relation to the minimum earthing conductor csa but those are the regulations that apply to the size of bonding conductors.
 

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