Outbuilding Submain

[Geoffsd]

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.

 

[silverfox1]

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 .

 

[Richard Burns]

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.