Earthing and bonding

[Ben Keer]

Hi all.

Following on from some research into the adiabatic equation, I realised that its ok using it to determine the main earthing size but I couldn't work out whether it gave me a size big enough to also support main bonding from it.

The scenario being if I were wanting to run a submain from my consumer unit in my house to a consumer unit in an outbuilding say which was to have a water service and i needed to have a main bonding conductor connected to the pipework, how could I calculate whether the earth in the submaim would be capable of providing the "main earth " and the "main bonding" combined.
Or am I just overthinking it, in that if the main earth is big enough then surely its big enough to accommodate the main bonding.

Discuss....

 

[Dustydazzler]

Wouldn’t you simply 10mm from the tap to the outbuilding consu unit ...?

 

[Lister1987]

Have you tested the tap to see if bonding is required? You can make an onstall MORE dangerous by installing bonding where not required.

 

[Julie.]

The adiabatic equation is not the answer to everything, it is merely one calculation of many which need to be done, so for example, with a remote service, you may have:
1) load current - given the flc and installation method works out at 1.5mm^2
2) protection device - the fuse/mcb/rcbo's rating may be higher than suitable for the above, so actually 2.5mm^2
3) fault level - in order to operate in say 0.4s the Zs may need to come down, so perhaps 4mm^2
4) the adiabatic equation -say 6mm^2
5) requirement for bonding, perhaps extraneous conductive part, therefore 10mm^2
6) volt drop, due to the length may have to go up to 16mm^2


And so on...

Obviously rather extreme, however when feeding small lighting loads a considerable distance for a hotel for example the cable ends up being stupidly large just for a small load due usually to voltage drop or zs.

 

[Wilko]

Hi - if your supply is PME then Reg 544.1.1 says the supply N sets the size of main protective bonding conductor in accord with Table 54.8 (10mm for 35mm or less N).

 

[Pretty Mouth]

As I understand it:

The min. required CSA of the earthing conductor for the installation can be calculated using adiabatic, PEFC and data for the DNO cut out. For PME, it must also be at least the CSA of any main bonding conductor. 542.3.1

All main bonding conductors would be sized according to the REQUIRED size of this earthing conductor (min 6mm2), or selected according to the size of the PEN conductor where applicable.

The min. CSA of the CPC to the sub main can be calculated using adiabatic, PEFC and data for the device protecting the cable from the MET to the sub main. This is likely to give a smaller minimum than that required for the earthing conductor.

To also use the CPC to the submain as a bonding conductor to any extraneous parts in the outbuilding, it must be at least the CSA of the main bonding conductor.

As Julie points out, there are other factors to consider

 

[Ian1981]

Hi - if your supply is PME then Reg 544.1.1 says the supply N sets the size of main protective bonding conductor in accord with Table 54.8 (10mm for 35mm or less N).

To be more accurate it’s the PEN conductor on the DNo’s side, not the neutral.

 

[Welchyboy1]

Hi - if your supply is PME then Reg 544.1.1 says the supply N sets the size of main protective bonding conductor in accord with Table 54.8 (10mm for 35mm or less N).

so If apparently we must now assume tns has also probably a pme link somewhere along its network nowadays(or soon probably will have) this table should just be used as standard now anyway surely?

and with TT the minimum is 6mm, but if the main rod conductor is unprotected needs to be upgraded(which it nearly always is)

we may as well just use table 54.8 for both tn earthing systems and for the sake of a bit of 10 mm TT systems too?

 

[Ian1981]

so If apparently we must now assume tns has also probably a pme link somewhere along its network nowadays(or soon probably will have) this table should just be used as standard now anyway surely?

and with TT the minimum is 6mm, but if the main rod conductor is unprotected needs to be upgraded(which it nearly always is)

we may as well just use table 54.8 for both tn earthing systems and for the sake of a bit of 10 mm TT systems too?

The minimum for the earthing conductor is 2.5 mm for TT subject to as you mentioned mechanical protection and protection against direct burial and thermal constraints but 6 mm will be more than adequate for bonding