Main incoming earth bonding

[EFLImpudence]

For non-PME, the minimum CSA of Earthing Conductor is given by (adiabatic calculation OR Table 54.7)

For PME, the minimum CSA of Earthing Conductor is given by the higher of (adiabatic calculation OR Table 54.7) AND (the CSA required for a PME main bonding conductor as given in 54.8 ).

No, it's not the higher of adiabatic OR 54.7

54.7 is only if you don't want to do the equation.

 

[JohnW2]

For non-PME, the minimum CSA of Earthing Conductor is given by (adiabatic calculation OR Table 54.7)
For PME, the minimum CSA of Earthing Conductor is given by the higher of (adiabatic calculation OR Table 54.7) AND (the CSA required for a PME main bonding conductor as given in 54.8 ).

No, it's not the higher of adiabatic OR 54.7 ... 54.7 is only if you don't want to do the equation.

I agree but I'm not saying that it is - maybe I should have put in yet a further level of brackets to make it 'foolproof' for you ... i.e:

higher of {(A OR B) AND (C)}

Is that clearer?

Kindest Regards, John

 

[EFLImpudence]

Not really - I don't think.

54.7 is irrelevant if you can do the equation.

 

[JohnW2]

Not really - I don't think. 54.7 is irrelevant if you can do the equation.

I don't really know how to make it any clearer . You can't really say that '54.7 is irrelevant'. The regs give one a choice as to whether to undertake an adiabatic calculation or use 54.7 - so it's reasonable to assume that at least some will choose each of those available options - so what is in my first pair of ordinary brackets is "(adiabatic OR 54.7)", depending on which approach the 'designer' has chosen to take!

As for "if you can do the equation", that might possibly be part of the issue - since, sad though it is, I'm sure there are a good few electricians (and even more 'electricians', and even more DIYers) who would struggle to undertake a adiabatic calculation - after all there are some who have difficulties with Ohm's Law.

Kind Regards, John

 

[EFLImpudence]

Yes, that is true but not really the determining factor in the sizing of earthing conductors.

It is not the larger of the two choices. It is one or the other.
If you discount the adiabatic for whatever reason then all that is left is 54.7.

It is no more than saying that to size tails you may look at Table 4D1A or (in domestic) use 25mm².

 

[JohnW2]

It is not the larger of the two choices. It is one or the other.

It's not the larger of the 'two choices' as to how one satisfies 543.1 (adiabatic or 54.7) - it is, as you say, 'one or the other', depending upon which approach the designer chose to use. However, if it is PME then it is the higher of the figures needed to satisfy 543.1 (whether determine by adiabatic calculation or 54.7) and the figure needed to satisfy the requirements (for PME) given in 544.1.1.

As an example, if (with PME) your adiabatic calculation said that 6mm² would be adequate, but 54.8 called for a minimum of 10mm², then the higher of those figures (10mm²) would have to prevail - it can't be 'one or the other' here. Conversely (although I can't imagine it would ever/often happen), if your adiabatic calculation said that you needed 16mm², but 54.8 only required a minimum of 10mm², then the higher of those figures (this time the 16mm² from adiabatic calc) would have to prevail

Kind Regards, John

 

[EFLImpudence]

I do see what you are saying.

However, whether PME or not, if anyone decides to use 54.7 then that is all that is needed because (for earthing conductors) 54.7 is never less than 54.8.

 

[JohnW2]

I do see what you are saying.
... However, whether PME or not, if anyone decides to use 54.7 then that is all that is needed because (for earthing conductors) 54.7 is never less than 54.8.

That is true - so, although it is theoretically true that, with PME, one needs to use the higher of 54.7 and 54.8, in practice 54.8 could be said to be 'irrelevant', since it's never going be higher than 54.7.

However, with PME the situation is very different if (as you seem to feel is what what most people "do") once chooses to use an adiabatic calculation, rather than 54.7. In that case, I imagine that the adiabatic calculation will commonly be the thing which is 'irrelevant', since it quite likely to produce a result which is less than is required by 54.8.

Kind Regards, John