Copper equivalence

[JohnW2]

Looking at it from the other direction, why is the k value for steel 51 and not 13 - 115/8.5?

Because k value is not just about resistivity. Far from it.

If this is because of other factors relating to the difference between steel and copper, then why cannot the 2.255 value be used for sizing bonding conductors as well as CPCs - or vice versa.

Because fault currents in CPCs are limited to a very brief duration by protective devices - hence adiabatic conditions pertain, and the 2.25 k1/k2 ratio is appropriate for adiabatic calculations. As for bonding conductors, as I keep saying, I haven't really got a clue as to what currents might flow (e.g. in "bernard scenarios"), or for how long - but the one thing I'm pretty sure of is that the current could flow for far too long for it to be an adiabatic process, so an adiabatic calculation (using any value of k) would probably not be appropriate.

Kind Regards, John.

 

[Risteard]

Having said all that, although I realise it's not a 'regulation', what about the "0.05Ω maximum" guidance for main bonding conductors?

It's not guidance either. Rather, it was a misunderstanding of a statement in Guidance Note 3.

GN3 was suggesting that a negligible resistance be measured (e.g. <0.05 Ohms) between an extraneous conductive part and the connection through e.g. BS 951 clamp to ensure that a satisfactory connection had been made to the extraneous conductive part.

It was never suggesting that a Main Protective Bonding Conductor should not have a resistance exceeding 0.05 Ohms.

 

[ban-all-sheds]

a bonding conductor of infinite impedance would....

...not be a bonding conductor.