Because k value is not just about resistivity. Far from it.Looking at it from the other direction, why is the k value for steel 51 and not 13 - 115/8.5?
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.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.
Kind Regards, John.