I am not an electrician but have a personal technical interest in understanding the Wiring Regulations.
I have become a bit confused by some contributions on the internet which seem to suggest that the adiabatic calculation will determine the size of CPC that will "cope with" a temperature rise to the maximum temperature permitted for a given cable type.
But as I understand it, it is the insulation which has to tolerate the temperature rise. This rise is caused by the heating effect (IsquaredR) of the current flowing in the CPC (and live conductor) under a dead short earth fault condition and which flows for the time it takes for the overcurrent protective device to disconnect the supply. Exceeding the maximum permitted temperature would permanently damage the insulation which I assume would no longer insulate.
Outside the parameters of the adiabatic conditions I assume that the conductors themselves could tolerate a very much higher and longer duration current, and hence temperature rise, before they themselves would become damaged. The only damage I can think of is that an unprotected conductor would melt like a fuse.
But for the adiabatic conditions of a known maximum level of earth fault current and a known OCPD disconnection time, the equation will determine the minimum size of CPC which would generate a temperature rise in the cable to the maximum permissible temperature for the insulation. In the case of a multi core cable like twin and earth, I assume that for an earth fault the heat contribution from the live conductor is not considered because the contribution from the smaller and hence higher resistance CPC will be much greater and more significant.
I will be pleased to hear from expert readers to confirm or otherwise clarify if I have understood the theory of this correctly . Thanks
