Cpc sizes

[Monkeyblaine]

Is the main reason for having proportional sized cpcs for cables to ensure incase of fault they dont over heat and burn. Say using a 1mm earth for a 25mm sub main in a hyperthetical fault would overheat very quickly? Also "hyperthetically" if you were to use the earth in the neutral terminal say in a standard pendant with lamp and switched it on, would it work? Seeing as neutrals and earths are basically at the same potential and even the same conductor in PME systems. Im sorry for odd questions. I really love to overthink scenarios and what ifs.

 

[telectrix]

you've got the basics right here. most equipment will work connected across L and E. RCDs don't like it though.

 

[Deleted member 26818]

The reason we have CPCs with a smaller CSA than the live conductors, is simply to save money.

 

[telectrix]

i think OP was more concerned about selecting cpc's of sufficient csa to withstand a PEFC without damage. as you rightly point out, spin, sometimes less csa than live conductors simply to save money.

 

[Monkeyblaine]

i think OP was more concerned about selecting cpc's of sufficient csa to withstand a PEFC without damage. as you rightly point out, spin, sometimes less csa than live conductors simply to save money.

But surely having a low enough resistance loop path during fault is the main concern over cost???

 

[telectrix]

as long as the cpc satisfies the adiabatic equation, then it's big enough

 

[Deleted member 26818]

CPCs are required to carry high fault currents for very short durations.
Hopefully, they will never have to do the task they are designed for.
Live conductors on the other hand have to be able to carry low to medium amperage currents for long durations, as well as being able to withstand high fault currents for short durations.
In general, the higher the current, the larger the conductor.
However because the speed of operation of the Circuit Protective Device increases as the current increases.
With CPCs the opposite applies. The higher the 'fault' current, the smaller the conductor.
As such, as long as the impedance of the fault path is low enough to allow the CPD to operate within the specified times, in order to save money, the smallest size of CPC is used.

 

[Monkeyblaine]

CPCs are required to carry high fault currents for very short durations.
Hopefully, they will never have to do the task they are designed for.
Live conductors on the other hand have to be able to carry low to medium amperage currents for long durations, as well as being able to withstand high fault currents for short durations.
In general, the higher the current, the larger the conductor.
However because the speed of operation of the Circuit Protective Device increases as the current increases.
With CPCs the opposite applies. The higher the 'fault' current, the smaller the conductor.
As such, as long as the impedance of the fault path is low enough to allow the CPD to operate within the specified times, in order to save money, the smallest size of CPC is used.

Thanks everyone. Clears everything up!