Calculating the correct cpc size

[johnduffell]

duplicate, sorry

 

[johnduffell]

I'm not sure the OP understands why the equation exists and what exactly it's trying to prevent. Basically in simple terms it's preventing a high current of short duration from damaging the cable insulation.

The energy per metre of the cable is what heats it and the volume is what buffers the heating due to thermal mass. I.e. twice as much copper takes twice as much heat to get to, say 100c.
This is measured as a constant number of joules "j" per cubic metre of copper required to heat the conductor by the required number of degrees. j joules per metre^3
This is the same as j watt seconds per metre^3
or j volt amp seconds per metre^3
or j ohm amp amp seconds per metre^3.

The resistance of the cable (ohms) is proportional to the length (metres) so we end up with amp amp seconds per metre^2 times another constant "r". Let's make a new constant "k" which is just r.j From that blurb we just derived the adiabatic equation:
CSA = k (I^2)t.

So what is I? the actual current that will flow in the worse case, an earth fault at the origin? ie. the PEFC.
So what is t? the time the current will flow before being cut off. You can read that from your protective device chart. Make sure you read the worse case, not the average.
Can't read t because the disconnection time is less than 0.1 seconds? There is another protective device chart, you have to check the energy class of the MCB, and look up based on the upstream protection. Then you can just read the I2t straight off the chart.

 

[Michael Davis-McCoy]

Hi Chaps,

Just been reading the thread. There seems to be some confusion.

The maximum disconnection time is 0.4 seconds. As it classed as a final circuit and is under 32A. See reg 411.3.2.3

You need the Zs of the circuit. That is the Ze +R1 +R2 for the length.

Then calculate the fault current. The nominal voltage 230/Zs = If

Then you need to check the disconnection time from the protective device curve, or the associated table on the curve figure. t

Now you can do the adiabatic equation.

S = minimum cpc size
If = the result of the above calculation
t = disconnection time from the figure
K = the constant for copper 115

Don't forget the squaring and square rooting. I think you maybe surprised how small the cpc can be calculated to.