What happens if the CPC is undersized.

Adiabatic just means 'neglecting heat flow' i.e. it is for short events where there is no time for the heat to escape to the environment.

An analogy may be boiling a kettle that takes 1 minute at 3kW, may be assumed to take half a minute at 6kW, but that does not mean it is safe to assume it will boil in 1000 minutes at 3 watts, more likely it will never boil at all, because the heat leaks out faster than that. The 3 watt case is not suitable for adiabatic analysis but the 6kW case is.

Steady state current ratings are based on the environment, so they differ for free air, burial in insulation and other in between states.

Adiabatic ratings are an energy limit - put in this many joules (watts times seconds) and based on the mass of metal and it will heat up so many degrees from resistance heating, if cooling off by heat loss is ignored.

But the limits in the regs are usually based on softening but not burning of plastic insulation, so a temperature rise of no more than perhaps 100 degrees.

Copper melts at over 1000 degrees C however so the step up in power from ' almost singes the insulation' to 'opens up like a fuse' is a power factor of about ten, or a current ratio of about 3 and a bit (power being an I² R thingy).

Those of us who like to have wire fuses that discriminate like a factor of 3 as well - the little one melts and the big one , very briefly gets almost, but not quite, hot enough to singe the paint..


A little undersize and a CPC will work fine. If you have a lot of faults, then after a while maybe the cable insulation nearest the CPC will brown off. A 1mm CPC on 30A fuse is in this category.


Badly undersize and  for some ranges of fault current that will operate the ADS eventually, the CPC gets hot enough to cut the wire like a cheese-cutter, and after the fuse blows,the cable will need replacement. 1mm CPC on 60A fuse is in this category.


Grossly undersize and the CPC blows and the fuse does not notice. A 1mm CPC on a 100A fuse is in this category.


RCDs are your friend, in operating much faster, so limiting the 'heating up' time.

Adiabatic just means 'neglecting heat flow' i.e. it is for short events where there is no time for the heat to escape to the environment.

An analogy may be boiling a kettle that takes 1 minute at 3kW, may be assumed to take half a minute at 6kW, but that does not mean it is safe to assume it will boil in 1000 minutes at 3 watts, more likely it will never boil at all, because the heat leaks out faster than that. The 3 watt case is not suitable for adiabatic analysis but the 6kW case is.

Steady state current ratings are based on the environment, so they differ for free air, burial in insulation and other in between states.

Adiabatic ratings are an energy limit - put in this many joules (watts times seconds) and based on the mass of metal and it will heat up so many degrees from resistance heating, if cooling off by heat loss is ignored.

But the limits in the regs are usually based on softening but not burning of plastic insulation, so a temperature rise of no more than perhaps 100 degrees.

Copper melts at over 1000 degrees C however so the step up in power from ' almost singes the insulation' to 'opens up like a fuse' is a power factor of about ten, or a current ratio of about 3 and a bit (power being an I² R thingy).

Those of us who like to have wire fuses that discriminate like a factor of 3 as well - the little one melts and the big one , very briefly gets almost, but not quite, hot enough to singe the paint..


A little undersize and a CPC will work fine. If you have a lot of faults, then after a while maybe the cable insulation nearest the CPC will brown off. A 1mm CPC on 30A fuse is in this category.


Badly undersize and  for some ranges of fault current that will operate the ADS eventually, the CPC gets hot enough to cut the wire like a cheese-cutter, and after the fuse blows,the cable will need replacement. 1mm CPC on 60A fuse is in this category.


Grossly undersize and the CPC blows and the fuse does not notice. A 1mm CPC on a 100A fuse is in this category.


RCDs are your friend, in operating much faster, so limiting the 'heating up' time.