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What happens if the CPC is undersized.

Ian Cheek
I’ve been thinking about the adiabatic equation, perhaps to much. While I’m quite happy with the use of the formula to calculate the CPC size. My thoughts have turned to what happens if the CPC is undersized.

Google has indicated that the adiabatic equation is something to do with heat. The heat produced in time t, when Ipf current flows.

So my questions is which of the following is the outcome, if the CPC is undersized

1. The heat generated during time t by the Ipf will cause the CPC to act like a fuse, leaving the Line conductor in its fault condition touching the Exposed conductive part.

2. The heat generated during time t by the Ipf will cause the CPC, to heat up to a dangerous value, i.e. melt something or cause a fire.

3. The heat generated during time t by the Ipf will cause the CPC, to increase its resistance, reducing the Ipf, leaving the fault condition on for longer, as the Protective Device is no longer operating on the instantaneous part of its curve.

4. Something else will happen.

5. Google has misled me.

Thank you
  • 0
    Often nothing much will happen, but the consequences of an undersized CPC CAN  result in the consequences described.

    CPC melts.

    CPC gets so hot that it starts a fire.

    CPC gets so hot that it melts the insulation on the live conductors.

    Or the fault current persists for longer than it should and class 1 appliances remain live for long enough to endanger anyone touching one.

    The prolonged fault current causes other problems.


    An undersized CPC will often be fine, but they should be correctly sized for worst case conditions.
  • 0
    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.

  • 0
    mapj1:

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


    So you could use a bit of wet string and all would be well?


    The surprising thing is without RCD how small you can go subject to the let through energy.


  • 0


    4. Something else will happen.


    It will change colour to black due to overheating.
  • 0
    Sparkingchip:


    4. Something else will happen.


    It will change colour to black due to overheating.




    If there is a big fault such as someone drilling through a concealed cable.


  • 0
    Thank you everyone for your replies, they have all been helpful. Looking forward to getting it right, and never seeing a blackened CPC.


    My interest in this topic, is with the current installation. The substation is very close, it’s in the basement of the building. So my Zs at the DB is well below the TN-S value of 0.8. In fact most Zs at the DB are well below 0.1, and within the tolerance of the tester at 0.05. This gives a very low circuit Zs, and therefore some circuits with a very high Ipf, into the 2Kamps.

    While most of the circuits have a CPC parallel paths back through the containment, so from a CPC sizing its only the short link to the socket face that could become undersized for the Ipf, if the fault is downstream of the socket.

    This parallel path of course doesn’t help, as the resistance of the circuit is lower than the R1+R2 resistance would be without the parallel path. In fact it just becomes R1 as R2 tends towards 0. So my thoughts turn to the Line conductor now having the potential of becoming undersized for the fault condition.

    All the circuits have additional protection with RCD’s, so perhaps my worry is for nothing.

    The job has overrun, so is now in the blame game phase, as to whose fault it is amongst the main contractor and the sub contractors. Perhaps I could test out my theory’s and just throw a spanner into the works.

    Once again thank you for your replies.

  • 0
    Appendix B of the On Site Guide gives guidance on minimum CPC size for a selection of fault levels.


    Previous postings have cautioned on the accuracy of loop impedance measured values and the need for careful zeroing out of lead resistance.
  • 0
    2000A is not an especially  high pssc, though above 6000 not all MCBs are happy (check rating) - and the whole point of fuses is that they cut off faster for higher fault currents, so saving your CPC. Breakers do this too of course but only up to a point, then unlike a fuse they stop getting faster with higher fault current, as the core of the actuating electro magnets saturate, and contacts can't be moved any faster.

    The wiring will almost certainly be OK however even in a dead short - and here are very few ways one of those will arise close to the d/b without the cable needing to be changed anyway - once it has been spiked with a silver dagger (or was that vampires ?)  it is considered a bit cheap to tape the damaged ends together hopefully, so it gets changed anyway.

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