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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
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  • 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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  • 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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