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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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    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.
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  • 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.
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