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What happens if the CPC is undersized.

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

Parents

0 Ian Cheek over 4 years ago

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.
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0 Ian Cheek over 4 years ago

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.
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What happens if the CPC is undersized.

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