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Checking if 1.0mm cpc on 2.5mm cable is acceptable for EICR and where the limit is

Alan B

Just want to check my calculations and conclusions are correct.
(been meaning to do this for a while)

Checking if a 2.5/1.0mm cpc is acceptable on a 32A  ring final for an EICR.

In this example looking at data for MK 32 A MCB MK data sheet

In the graphs I am assuming the horizontal is PSCC/PFC. In my example I have a PFC of 700A, therefore I2T = about 5000 A/sec

Energy withstand for 1.0cpc  K2S2 = 115*115 * 1.0 = 13,225

I2t needs to be less than K2S2

5000 < 13,225  therefore I am ok.

If I look at the graph and find I2t of 13,000 it looks like I am ok for pscc/pfc  up to about 3K, bit difficult to read with linear graduation on a log scale.

In reality the B type actually switches in 10msec with 5 * rated current rather than the 100msec typically used in calculations.


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  • 0

    Which tallies with the conclusion we came to when doing the calculations at college twenty years ago.

    ”The general consensus after doing those calculations twenty odd years ago was that a BS60898 B32 MCB could be okay, a BS3036 30-amp fuse will probably be inadequate, but the best option is probably the BS1361 30-amp fuse.”.

  • 0 in reply to Sparkingchip
    Sparkingchip said:
    ”The general consensus after doing those calculations twenty odd years ago was that a BS60898 B32 MCB could be okay, a BS3036 30-amp fuse will probably be inadequate, but the best option is probably the BS1361 30-amp fuse.”.

    Probably also worth considering that a couple of things have changed since those calculations were done (and anyone who did 16h Ed calculations)

    First, we now calculate at 230 V. Second, we should now apply either Cmax (1.1 factor) or Cmin (0.95 factor) to our calculations associated with thermal effects. Which you choose depends on what effect it has on the outcome. BS 7671 requires thermal effects to be considered for all prospective fault currents along the length of a circuit, from the largest at the origin, to the lowest (typically the furthest point of the circuit), which is why the 'line plot' option is always the best approach for fuses, but basically:

    • Cmin is used where we are calculating the lowest prospective fault current (typically the furthest point of the circuit) and has an effect of lowering the lowest prospective fault current by 5 % over the "old" calculations.
    • Cmax is used in calculating the prospective fault current at the origin of the circuit, and has the effect of raising the highest prospective fault current by 10 % over the "old" calculations.

    Usually (but not always) with fuses, it is the lowest prospective fault current that causes us a problem and Cmin would be applied for that.

    With circuit-breakers it's slightly more tricky, but if a circuit-breaker is providing ADS, then typically the breaking time is 0.1 s or less, so we are considering using let-through energy calculation as required by BS 7671, and this typically (but not necessarily always) means that Cmax would provide the worst-case.

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  • 0 in reply to Sparkingchip
    Sparkingchip said:
    ”The general consensus after doing those calculations twenty odd years ago was that a BS60898 B32 MCB could be okay, a BS3036 30-amp fuse will probably be inadequate, but the best option is probably the BS1361 30-amp fuse.”.

    Probably also worth considering that a couple of things have changed since those calculations were done (and anyone who did 16h Ed calculations)

    First, we now calculate at 230 V. Second, we should now apply either Cmax (1.1 factor) or Cmin (0.95 factor) to our calculations associated with thermal effects. Which you choose depends on what effect it has on the outcome. BS 7671 requires thermal effects to be considered for all prospective fault currents along the length of a circuit, from the largest at the origin, to the lowest (typically the furthest point of the circuit), which is why the 'line plot' option is always the best approach for fuses, but basically:

    • Cmin is used where we are calculating the lowest prospective fault current (typically the furthest point of the circuit) and has an effect of lowering the lowest prospective fault current by 5 % over the "old" calculations.
    • Cmax is used in calculating the prospective fault current at the origin of the circuit, and has the effect of raising the highest prospective fault current by 10 % over the "old" calculations.

    Usually (but not always) with fuses, it is the lowest prospective fault current that causes us a problem and Cmin would be applied for that.

    With circuit-breakers it's slightly more tricky, but if a circuit-breaker is providing ADS, then typically the breaking time is 0.1 s or less, so we are considering using let-through energy calculation as required by BS 7671, and this typically (but not necessarily always) means that Cmax would provide the worst-case.

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  • 0 in reply to gkenyon

    The physics cannot have changed, but are you saying that the understanding of it has improved? Or have the circuit protective devices changed?

    I recall one of my tutors saying that cleverer people had decided that a reduced CPC was safe, so just accept it.

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