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

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.


  • Well those are good shots for the rogues gallery !!

    That upper board is a super example  of what I meant by 'brige wire' design - complete with charred asbestos or possible glass mat pads, Still it looks like it was installed in the era of PVC insulation , so it maybe rated to some modern-ish spec. Though I note that it is the version where the heads of the screws in the removable holders are not well covered - later ones used dobs of epoxy, but at least one can tell if the fuse wire is failed without opening by a volt test on the exposed live bolt heads once the origina plug has fallen out !  I wonder if the conduit is tight enough  to give  decent Zs with no star washers anywhere. And its a metal box not a plastic one, so that part is to current regs, in a back to the future sort of way.

    The whole thing designed before HASWA, the asbestos regs,  and all the attendant safety stuff was a thing, and is shows rather.

    And that fuse is an advert for installing MCBs or RCBOs in all settings where folk have more enthusiasm than skill....

    But I bet  'its never given us any trouble until you fitted that new RCD thing'

    Mike