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

Question

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.

mapj1 · Accepted Answer

yes, again the 1mm2 will be fine. As you note most breakers are open, or at least well on the way to open, in a tenth of the assumed period, and even while opening, the voltage drop across the stretching arc is eating into the power available to drive the full PSSC which is therefore falling.. Note that breaker makers sometimes publish a let-through figure I2t specific to the MCBs or whatever, or there is an upper bound in the standards. ABB publish this helpful aide memoire 
 from here The curves are far more revealing than the single figures often quoted, and explain some of the physics. 1) The dotted fuse curves show that at high fault currents, fuses become more or less constant energy devices - the number of joules to melt and the vaporize a fixed length of fusewire is more or less constant so long as you do it fast enough that no energy escapes out the side.... 2) The solid slopes for breakers however show how at high currents they behave almost like constant speed devices - if you double the PSSC you almost quadruple the damage energy. Perhaps unsurprisingly in a device with moving parts where beyond a certain point, it can't go any faster. It is worth realizing that in many ways cables are better protected against really big faults by fuses. 
 (and yes, 1mm2 cable really will be protected by a 60A gL or similar fuse so long as you have available PSSC of 6000A plus, but not in a higher resistance setting.. ) Mike

SRI LANKA · Answer

correct decision

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

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