Adiabatic equation infinite loop

[Ian1981]

So as promised I eventually looked into this, it seems like we were both right!
I found an interesting thread on IET to explain IET Forums - Understanding the adiabatic equation - https://www.------.org/forums/forum/messageview.cfm?catid=205&threadid=65487&enterthread=y
The gist of it is for fuses the worst case is indeed as you state, where the I is lower the I2t will be higher, so you have to check the distant end.
However for MCBs the situation reverses when the magnetic trip comes into play - when the I is higher the I2t will be higher, so you have to check the head end instead.
Cheers, I learnt something today!

However, after all that, the OPs situation, as far as I can tell, he still needs to use the actual PEFC rather than the Cmin, and he still needs to use the head end and the tabulated I2t values due to the magnetic trip being relevant.
And also not forgetting that the other conductors are also subject to the same process - which may apply if PEFC and PSSC are not the same.

In the case of a disconnection time lower than 0.1 seconds like in the op case, yes he needs the let through energy of device, this can be used to make sure K2S2 is e equal to or greater than the let through value.

 

[Zdb]

I overcame the problem by removing a PIR and having a light switch instead. This increased the circuit length from 3 metres to 12+ metres which increased the Zs.

I've taken all comments on board and in real life would use the manufacturers data for the protective device disconnection times but for the purpose of this project I wanted to stick with the information found in BS7671.

Thanks for all of your replies

 

[johnduffell]

I overcame the problem by removing a PIR and having a light switch instead. This increased the circuit length from 3 metres to 12+ metres which increased the Zs

See the list directly above yours which is spot on.
Based on that please also do the calculation assuming the fault occurs within 1m of the DB. Then make sure the cables still comply with adiabatic.
Bs7671 doesn't contain all the data about class 3 protective device let through, but that information is necessary.