Significance of 2.18ohns

[John-]

Yes

But using the actual disconnection time from your Zs, 95% voltage and when hot.

Thanks for coming back Julie. So i took the measured PSCC at the end of the circuit, 105A. K of 115, CSA 2.5mm
7.49seconds. Looking at the current graph, for a 20A C type MCB is around 18s. It will overheat. Correct? If i downgrade to B type, it,s less than .1. How would i apply the worst case scenario’s you describe?

 

[Julie.]

Thanks for coming back Julie. So i took the measured PSCC at the end of the circuit, 105A. K of 115, CSA 2.5mm
7.49seconds. Looking at the current graph, for a 20A C type MCB is around 18s. It will overheat. Correct? If i downgrade to B type, it,s less than .1. How would i apply the worst case scenario’s you describe?

The issue is to find the worse case, it may be when the Zs is 230V/105A - use the trip time for 105A - compare the time from the adiabatic to the trip time for 105A, but also when the same Zs is on 95% voltage, and hot (1.2x Zs) - which means actually a lower fault current, this would give a longer acceptable time from the adiabatic, but would also result in a longer time to trip.

You would have to calculate both cases as you can't really predict which will be worse. Higher currents are bad from the adiabatic point of view, but result in quicker trip times, so actually can help overall, but in other cases the lower current allows longer adiabatic time, which may actually be better even though it results in longer trip times.

The problem with 105A though, I think it will be the same trip time (or very similar) for both type b and type c

So actually unless you can drop the protection down (16A?), then you must increase the cable size, which not only reduces the Zs, but increases the capacity via the adiabatic.