Is it okay to consider only phase to phase faults for the end of line fault as it produces the minimum fault. Is there any probability of phase to earth or 3 phase faults getting lower than phase to phase faults.
There's certainly a possibility of L-PE faults having lower fault currents than a L-L fault - TT systems being the classic but even in TN systems a reduced c.p.c. or differing material for the c.p.c. (say steel armour rather than copper) will often result in higher impedance and hence lower currents for a L-PE fault. Also L-PE faults are normally driven by a lower voltage than L-L faults (say 230V rather than 400V) so will result in lower current even if the paths have the same impedance.
This question, together with your other question about a distributed neutral.... I get the feeling your thinking more about transmission system faults rather than 400V system faults?
I tend to think that if the breaker is of current limiting type / fuses, the maximum length of the cable need not be verified. Cable size based on let-through energy is not dependent on the length of the cable. Indeed as the fault location is further away from the breaker, the let-through energy decreases which improves the situation.
I tend to think that if the breaker is of current limiting type / fuses, the maximum length of the cable need not be verified. Cable size based on let-through energy is not dependent on the length of the cable. Indeed as the fault location is further away from the breaker, the let-through energy decreases which improves the situation.
Certainly not always - for most fuses once you get into slower disconnection times the energy let-though can increase with reduced fault currents. Hence the traditional text-book approach of calculating for the furthest point of the circuit.
With MCBs it's usually the start of the circuit that's the worst case - but again not always - in a few cases - e.g. where the fault current is small doesn't trigger instantaneous operation of the MCB but still operates it within the required disconnection time (typically D-type MCBs and 5s disconnection times) you might again be looking at higher energy-let throughs further down the circuit than at the start.
