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Minimum and Maximum Prospective Fault current calculations
Former Community Member
hello
I am studying IET Electrical Installation and Design Guide 4th edition. and I am confused about using Cmin and Cmax to calculate Ipf and Ief. When do we use Cmin and Cmax to calculate the Minimum Prospective fault current (Ief) ??
Cmax is used in BS 7671 to determine Im and RA ev in section 722, from Amendment 1:2020.
Ah, good spot - I'd not noticed that one!
BUT I could argue that Cmax should be used to calculate I where adiabatic is used (S being proportional to I).
Humm, but not just proportional to I - there a t in there too and t tends to vary with I. For MCBs I'd agree that t doesn't reduce much as I increases so using Cmax is likely to give the worst case, but for fuses t tends to decrease quite rapidly as I increases - so often the worst case energy let-through is for lower fault currents - so perhaps in some cases Cmin would be the defensive choice.
Cmax is used in BS 7671 to determine Im and RA ev in section 722, from Amendment 1:2020.
Ah, good spot - I'd not noticed that one!
BUT I could argue that Cmax should be used to calculate I where adiabatic is used (S being proportional to I).
Humm, but not just proportional to I - there a t in there too and t tends to vary with I. For MCBs I'd agree that t doesn't reduce much as I increases so using Cmax is likely to give the worst case, but for fuses t tends to decrease quite rapidly as I increases - so often the worst case energy let-through is for lower fault currents - so perhaps in some cases Cmin would be the defensive choice.
