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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) ??
Cmin and Cmax I presume are the factors for variability of the voltage of the source (differences in transformer tap settings etc). As far as I recall BS 7671 only mentions Cmin and then only in the context of ADS. So current calculations for other purposes as far as I know only need to work from the nominal voltage directly.
Cmin and Cmax I presume are the factors for variability of the voltage of the source (differences in transformer tap settings etc). As far as I recall BS 7671 only mentions Cmin and then only in the context of ADS. So current calculations for other purposes as far as I know only need to work from the nominal voltage directly.
- Andy.
Cmax is used in BS 7671 to determine Im and RA ev in section 722, from Amendment 1:2020.
In most other cases, the nominal voltage value or one using Cmin is OK, BUT I could argue that Cmax should be used to calculate I where adiabatic is used (S being proportional to I).
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.
