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Minimum and Maximum Prospective Fault current calculations

Former Community Member
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) ??
  • 0
    The minimum credible fault current needs to be at least enough to operate the fuse or circuit breaker reliably


    The maximum needs to be  not enough to cause damage to the wiring or fuse or circuit breaker or , in the time it takes the latter it to operate.


    you may find this previous discussion helpful

    regards Mike.
  • 0
    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.
  • 0
    AJJewsbury:

    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).


  • 0
    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.


      - Andy.

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