Hello, can anyone explain what's going on here with 3 phase fault currents...
From BS7671 Appendix 14 it states, in a 3ph system, the highest prospective fault current occurs with a simultaneous fault across all 3 phases and that an 'approximation' of this is determined by measurement between live and neutral multiplied by 2... i am presuming the measurement between live and neutral being what we'd normally measure with an earth loop impedance meter eg Zs. I do appreciate this factor of 2 is a simplification and over approximation because fault currents (particular 3phase systems) are really quite a complicated beast!
but my query relates from the IET 5th edition Electrical installation design guide [page 81]. The design guide appears to agree that "A fault across the three phases is considered the worst case etc.." but then goes on to give the formula [Ipf] approximately equalling Cmax Uo / Zx + ZD
No mention of the multiplication of 2 or am i getting my wires crossed.
Note 1 from design guide [page 84] gives an example whereby a circuits total line impedence Zpf = sqrt(r^2 + x^2) = 0.0204 ohms ... in simple terms is this the same as Zs?
the design guide goes on to give
Ipf = 1.1 * 230/0.0204 =12.4kA
but from BS7671 appendix 14's version of events, wouldn't Ipf approximate as
Ipf = (1.1 * 230/0.0204) *2 = 28.8kA ?
Thanks