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Network Fault Level

The fault level at the source of a cable due to all the upstream elements is the network fault level (or external fault level).  Network fault level can be specified either as an apparent power, impedance or fault current and power factor.  The relationship between the various quantities is given by:

Three Phase a.c System

Ik=VA3Un                or        Ik=Un3Ze

VA = 3IkUn        or        VA=Un2Ze

Ze=Un3Ik                or        Ze=Un2VA

Single Phase a.c. System

Ik=3VAUn                or        Ik=Un3Ze        or         Ik=VAULN

VA = IkUn3              or        VA=Un23Ze        or        VA=IkULN

Ze=Un3Ik                or        Ze=Un23VA        or         Ze=ULNIk

d.c. System

Ik=VAUn                or        Ik=UnZe

VA = IkUn           or        VA=Un2Ze

Ze=UnIk                or        Ze=Un2VA

Note: for a.c. systems the values are phasor quantities and the use of absolute values will lead to some error.  The easiest way to take this into account phasor relationships is by carrying out the calculations with numbers in complex form (ensuring that both resistance are reactance are considered appropriately).  

Symbols

Ik               - network/external fault current, A
VA             - network/external fault apparent power, VA or MVA (x by 10-6 to convert to VA)
pf              - power factor of the network/external fault
ULN           - nominal voltage, line-neutral (single phase circuit), V
Un             - nominal voltage, line-line (or absolute for d.c.), V
Ze             - network/external fault impedance, Ω

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