7.13.11Calculating external earth fault loop impedance#
External impedance is the impedance of the circuit from the Electricity Entity’s transformer through the network distribution cables to the electrical installation’s point of supply.
From its network data, the electricity entity should be able to provide:
- transformer type and size, and
- cable type and length between transformer and point of supply.
Using these data and the transformer and distribution cable impedance characteristics can be calculated as indicated. Once the transformer R and X values have been calculated, they can be added to the cable R and X values to determine the total external R and X; hence:
RExternal = RTransformer + RCable1 + RCable2 + .... RCablen
XExternal = XTransformer + XCable1 + XCable2 + .... XCablen
The external impedance at the point of supply is:
ZExternal = √(RExternal² + XExternal²)
From Ohm’s Law, the prospective short circuit current at the point of supply can then be found using ZExternal and the design voltage (400 / √3 V).
Isc = V / ZExternal
