Typical fault level

The following earthing systems and design fault currents (3-phase symmetrical) are in use throughout the distribution network.

Voltage	Type of earthing	Fault current	Fault level
132KV	Slid earthing	21.9 KA	5000 MVA
33 KV	Impedance earthed	17.5 KA	1000 MVA
11 KV	Impedance earthed	13.1 KA	250 MVA
6.6 KV	Impedance earthed	21.9 KA	19.4 MVA
< 650V	Slid earthing	27 KA

Where the network is impedance earthed the typical neutral earth resistor will limit the earth fault current to 1000A per transformer, i.e. giving a maximum earth fault level of up to 3kA.

Design earth fault clearance times are:

VOLTAGE    CLEARANCE TIME

132 KV           200ms ( mili second)

33 KV                         500ms

6.6/11 KV       1s

Short circuit current, Isc (kA) at the LV terminals of a 3-phase HV/LV transformer.

Transformer rated power (kVA)	100	315	500	800	1000	1250	1600	2000	2500
Transformer rated (LV) current (A)	137	433	687	1100	1375	1718	2199	2749	3437
Psc = 250MVA	33.1	8.38	13.2	17.4	20.4	21.5	26.4	40.4	49.1