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Friday, 20 July 2018

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











                                                                                           

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