7SR18 Applications Guide
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Example - 33kV Earth Fault on Resistance Earthed Cable Feeder
Figure 7.5.2-1 Earth Fault with load bias for Resistance Earthed System
Circuit Parameters:
33 kV Cable Length = 7 km
Load prior to Fault inception = 700 A
Load during Earth Fault ≈ 600 A
Circuit Rating = 800 A
CT Ratio = 1200/1 A
Maximum Feeder Earth Fault level = 800 A (two transformers)
Minimum Feeder Earth Fault Level = 400 A (one transformer)
Charging Current per cable = 4.5 A per km
At the fault point the phase to neutral voltage may not fall significantly and therefore load current will continue to
flow through the radial cable to the load during the earth fault. The load will usually reduce, but the full circuit
rating is used to calculate the fault position on the relay bias characteristic, as this will test the relay setting for the
worst case. At one end of the feeder only the load current will be measured. At the other end the load and the
superimposed fault current will flow.
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