7SR18 Applications Guide
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Cables in particular will reduce the through-fault X/R ratio, as they have small X/R ratios in the 5 to 0.3 range.
Higher voltage single phase cables tending towards the higher figure and lower voltage trifoil cable tend towards
the lower end of this range. If the cable feeder is longer than a few miles then it is fairly safe to use the X/R of the
cable. Using more than one cable per phase will of course reduce this affect and increase charging current. In all
cases it is better to calculate the X/R and fault level of the through-fault if the data is available. Where multiple
sources are present to feed the fault this has a magnifying affect in reducing the overall X/R of the external fault
as the feeder circuit impedance will dominate.
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