Ben asks this question about whether he can calculate the supply impedance of a system comprising LV panels and supply transformers as his meters are not rated to the currents concerned. It has been answered by the ECA (Theme – Distribution Equipment including Cable Management):
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Question: We’ve been asked as a company to service low voltage panels for J.Sainsbury’s depots across country.
Some of the supply transformers are in excess of 2,000 kVA rating, with the fault current exceeding 45 kA at full load.
Our testing equipment is the standard Megger brand multimeter and is not rated to these fault currents – i.e. the worst-case scenario. It has also been deemed not possible to isolate these supplies to gather a true Ze reading (disconnect main equipotential bonding conductors) and, remembering back in my past, it was possible to calculate it, knowing the Z values of the cable and Z value of the transformer from the nameplate.
I would like your knowledge on this - to have the known impedance of the supply without having to measure using a meter.
Answer: The values of the transformer ratings, per unit impedance and phase / line voltage will usually be mentioned on the transformer rating plate / data sheet. Take a look at that.
As an example, a 500 kVA, 11 kV / 400 V three-phase / 50Hz transformer having 5% P.U. impendence will have the following fault levels on the secondary side:
Fault level = 500 / (5%) = 10,000 kVA
S = 1.732 * VPP * IP
Fault current = 500 / (5% x 400 x 1.732) = 14.4 kA.
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