Engineer's Notes: fault level and short circuit
Start with the utility data
Every short-circuit study starts with the fault level and X/R the network operator declares at the point of supply, both maximum and minimum. The maximum sets equipment ratings; the minimum checks that protection still trips fast enough.
The transformer sets the LV fault level
Seen from the LV side, upstream HV impedance shrinks by the square of the voltage ratio. The transformer impedance dominates, so transformer size and uk decide whether the LV board needs 50, 65 or 80 kA.
Peak current breaks things
The first peak, including the DC offset, can be over 2.5 times the rms fault current. It sets the making capacity of breakers and the mechanical force on busbars and supports, which grows with the square of the current.
Fault levels grow
New generation, network reinforcement and bigger transformers raise fault levels over the life of a building. Leave a margin and check again whenever a generator is paralleled with the grid.
Frequently asked questions
How do you calculate the fault level at an LV switchboard?
Add the source impedance from the utility fault level, the HV cable, the transformer impedance and the LV busduct, all referred to the LV voltage. Then Ik'' = c x Un / (sqrt(3) x Zk). The transformer impedance dominates, so a 1500 kVA, 6 % transformer gives roughly 35 kA at 400 V.
What is the difference between Ik'', ip, Ib and Ith?
Ik'' is the initial symmetrical rms current. ip is the highest instantaneous peak including the DC offset and sets the making and mechanical ratings. Ib is the current when the breaker contacts part and sets the breaking rating. Ith is the rms current with the same heating effect over the fault duration.
Why does the X/R ratio matter?
X/R sets how slowly the DC offset decays. A higher X/R gives a higher peak current and more DC at contact separation. IEC switchgear is tested with a 45 ms time constant, about X/R 14 at 50 Hz, so higher values need a special check.
Does a generator increase the fault level?
Yes. A synchronous generator running in parallel with the grid feeds its own fault current, typically 5 to 8 times its rated current at first. Every switchboard rating must be checked again before parallel generation is connected.
Why can an LV earth fault be higher than the three-phase fault?
With a Dyn transformer the zero-sequence impedance seen from the LV side is close to the transformer impedance only, while the HV source adds to the positive and negative sequence. Close to the transformer the single-phase fault can therefore equal or slightly exceed the three-phase value.
Topics: fault level · IEC 60909 · Ik'' · peak current · kappa factor · X/R ratio · DC time constant · breaking current · thermal equivalent current · switchgear kA rating · Icw and Ipk · transformer impedance · generator contribution · motor contribution · busbar forces · earth fault current · NER
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