Two identical 500 kVA gensets running in parallel — how much fault current does each one contribute to a common bus fault, and what does that mean for switchgear rating?
| System voltage | 415 V line-to-line |
| Genset 1 | 500 kVA, Xd'' = 0.12 p.u. |
| Genset 2 | 500 kVA, Xd'' = 0.12 p.u. |
| Example bus fault rating | 15 kA (for comparison) |
The subtransient reactance Xd'' represents the generator's lowest impedance immediately after a fault occurs — the highest current it will momentarily deliver.
This first-approximation method adds each machine's own-base contribution arithmetically, ignoring the (usually small) impedance of the bus and cabling between the two machines.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Fault contribution per generator | n/a (informational) | 5.80 kA each | ✓ PASS |
| Total fault current at common bus | n/a (informational) | 11.59 kA | ✓ PASS |
| Switchgear interrupting rating | ≥ 11.59 kA | 15 kA rated (example) | ✓ PASS |
Key insight: Fault contribution scales with the number of paralleled machines, not just total installed capacity in isolation — adding a third identical generator would push total fault current toward 17.4 kA, which is exactly the kind of check that needs revisiting every time generation capacity is added to a paralleled bus, since existing switchgear may not have been rated for the new total.
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Open Parallel Generator Fault Contribution calculator →Immediately after a fault, a generator's effective internal impedance is at its lowest (the subtransient value, Xd''), producing the highest instantaneous fault current — this is the value protective devices and switchgear must be rated to interrupt, even though the current decays toward a lower steady-state value over subsequent cycles as the transient and subtransient effects decay.
It's a simplifying assumption that's usually conservative for switchgear rating purposes (real bus/cable impedance between machines would slightly reduce the combined fault current), but it does not account for network impedance beyond the immediate bus, so it isn't a substitute for a full short-circuit study when sizing protection for a larger, more complex paralleled system.