In short. A diesel generator supplies far less fault current than the DISCOM transformer it replaces, and the current decays within cycles. The first-cycle value is set by the alternator’s sub-transient reactance and is typically five to ten times rated current. Within a second it falls to a sustained value set by the excitation system: about three times rated current with PMG excitation, much less with shunt excitation. Protection must be assessed against this decaying contribution.
Why fault current on a generator is a different problem
On a utility supply the source behind the 415 V board is a large transformer on an 11 kV network with a fault level in the hundreds of MVA. The fault current is high and roughly constant for as long as the fault persists. A generator is a rotating machine whose internal reactance is not constant. In the first cycle after a short circuit the damper and field windings hold the flux, so the current is high. As the damper currents die away the effective reactance rises and the current falls. As the field flux decays it falls again, and what remains depends on whether the excitation system can force field current while terminal voltage is collapsed. The same board therefore sees two different fault environments depending on the source, and the protection must work in both.
The three periods of a generator fault
| Period | Governing reactance | Typical duration | Typical magnitude | What it decides |
|---|---|---|---|---|
| Sub-transient | X”d, usually 0.10 to 0.20 pu | First 1 to 3 cycles | 5 to 10 times rated current | Breaker making capacity, peak forces on busbars, instantaneous relay pick-up |
| Transient | X’d, usually 0.15 to 0.35 pu | Up to about 1 second | 3 to 6 times rated current | Breaker breaking capacity, short-time relay elements |
| Sustained | Xd with excitation response | Until cleared | About 3 times rated for PMG or forced excitation for up to 10 seconds; may fall below rated for shunt excitation | Time-delayed overcurrent, fuses, downstream MCCB thermal elements |
The values are typical of low-voltage salient-pole alternators on diesel sets and must be confirmed from the datasheet for any real study. The alternator reactance and short-circuit page explains where each figure comes from.
Estimating the first-cycle current
I”k = In ÷ X”d
Here I”k is the initial symmetrical three-phase fault current at the terminals, In is the alternator rated current and X”d is the saturated sub-transient reactance in per unit on the alternator’s own base. Rated current is kVA multiplied by 1,000 and divided by √3 times the line voltage. Consider a 500 kVA, 415 V set with a quoted X”d of 0.14 per unit (illustrative). Rated current is 696 A, so the initial symmetrical fault current is about 4,970 A, roughly 5 kA. The asymmetrical first-half-cycle peak can be 1.5 to 1.8 times higher because of DC offset, which is the figure for breaker making duty and busbar bracing. A 1,000 kVA, 5 percent impedance utility transformer on the same board delivers about 28 kA sustained; the generator supplies less than a fifth of that, and only for a few cycles. The generator short-circuit contribution page and the IEC short-circuit calculation page take the same method through to a full study under IEC 60909.
What the excitation system does to the sustained value
The sustained fault current is where generators differ most. A shunt-excited alternator takes its field power from its own terminals. When a close-up fault pulls the voltage down, the AVR loses its supply and the field collapses, so the sustained current may fall below rated current within a second or two. A time-delayed overcurrent relay set above rated current then never operates, and the fault persists until something burns. A PMG-excited alternator, or one with an auxiliary winding for fault support, keeps the AVR supplied regardless of terminal voltage and holds around three times rated current for up to ten seconds, long enough for downstream protection to clear the fault. For any generator feeding a distribution system with more than one level of protection, we regard PMG or equivalent excitation as the correct specification. The PMG excitation page covers the hardware.
Fault current away from the generator terminals
The figures above are terminal values. Every metre of cable between the generator and the fault adds impedance and reduces the current further, so a 415 V fault at the end of a long run to a remote board may draw little more than a heavy overload from the generator. The protective device there must still see enough current to operate within the time the cable can tolerate. The cable sizing guide deals with cable thermal withstand, and the protection coordination guide explains how to confirm that the minimum fault current still reaches the device’s pick-up. Minimum fault current, not maximum, is usually the limiting case on generator supply.
Multiple generators and other sources
When sets run in parallel their contributions add at the common bus. Four 500 kVA sets with the same X”d deliver about four times the single-set first-cycle current, approaching 20 kA in the example above, and the switchgear must be rated for the largest combination that can be online. Large induction motors also feed current back into a fault for a few cycles, and UPS or BESS inverters contribute a small, current-limited amount. All of these belong in the maximum-duty case and are excluded from the minimum-duty case used for protection sensitivity.
Which value to use for which decision
- Breaker making capacity and busbar bracing use the first-cycle asymmetrical peak from the sub-transient value, including all parallel sources and motor contribution.
- Breaker breaking capacity uses the symmetrical current at the contact-parting time, which for a low-voltage breaker opening within a few cycles sits between the sub-transient and transient values.
- Instantaneous and short-time relay elements are checked against the sub-transient and transient values to confirm they pick up but remain selective.
- Time-delayed overcurrent and fuse operation are checked against the sustained value from the weakest source, usually a single generator at the end of the longest cable.
- Earth-fault current depends on the neutral earthing method rather than on the alternator reactances alone, as explained on the earthing and neutral grounding page.
Related photographs and videos
Common mistakes
- Applying the utility fault level to a board that will also run on generator, so the protection is set for a current the generator can never deliver and faults are not cleared.
- Using a generic multiplier such as ten times rated current for every machine, which overstates the current from large low-reactance alternators and understates the risk from shunt-excited ones.
- Forgetting that parallel sets add, and installing switchgear rated for one generator on a bus that can see four.
- Specifying shunt excitation on a set that feeds a distribution network, so the sustained fault current collapses below relay pick-up.
What to send us
Send a photograph of the alternator nameplate and, if you have it, the alternator datasheet showing X”d, X’d, Xd, the time constants and the excitation type. Add the single-line diagram with cable sizes and lengths, the number of sets that can run in parallel, the ratings of the existing breakers and any fault level letter from the DISCOM. Send these through the application engineering enquiry and we will tell you whether a short-circuit study is needed.
Questions engineers ask
How much fault current does a 500 kVA diesel generator produce?
With a typical sub-transient reactance of 0.12 to 0.16 per unit the first-cycle symmetrical current is about 4 to 6 kA at 415 V, falling within a second towards roughly three times rated current, or about 2 kA, if the alternator has PMG excitation. A shunt-excited alternator may sustain much less. The exact figures must come from the alternator datasheet.
Why does generator fault current decrease during the fault?
The alternator’s effective reactance increases as the damper winding currents and then the field flux decay after the fault begins. The current therefore falls from the sub-transient value through the transient value to a sustained level set by how hard the excitation system can force the field while the terminal voltage is depressed.
Can I use the same breakers on generator and utility supply?
Usually the breakers are rated for the higher utility fault level and are therefore adequate for the generator in terms of making and breaking duty. The question is whether their trip settings still operate on the much lower generator fault current, especially at the end of long cables. That requires a coordination check for the generator operating mode.
Is an earth fault on a generator smaller than a three-phase fault?
Not necessarily. A three-phase fault is limited by the positive-sequence reactances described here, while a single-phase-to-earth fault is limited by the sum of positive, negative and zero-sequence impedances plus any neutral earthing resistor. On a solidly earthed low-voltage alternator the zero-sequence reactance is small, so the earth-fault current can exceed the three-phase value.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.