In short. Most electrical installations are designed and checked on mains supply, where the transformer can deliver tens of thousands of amps into a fault. A generator of similar kVA delivers far less — and on sets without excitation support that current falls away within about a second. A breaker whose magnetic (instantaneous) trip is set for mains conditions may simply not see a fault on generator supply. The fault then persists until a slow thermal element acts, or the generator trips itself and blacks out the whole site. This page gives a practical check, circuit by circuit, and the fixes for circuits that fail it.
Who this page is for: For electrical engineers, contractors and facility managers: confirm that the protective devices designed for mains supply will still clear a fault when the site is running on its generator — and fix the circuits that will not.
Key facts
- A generator’s fault current at its own terminals is typically a few times its rated current, compared with many times more from a transformer of the same kVA.
- The breaker’s instantaneous trip must operate below the minimum fault current at the far end of its circuit — and on generator supply that minimum is much lower.
- Breaker trip settings have a tolerance, commonly about ±20 % for magnetic trips; the check must use the upper end.
- Excitation matters: sets with permanent-magnet or auxiliary-winding excitation sustain fault current; simple self-excited sets may let it collapse.
- Earth-fault protection (residual current devices or earth-fault relays) is often the simplest way to make generator-fed circuits safe.
The theory of generator fault current — the sub-transient, transient and sustained periods — is on our Fault Current page, and the full coordination study on our Protection Coordination page. This page is the field check.
Why this matters
- Fire risk: a fault that persists for seconds or minutes heats cables and terminations far beyond their limits.
- Shock risk: during an earth fault, exposed metalwork stays at a dangerous voltage until the circuit is disconnected. Disconnection-time rules exist for this reason.
- Loss of the whole site: if the branch breaker does not clear the fault, the generator’s own protection eventually trips — and every load goes dark, not just the faulty circuit.
- Alternator damage: sustained fault current overheats the windings.
The field check, step by step
This is a deliberately simple screening method: it adds resistance and reactance arithmetically and ignores the generator’s zero-sequence behaviour, which errs on the safe side for most low-voltage sets. Borderline circuits need a proper calculation or a measurement.
Worked example (illustrative)
Illustrative figures to demonstrate the method. A 250 kVA, 415 V generator with transient reactance 25 % backs up an office building that is normally fed by a 1,000 kVA, 5 % transformer.
- Rated current: 250,000 ÷ (1.732 × 415) ≈ 348 A.
- Generator impedance: 0.25 × 415² ÷ 250,000 ≈ 0.172 Ω.
- Fault current at the generator terminals: about 348 ÷ 0.25 ≈ 1,391 A.
- A feeder of 100 m of 35 mm² copper, protected by a 125 A MCCB with its magnetic trip at 10 × In (1,250 A). Loop resistance hot ≈ 2 × 0.1 km × 0.524 Ω/km × 1.2 ≈ 0.126 Ω.
- Fault current at the far end on generator supply ≈ 240 ÷ (0.172 + 0.126) ≈ 804 A.
- Required for a certain magnetic trip: 1,250 × 1.2 = 1,500 A. The fault current is far below it. The breaker will not trip instantaneously on generator supply.
- On mains supply, the same far-end fault is about 1,783 A — which does trip it. The circuit was correctly designed for mains and is unprotected on DG.
In fact, with its magnetic trip at 10 × In, this 125 A breaker cannot trip instantaneously anywhere on this generator: even a fault at the generator terminals gives less than 1,500 A.
The fix options for this circuit
- Lower the magnetic setting where the breaker allows it — at 5 × In (750 A with tolerance) it would just trip on this feeder (804 A against 750 A) — a borderline margin that should be confirmed by calculation or measurement, and checked against motor starting and inrush.
- Add earth-fault protection — a residual current device or earth-fault relay — so the most common fault type is cleared at a low current regardless of the generator.
- Use an electronic trip unit with a short-time element set for generator conditions, or a second settings group switched in when the generator is running.
- Increase the protective conductor size to reduce loop impedance on long circuits.
- Sustain the fault current with PMG or auxiliary excitation, so the generator keeps supplying about three times rated current long enough for time-delayed elements to work.
Maximum circuit length on this generator (illustrative)
Maximum length of a copper circuit for which the magnetic trip still operates on the example 250 kVA set, with the protective conductor the same size as the phase conductor. Illustrative screening values only.
| Breaker and magnetic setting | Current needed (with 20 % tolerance) | 16 mm² | 35 mm² | 70 mm² |
|---|---|---|---|---|
| 63 A, magnetic at 10 × In | 756 A | about 50 m | about 115 m | about 220 m |
| 125 A, magnetic at 5 × In | 750 A | about 50 m | about 115 m | about 225 m |
| 125 A, magnetic at 10 × In | 1,500 A | will not trip at any length | will not trip at any length | will not trip at any length |
| 250 A, magnetic at 5 × In | 1,500 A | will not trip at any length | will not trip at any length | will not trip at any length |
The pattern is what matters: on a generator, larger breakers with high magnetic settings may not trip at any length, and smaller breakers trip only on shorter circuits. Larger generators raise every figure; long, thin circuits lower them.
Measuring rather than calculating
An earth-loop impedance test made while the site is running on generator supply gives a direct reading of the circuit’s loop impedance including the source. It is a valuable check, but treat the result as a best case: the instrument’s test pulse is very short and sees the generator’s lowest (sub-transient) impedance, while a breaker needs current to be sustained long enough to trip. Keep a margin, and use the test to confirm calculations rather than replace them.
Which circuits to check first
- Large feeders with high magnetic settings near the generator panel.
- Long circuits: outbuildings, pump houses, street lighting, car parks, guard rooms.
- Circuits feeding outdoor or wet areas where shock risk is highest.
- Rising mains in tall buildings.
- Any circuit where the breaker was replaced or the setting changed after the generator was installed.
Field check sheet
| Circuit | Breaker / curve / magnetic setting | Cable size & length | Calculated or measured fault current on DG | Needed (setting × 1.2) | Pass / fail | Fix |
|---|---|---|---|---|---|---|
Generator data used: kVA ______ Voltage ______ X′d ______ Excitation type ______ Checked by ______ Date ______
Common mistakes
- Checking protection only for mains supply and assuming the generator case is covered.
- Using the breaker’s nominal magnetic setting without its tolerance.
- Lowering a magnetic setting without checking motor starting and inrush on that circuit.
- Relying on a loop-impedance reading on DG supply as if it were the sustained value.
What to send us
Send the generator’s kVA, voltage, alternator transient reactance and excitation type, and for each circuit of concern the breaker type, rating and magnetic setting, cable size and length, and the protective conductor size. Use the application engineering enquiry, or WhatsApp the details and photographs.
Questions engineers ask
Why would a breaker trip on mains but not on generator supply?
Because the generator supplies much less fault current than the transformer, and that current falls within a second. A breaker’s instantaneous trip set for mains fault levels may be above anything the generator can deliver.
How much fault current does a diesel generator give?
At its terminals, typically a few times rated current in the first fraction of a second, falling after that. With PMG or auxiliary-winding excitation it can sustain about three times rated current for around ten seconds; simple self-excited sets may fall much lower.
Is it enough that the breaker’s thermal element will eventually trip?
Usually not. Thermal tripping at modest overcurrent can take seconds to minutes, which may exceed the disconnection time needed to prevent shock and the time cables can survive the fault. It may also let the generator trip first and black out the site.
Will an RCD solve the problem?
For earth faults — the most common type — a residual current device or earth-fault relay clears the fault at low current regardless of the generator. It does not solve phase-to-phase faults, which still need correctly set overcurrent protection.
Can I lower the magnetic setting of my MCCB?
If the breaker has an adjustable magnetic trip, often yes — but check that motor starting, transformer inrush or capacitor switching on that circuit will not cause nuisance tripping at the new setting.
Do we need to repeat this check?
Yes, whenever the generator, breakers or circuits change, and as part of a periodic electrical inspection.
Related pages
- Fault Current
- Protection Coordination
- Generator Cable Sizing
- Earthing & Neutral Grounding
- PMG & Excitation
- Cable Protection & Coordination
- ACB, MCCB & MCB Selection
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 3 November 2026.