Breaker scope is often treated as an accessory in diesel generator quotations, but it can decide whether the package connects cleanly to the project switchboard. The purchaser should know who selects the breaker frame, who sets the trip unit, and which protection documents are expected at handover.
What buyers should know first
A diesel generator breaker is not selected by output kVA alone. Voltage, current, short-circuit expectations, cable distance, installation method, and downstream protection coordination can all affect the breaker and panel scope. Yangzhou Tesla Power Equipment Co., Ltd. can quote generator package options, but final protection coordination should be reviewed by the project electrical engineer.
Breaker information to align
| Item | Why it matters | Information to include |
|---|---|---|
| Breaker rating | Must match generator output and project voltage. | Ampere rating, voltage, poles, and enclosure. |
| Trip settings | Incorrect settings can cause nuisance trips or poor protection. | Who provides settings and who approves them. |
| Fault study | Generator fault current differs from utility supply. | Available project fault calculations if any. |
| Cable interface | Terminal size and cable route affect installation. | Cable quantity, size range, gland plate, entry direction. |
| Test documents | Settings should not exist only as verbal comments. | Factory record, drawing, and commissioning checklist. |
In practice
Two generator sets with the same kVA can have different breaker requirements if one feeds a simple manual panel and the other feeds a coordinated factory switchboard. For export projects, the RFQ should also state whether the breaker is inside the generator-mounted panel, in a separate output cabinet, or excluded from the supplier scope.
Before locking the order, compare this topic with cable entry and breaker scope checks and short circuit and protection scope checks. For higher output comparison, see the 800kW 1000kVA Cummins diesel generator with ATS option.
Before requesting a quotation
- State the project voltage, frequency, phase, and maximum load current.
- Confirm breaker location and whether a trip unit is required.
- Ask who is responsible for protection coordination with downstream equipment.
- Send cable entry direction, cable sizes, and termination expectations.
- Request drawings showing breaker, terminals, controller, and output interface.
Questions purchasers often ask
Can the supplier set the breaker trip values?
The supplier can provide typical settings if enough project data is available, but the project electrical engineer should approve final coordination.
Is a generator breaker always included?
No. Some quotations include only the generator-mounted breaker, while others require a separate output panel or contractor-supplied switchgear.
Why does cable distance matter?
Cable distance can affect voltage drop, termination arrangement, and how protection coordination is reviewed.
What should be checked during commissioning?
Check breaker labeling, trip-unit settings, terminal tightness, phase sequence, insulation test records, and load-test behavior.
What mistake delays factory projects?
A common delay is discovering that the ordered breaker does not match the actual switchboard, cable, or protection-study requirement.
Why breaker scope belongs in the quotation
The output breaker is the boundary between the generating set and the site switchboard, and it is the item most often left
undefined in an enquiry. When it is not specified, the supplier prices the set without a breaker, the electrical contractor
discovers the gap at delivery, and the project waits while a frame is sourced locally. Worse, protection that is sized only by
the generator’s kVA can be wrong in both directions: too small and it trips during a legitimate motor start, too large and it
fails to clear a fault before the alternator is damaged. Agreeing the frame, the trip unit, the settings philosophy and the
documents at quotation stage removes a delay that typically costs more than the breaker itself.
Protection elements and typical setting ranges
| Protection element | What it protects against | Typical starting range | Comment |
|---|---|---|---|
| Long-time pickup (Ir) | Sustained overload of the alternator | 1.0 – 1.1 × generator rated current | Must sit above the continuous rating but below the alternator damage curve |
| Long-time delay | Allows harmless short overloads while motors start | 10 – 30 s at 3 × Ir | Set against the longest expected motor start |
| Short-time pickup (Im) | Faults beyond the feeder, with intentional delay | 2 – 5 × Ir, delay 0.1 – 0.4 s | The delay is what preserves selectivity with downstream devices |
| Instantaneous (Ii) | Bolted short circuits at the terminals | 6 – 12 × Ir | Sometimes disabled on a generator breaker so downstream devices clear first |
| Ground fault (Ig) | Phase to earth faults | 0.2 – 1.0 × Ir, coordinated delay | Depends on the site earthing arrangement |
| Reverse power / loss of mains | Import or back-feed when paralleled with the utility | Per utility agreement | Required for any parallel or peak shaving scheme |
Ranges above are starting points for discussion only. Final settings must be produced by a protective device coordination
study carried out by the project’s electrical engineer, then verified by injection testing during commissioning.
Information to fix before the breaker is specified
- Generator rated current, from kW, voltage and power factor, not from kVA alone.
- Available short-circuit current at the point of connection, so the breaker’s breaking capacity is adequate.
- Cable route length, size and installation method, for both fault level and voltage drop.
- Whether the set is a single unit or one of several in parallel, and whether it ever runs in parallel with the utility.
- Downstream protection devices, so selectivity can be demonstrated rather than assumed.
- The applicable standard for the destination market, generally IEC 60947-2 or UL 489.
Standards and documents commonly referenced
- IEC 60947-2 – Low-voltage switchgear and controlgear, circuit breakers.
- UL 489 – Molded-case and circuit breakers for North American installations.
- IEC 60204-1 – Electrical equipment of machines, relevant to control and interface wiring.
- NFPA 110 – Emergency and standby power systems, where the set supports life safety loads.
- ISO 8528-1 – Generator rating definitions that determine the rated current the breaker must carry.
Frequently asked questions
Who is responsible for setting the generator breaker trip values?
Responsibility should be stated in the enquiry. In practice the supplier selects and supplies the breaker frame and trip unit, the project electrical engineer defines the required settings based on a protection coordination study, and the commissioning engineer enters and tests them on site. If the enquiry is silent, the breaker usually arrives on factory default settings, which are safe for the breaker but rarely selective with the site’s downstream devices.
Is a generator output breaker always included in the package?
Not always. Many quotations include only a controller with protection functions, leaving the output breaker, the cabling and the switchboard interface to the buyer’s electrical contractor. This is a frequent cause of delay because the omission is discovered at delivery. The enquiry should state explicitly whether the breaker, the cable run and the terminations are inside the supplier’s scope.
What trip unit settings are typical for a generator breaker?
As a starting point only, a long-time pickup around 1.0–1.1 × generator rated current, a short-time pickup around 2–5 × rated current with a delay of 0.1–0.4 s, and an instantaneous element around 6–12 × rated current that is sometimes disabled to preserve selectivity with downstream feeders. Ground fault settings are normally a fraction of rated current, coordinated with the site earth fault scheme. Final values must come from a coordination study.
Why does cable distance matter to the breaker specification?
Cable impedance limits the fault current that reaches the breaker and adds voltage drop under load. A long run can reduce the fault level enough to change the instantaneous setting, and it can push the voltage drop beyond the accepted limit. Supplying the route length and cable size with the enquiry lets the supplier confirm both the trip settings and the voltage regulation at the far end.
What should be checked during commissioning of the protection?
Confirm the entered settings against the coordination study, carry out primary or secondary injection testing of the trip unit, verify the breaker’s short-circuit rating against the available fault level, check the shunt trip and auxiliary contacts, and record the results in the handover documentation so that future maintenance has a baseline.
Which standards apply to generator breakers?
Air circuit breakers and moulded case breakers are generally built to IEC 60947-2 or, for North America, UL 489. Generator protection and control interfaces are influenced by IEC 60204 for machinery and by NFPA 110 where the set supports life safety systems. The destination market and the local electrical code decide which of these governs.