A load test proves that a genset can deliver its rated output with stable voltage and frequency. The builder connects the set to a load bank, applies load in steps up to full rating and records how the set behaves.
A no-load run only proves the engine starts. A load test is the only way to know the set will hold its rating in service.
What a good load test covers
- Steady runs at stepped loads, often 25%, 50%, 75% and 100% of rating, with readings at each step.
- A short overload run where the rating allows it, as stated in the builder's data.
- Voltage, current, frequency, power and power factor at each step.
- Coolant, oil and exhaust temperatures, and oil pressure, until they stabilise.
- Transient response when load is switched on and off suddenly, measuring voltage and frequency dip and recovery time.
- Operation of protections, such as low oil pressure, high temperature and overspeed shutdowns.
Standards and load banks
ISO 8528-6 sets out test methods for generating sets. ISO 8528-5 defines performance classes G1 to G4, which set limits for voltage and frequency deviation. Agree the class in your specification, then check the test against it.
Ask what load bank the builder uses. A resistive load bank tests the engine at unity power factor. A resistive-reactive bank tests the alternator at 0.8 power factor, which is closer to real loads. If the builder has only a resistive bank, the alternator's full kVA capacity has not been tested.
Witnessing the test
You or your inspector should witness the test on the actual sets you are buying, not a sample from stock. Check that the serial numbers on the engine, alternator and panel match the test sheet. Check that the test instruments are calibrated, and note the room temperature during the run.
Ask for the full run time, not just one reading. Temperatures need time to settle, and many faults appear only after a long run. The broader idea is covered in factory and site acceptance tests.
Record fuel use during the test if it matters to your buyers. See genset fuel consumption for how to read the figure.
Faults a load test often finds
- Sets that cannot hold full rating because the engine is too small for the alternator.
- Overheating inside the canopy once temperatures settle.
- Voltage or frequency that drifts outside the agreed class when load changes.
- Loose connections that heat up in the control panel under current.
- Oil, fuel or coolant leaks that appear only after a long, hot run.
- Protections that do not trip, or trip at the wrong setting.
Next step
We witness load tests, check instruments and serial numbers, and send you the readings before the set is packed. To arrange this for your order, book a free sourcing review.