What buyers should know first: a generator can start perfectly, sound healthy, and still be unable to carry its load. Running an engine at no load proves the engine turns. It does not prove the alternator delivers, the cooling system copes, or the controls behave when a block load lands. Load bank testing is the only practical way to find that out before you need the set.
It is also the commissioning activity most often reduced in scope to save a day. That saving is usually spent several times over later.
Why no-load running is not a test
Diesel engines run at light load accumulate deposits and can wet-stack; more to the point for a buyer, nothing in the electrical or cooling chain is being exercised. A set that has only ever idled has unproven temperature behaviour, unproven voltage regulation under step load, and unproven protection settings.
What a useful load bank test covers
Load steps, not just a steady run
Applying load in steps shows how the set recovers: voltage dip, frequency dip, and recovery time. This is what tells you whether the governor and AVR are set up for your load profile. A single steady figure hides all of it.
Sufficient duration
Cooling systems need time to reach equilibrium. A short run at full load may finish before the temperature has stabilised, which means the thing you most wanted to check was never actually tested.
Measurements recorded, not observed
Ask for recorded values: voltages, currents, frequency, temperatures and pressures at each load step. Recorded data can be compared with the next test; a verbal “all fine” cannot.
Factory test versus site test
| Factory acceptance test | Site load bank test | |
|---|---|---|
| What it proves | The set itself meets its design figures | The installed set works in its actual environment |
| Scope | Controlled load, controlled conditions | Real cabling, real ventilation, real fuel |
| What it misses | Installation, ducting, room airflow, wiring | Assembly defects upstream of installation |
| Buyer value | Confirms the product before shipment | Confirms the installation before handover |
| Typical gap | Cannot catch a bad room | Cannot catch a bad alternator build |
They are complementary, not interchangeable. Asking for one and assuming it covers the other is the most common commissioning gap.
Procurement checklist
- Specify load bank testing as a separate, priced line item rather than assuming it is included.
- State the load steps you want and the duration at each, including a period at full load.
- Ask for recorded readings at each step, not a pass/fail statement.
- Confirm whether the test is on a temporary load bank or on the building load, and accept the limitations of each.
- Ask what happens if the set fails: who pays for retesting.
- Require the results in the handover documentation, along with the factory test certificate.
What to put in the RFQ
Ask for the load test to be described as a procedure: number of steps, size of each step, duration, parameters recorded, and the acceptance criteria. If your load has large motors or a significant step component, say so, because the test should reflect the profile the set will actually see. Our notes on factory acceptance testing before shipment and on step load acceptance cover the two halves of this in more detail.
FAQ
Can we test on the building load instead of a load bank?
Sometimes, if the building can present a sufficient and controllable load. The limitation is that you usually cannot choose the steps or hold a steady full load, so the test is less informative.
How long should a load bank test run?
Long enough for temperatures to stabilise at each significant step. The right duration depends on the set size and cooling arrangement; ask the supplier to justify the proposed figure.
Why does the set dip and recover on each step?
That is normal and is exactly what the test should measure. The size of the dip and the recovery time are the useful numbers, not the fact that a dip occurred.
Should standby sets be tested regularly after handover?
Yes. Commissioning proves the installation once; a periodic programme proves it still works. Ask about a testing regime as part of the maintenance scope.
Is a load bank test included as standard?
Do not assume it. It is frequently an optional extra, which is why it should appear as its own line in the quotation.
Next step
Ask for load bank testing to be quoted with a written procedure. Browse the diesel generator sets suited to your rating, and request a quote specifying the load profile the set will actually see.