Guide
Generator load bank testing: why starting is not the same as working
Updated
The most expensive words in standby power are: it started fine last month. Starting proves the battery, the starter and the fuel line. It does not prove the set will carry your load.
What a load bank test does
A load bank applies an artificial electrical load to the generator so it can be run up towards its rating without needing the real building load or an outage. That exercises the engine and alternator properly, brings the engine to full operating temperature, and reveals the faults that a twenty minute off-load run never will: cooling capacity, fuelling under load, exhaust and turbo condition, alternator regulation, and the point at which something trips.
Why lightly loaded standby sets need it
- Light loading is bad for diesel engines. Running a set repeatedly at a small fraction of its rating leads to incomplete combustion and unburnt fuel and oil accumulating in the exhaust system, commonly called wet stacking. A properly loaded run is the standard remedy.
- The building load is often not representative. Testing on the real load only proves the set can carry the load that happened to be running that day.
- Faults hide at low load. Cooling, fuelling and governing problems appear as the engine approaches its rating, not while it idles.
- It is the only honest evidence. A dated load test report with readings is what you want on file when the set has to work for real.
What to specify when you ask for a quote
- The load profile: what percentage of rating, in what steps, and for how long at each step.
- Who supplies the load bank, how it gets to the set, and what cabling and connection work is needed.
- Whether the building load is used first, with the load bank topping up to rating, which is often the cheaper and more realistic approach.
- What is recorded: voltages, currents, frequency, temperatures, pressures and fuel consumption against time, not a pass or fail line.
- What happens if it fails, because a load test that finds a fault is a success, and you want the repair quoting process agreed in advance.
How often
There is no statutory interval for load bank testing, as there is none for generator servicing generally. Frequency is set by the manufacturer's guidance, by what the set protects, and by the risk assessment behind your maintenance regime. Sets that carry critical or life safety loads, and sets that rarely see real load, are the ones where more frequent proving is easiest to justify.
If your set is large enough to fall inside the medium combustion plant or specified generator permitting regimes in England and Wales, testing regimes can interact with emissions obligations. Check whether your plant is in scope on gov.uk before planning a testing programme.