A load bank test connects a generator to an artificial electrical load so it can be run at a controlled percentage of its rated output. It proves the generator can carry the load it was bought for, and it burns off deposits left by light-load running. For most standby diesel generators it is a normal part of periodic testing, not an emergency measure.
Why building load is not enough
Standby generators are usually sized with margin for future growth, motor starting and worst-case conditions. On a normal monthly test, the actual building load may be a small fraction of the generator's rating. That proves the set starts and transfers, but it does not show whether it can deliver full output, and it does not get the engine hot enough to run cleanly.
Diesel engines that run at light load for long periods suffer from wet stacking. Fuel does not burn completely, and unburned fuel, oil and carbon collect in the exhaust system, on injectors and around piston rings. Signs include black, oily residue at exhaust joints, heavy smoke on start-up and falling performance. Running the engine at a substantial load for a sustained period helps clear these deposits.
Standards and requirements
For emergency generators in buildings, CSA C282 sets out testing requirements that include periodic tests at a specified load, and allows a load bank to be used where the building load is too low. The National Fire Code of Canada and provincial fire codes reference these requirements. Check the current edition and what your province and local fire authority require, since the required load level, duration and frequency are set there, not by the testing contractor.
Data centres, hospitals and industrial sites often have their own internal standards or insurer requirements that call for more frequent or longer tests.
Types of load banks
| Type | What it does | Typical use |
|---|---|---|
| Resistive | Converts power to heat through resistor elements, loading the generator at unity power factor | Most routine load bank tests; tests the engine and cooling system |
| Reactive (inductive or capacitive) | Adds reactive load so the test runs at a lagging or leading power factor | Combined with resistive load to test the alternator and voltage regulator under realistic conditions |
| Resistive-reactive | Combines both in one unit | Commissioning and more thorough periodic tests, especially for larger sets and paralleled systems |
Load banks may be portable units brought to site on a trailer, or permanently installed units with a dedicated connection point. A permanent connection point makes regular testing faster and safer, since it avoids opening equipment to connect temporary cables.
How a test is run
- Planning. The contractor reviews the generator rating, the test requirements, the connection method and site conditions such as where the load bank will sit and where its hot exhaust air will go.
- Pre-test checks. Fluid levels, batteries, fuel quantity, exhaust and visible condition are checked. Enough fuel must be on hand for the full test.
- Connection. A qualified electrical worker connects the load bank following the site's electrical safety and lockout procedures.
- Stepped loading. Load is applied in steps, often starting low and increasing to the target level, with the engine allowed to stabilize at each step.
- Monitoring and recording. At set intervals the tester records voltage, current, frequency, kilowatts, oil pressure, coolant temperature, exhaust temperature and any alarms.
- Cool-down and reconnection. Load is removed, the engine runs through its cool-down, the load bank is disconnected and the system is returned to automatic.
Safety during testing. Load banks produce large amounts of heat and work at full generator voltage. Keep people clear of the discharge air and cable runs, and make sure connection and disconnection are done only by qualified electrical workers under your site's procedures. See arc flash basics.
What the results tell you
A good test report shows readings at each load step and highlights anything that drifted outside normal limits. Things to look for:
- Voltage and frequency stable and within limits at every load step
- Coolant temperature levelling off rather than climbing, which points to a healthy cooling system
- Oil pressure steady throughout
- Exhaust temperatures balanced across cylinders where measured
- No leaks, abnormal smoke, vibration or alarms
- Recovery after each load step within the expected time
Problems found under load, such as overheating, a voltage regulator that hunts or an engine that cannot reach its rated output, are far better discovered on a planned test day than during an outage.
Fitting load bank tests into your program
Load bank testing works best as one part of a planned schedule that includes frequent checks, regular run tests and periodic service. Our guide to standby generator maintenance sets out how the pieces fit together. Many owners schedule the load bank test right after annual service, so fresh filters and fluids are in place and any issue found can be fixed in the same visit.
How ehZee can help
As part of our diesel generator inspections, we review generator condition, test records and load test results against the requirements that apply to your building, and help you plan and coordinate load bank testing with your service contractor. We explain the findings in plain terms and set out what to fix first.
Common questions
Does a load bank test interrupt building power?
Usually not. Most load bank tests are run with the generator disconnected from the building load, so normal utility power stays in place. Your contractor should confirm the arrangement before the test.
How long does a load bank test take?
It depends on the load steps and duration required by the applicable standard or your own program, plus connection and cool-down time. Ask your contractor for a written test plan.
Can a load bank test damage a generator?
A generator in good condition should handle a test at its rated output. A test can reveal existing weaknesses, which is the point. Stepped loading and close monitoring keep the risk low.