Generator load bank testing

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

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

1. 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.
2. Pre-test checks. Fluid levels, batteries, fuel quantity, exhaust and visible condition are checked. Enough fuel must be on hand for the full test.
3. Connection. A qualified electrical worker connects the load bank following the site's electrical safety and lockout procedures.
4. 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.
5. Monitoring and recording. At set intervals the tester records voltage, current, frequency, kilowatts, oil pressure, coolant temperature, exhaust temperature and any alarms.
6. 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.

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.

What a written test plan should include

Ask for the plan before test day. It saves time on site and makes sure everyone agrees on what a pass looks like.

• The generator's nameplate rating and the standard or program the test is meant to satisfy
• The load steps, the time at each step and the total duration at the target load
• The type of load bank (resistive, or resistive-reactive) and its capacity
• How and where the load bank connects, and whether the building load will be disconnected
• Who does the connection, under which lockout and electrical safety procedures
• Fuel required for the test and confirmation that enough is on site
• Where the load bank will sit, where its exhaust heat goes, and how the area will be barricaded
• The readings to be recorded, how often, and the limits that would stop the test

A typical case

A building's generator passes its monthly tests without trouble, but the building load during those tests is a small share of the generator's rating. The first load bank test in several years starts well. At the first step, black smoke and oily drips appear at the exhaust joints, which is typical of wet stacking, and clear as the load increases. At the higher steps the coolant temperature keeps climbing instead of levelling off. The tester stops increasing load, holds at a lower step, and later finds a radiator core partly blocked with dirt and debris on the air side.

The test did its job. The radiator was cleaned, the test was repeated and the set carried its target load with stable temperatures. Had the first time at high load been during a real outage, the generator would likely have shut down on high temperature with the building depending on it.

Common mistakes

• Not having enough fuel. A long test at high load uses far more fuel than a monthly run. Running low mid-test can draw sediment from the bottom of the tank.
• Skipping the cool-down. Shutting down a hot engine straight from high load is hard on turbochargers and other components.
• Leaving the system in manual. Confirm the generator and transfer switch are back in automatic before the contractor leaves.
• Filing the report unread. A pass with notes about rising temperatures or slow recovery is a warning worth acting on.

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