Anywhere there are critical standby or life-safety electrical systems, a load bank may be desirable or even mandated by code to help ensure the reliability of those systems. Although the concept of a load bank as an artificial electrical load is relatively straightforward, consideration must be given to the detailed needs of the site when load banks are specified and installed. 

Why load banks?

The only way to verify that a backup power system will perform during an outage is to periodically test it under load. Both generators and UPSs may appear to run fine when lightly loaded, but may fail to deliver full load power if they are not regularly tested to ensure they are up to the task.

Diesel generators are susceptible to a condition known as wet stacking when operated at light loads for extended periods. Wet stacking occurs when carbon or unburned fuel oil accumulates on the injectors, on the exhaust valves, or in the exhaust system. Readily observable as billowing black smoke or as an oil leak (engine slobber), the generator output will be reduced as combustion gasses leak past the valves. Permanent damage due to cylinder scarring is possible although unlikely. Fortunately, the condition is usually corrected rather easily by running the engine for a few hours under sufficient load—typically 30% to 40% of rated load—to bring the engine to nominal operating temperature and burn off the deposits. Load banks may be required to achieve this load if normal building load is not sufficient.