This usually occurs in high-rise buildings, or in systems that supply a large volume of water, such as in storage warehouses. Fire pumps are also needed if the water supply is provided from a ground level storage tank. The fire pump starts when the pressure in the fire sprinkler system drops below a threshold usually caused by one or more fire sprinklers releasing water or other firefighting connections being opened, causing a pressure drop in the fire fighting main. Fire pumps are usually powered either by an electric motor or a diesel engine. If necessary, an electric motor powered fire pump may require the installation of an emergency generator.