When a pumper is taking its water supply from a hydrant the engineer must make sure the pump is not taking every last bit of the pressure the hydrant is able to provide. Some residual pressure must be allowed to remain to provide a buffer so the pump does not accidentally over-draw the hydrant and potentially damage or collapse the water mains leading to the hydrant. It may also refer to the water utility boosting water pressure in the area of a working fire so that "residual pressure" remains in the entirety of the municipal water system despite the water being drawn for fire-fighting.