Fire due to fire hydrant water, and pipe network under pressure down to a pressure P1 when the value of P4, start the main fire pumps supply water to meet fire safety requirements and pressures.

• 消防时，因消火栓出水量大，管网压力下降到P1以下某个压力值P4时，启动主消防水泵供给满足消防要求的用水量和压力。