Fire pump systems contain a sensor which monitors the water pressure in the sprinkler pipes. When the sensor detects a small drop in the water pressure which is not enough to indicate a sprinkler was activated, it will start the jockey pump in order to drive the pressure up to the required standard. If a larger pressure drop is detected, and the jockey pump can’t keep the correct pressure, the main fire pump automatically starts up and feeds water under high pressure into the system.
when is a fire pump required? – http://www.ehow.com/facts_7719685_fire-pump-required.html