Diesel Engine Pump are used in fire fighting systems to maintain the pressure in pipe line. Normally jockey pump starts if water pressure goes below 120 and it maintain the pressure but if at anytime 2 – 3 sprinklers start sprinkling water due to high temprature sensing and pressure goes below 110. Then it become difficult for jockey pump to maintain the pressure at this stage main fire pump also starts to maintain the pressure above 140. During  special case if main fire pump is also not able to maintain pressure then at third stage diesel engine pump starts now these 3 pumps : jockey, main fire pump & diesel engine pump maintain the pressure. In some cases during fire in a building the main electricity  is switched off to avoid fire expansion then only diesel engine pump starts to supply water to sprinkle the water during fire.