There are two kinds of crankcase explosion, one a primary explosion and other the secondary explosion.
When the primary explosion takes place because of the oil mist and the appearance of hot spot in the crankcase, there is a high-pressure rise in the crankcase which is released into the engine room atmosphere with the help of crankcase relief valves. At the same time, the non-return valve will shut and not allow the new air to enter into the crankcase. The crankcase atmosphere will be highly rich and hence there is a less possibility of exposure. Under such circumstances if by mistake the crankcase door is opened, fresh air will enter the crankcase and bring the crankcase atmosphere in the range of explosive limit. When this happens a heavy secondary explosion may take place.
