A massive fire broke out at a three-storeyed plastic factory in Narela, an industrial area in Delhi. The cause of the fire is suspected to be an electric short-circuit, police said.   Even as 40 fire tenders fought the blaze, a boiler in the building exploded, trapping four persons, including two firemen. The building caved-in injuring four workers and two firefighters.Fire officials said that building has major safety issues such as closed exits, and contained hazardous chemicals.