EE WAS SUPERVISING 3 MAINTENANCE WORKERS TO REPLACE AN IMPELLER ON PUMP. PUMP WAS LOCATED IN A CONCRETE CHAMBER BELOW GROUND LEVEL. ONE OF THE WORKERS HAD ATTACHED THE CHAIN WITH A NUT AND BOL T FROM THE 1-TON HOIST TO THE TOP CENTER LIFT RING OF THE PUMP HOUSING IN ORDER TO TIP THE HOUSING AWAY FROM THE PUMP. WHEN IT DID SO, THE 120# HOIST FELL APPROX. 12' DIRECTLY ON TOP OF SUPER