Abstract: SEVEN POWER GROUP DEPARTMENT EMPLOYEES WERE ASSIGNED TO PERFORM MAINTENANCE ON A 6.9-KILOVOLT, 25-CYCLE TRANSFORMER BANK. THE MAINTENANCE WAS TO INCLUDE INSPECTING THE EQUIPMENT, CHANGING THE CIRCUIT BREAKER OIL, AND CLEANING THE TRANSFORMER BUSHINGS. THE WORK WAS TO BE COORDINATED SO THAT THE BUILDING HOUSING THE OIL CIRCUIT BREAKER COULD BE REPAIRED BY CARPENTERS AT THE SAME TIME. BEFORE THE MAINTENANCE WAS STARTED, IT WAS NECESSARY TO ISOLATE THE EQUIPMENT FROM THE POWER SOURCE. EMPLOYEE #1 CLOSED THE 230-VOLT TIE DISCONNECTS AND OPENED 6 DELTA DISCONNECTS TO TAKE THE LOAD OFF THE SECONDARY POWER SIDE. HE PLACED RED TAGS ON EACH DELTA DISCONNECT. EMPLOYEE #1 THEN CHECKED CURRENT TO THE TRANSFORMERS WITH AN AMMETER. THE CHECK SHOWED NO LOAD. HE THEN ATTEMPTED TO OPEN THE 6.9-KILOVOLT OIL CIRCUIT BREAKER. AS HE OPENED THE BREAKER, A LARGE FLASH OCCURRED, AND THE BUILDING CAUGHT ON FIRE. (THE OIL CIRCUIT BREAKER WAS A GENERAL ELECTRIC FK BREAKER #S DKL 140569 59406, WSF 23308; RATED 300 AMPERES, 15 KILOVOLT, 3-PHASE; A SINGLE-TANK UNIT OF 1920 VINTAGE.) EMPLOYEES # 1, 2, AND 3 WERE INSIDE THE OIL CIRCUIT BREAKER BUILDING, AND EMPLOYEES #4, 5, 6, AND 7 WERE OUTSIDE THE BUILDING WHEN THE FLASH OCCURRED. SIX EMPLOYEES SUFFERED BURNS. ONE HAD LACERATIONS AND CONTUSIONS. THE BUILDING HOUSING THE BREAKER WAS KNOWN TO HAVE A LEAKING ROOF. THIS COULD HAVE ALLOWED WATER TO CONTAMINATE THE OIL IN THE TANK, RESULTING IN A DIMINISHED ARC-QUENCHING CAPABILITY OR RESULTING IN A FAULT TO GROUND. ADDITIONALLY, THE OIL CIRCUIT BREAKER HAD NOT RECEIVED SCHEDULED SERVICE OR REPAIR SINCE 1971 (13 YEARS EARLIER).