Abstract: TWO EMPLOYEES WERE MEASURING LINE-TO-LOAD DISTANCES FOR RETROFITTING A 480-VOLT, 100-AMPERE BREAKER ONTO 200-AMPERE BUS FINGERS ON THE SECONDARY SIDE OF AN ISOLATED TRANSFORMER INSTALLED INDOORS. THEY WERE TAKING THEIR MEASUREMENTS AT AN UNGUARDED, ENERGIZED PANELBOARD IN A MAIN SUBSTATION. AN EXPLOSION AND FIRE OCCURRED, AND BOTH EMPLOYEES WERE SEVERELY BURNED. TWO 6-FOOT WOODEN FOLDING-TYPE RULES WITH BRASS TIPS AND HINGES WERE FOUND AT THE ACCIDENT SCENE. ONE OF THE RULES WAS SIGNIFICANTLY BURNED. IT WAS BELIEVED THAT AN ELECTRIC ARC OCCURRED WHEN THE TIP OF THE RULE CONTACTED LINE VOLTAGE. THIS CAUSED A FAULT AND AN EXPLOSION. A SECOND EXPLOSION WAS HEARD, AND IT WAS BELIEVED TO HAVE BEEN CAUSED BY A FAULT IN A TRANSFORMER COIL. SEVERE FIRE DAMAGE WAS EVIDENT AT THE OPEN PANELBOARD SECTIONS. THE FAULT CONTINUED THROUGH TWO SECTIONS OF THE 100-AMPERE PANELBOARD, ENDING AT AN ENCLOSED PANELBOARD SECTION. THE FAULT BLEW A HOLE THROUGH THE PANELBOARD COVER. THE 125E PRIMARY FUSES CLEARED THE FAULT. THERE WAS NO SECONDARY FUSE PROTECTION INSTALLED. BOTH EMPLOYEES WERE HOSPITALIZED FOR THEIR INJURIES. (A THIRD EMPLOYEE WAS ALSO INJURED AS A RESULT OF THE ACCIDENT, BUT THE NATURE AND CAUSE OF HIS INJURIES WERE NOT GIVEN IN THE REPORT.)