LABORER WAS DIGGING A SMALL EXCAVATION (~30"" WIDE X 36"" LONG X 14-21""DEEP) USING PICK AND SHOVEL HE HIT A LARGE ROCK AND USED A JACKHAMMER TO BREAK THE ROCK. WHILE USING THE JACKHAMMER AN UNDE RGROUND ELECTRICAL CONDUIT WAS PENETRATED CAUSING A FLASH FIRE.