AT 7 AM THE LOWER PART OF THE DEEP WELL PUMP #4 SHROUD WAS BEING LOWERED INTO THE HOLE. THE OTHER END WAS LOWERED TO CONNECT THE 2 PARTS TOGETHER. AT 705, A METHANE TEST WAS MADE & 0.0 WAS ALL THAT THE SPOTTER READ. AT THAT TIME THE PREP PLANT MECHANIC STARTED WELDING. THAT IS WHEN HE FELT HEAT ON HIS HAND. HE STOPPED, LOOKED AROUND & STARTED AGAIN. IGNITION OCCURRED.