On June 23, 2002, about 1300 eastern daylight time, a Cessna 150D, N4313U, registered and operated by Harrier Soaring Club Inc., experienced an in-flight electrical fire, and burst into flames after landing at the Ball Airport, Louisburg, North Carolina. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 glider towing flight. The airplane was substantially damaged. The commercial-rated pilot reported no injuries. The flight had originated from the same airport at 1248. According to the pilot, he had dropped a glider at 2,500 feet above ground level, and was returning to the airport. He turned base to final about 1/2 mile from the airport when he noticed "an odor" in the cockpit that smelled like "hot plastic." He thought that a wire was overheating, and as he entered the left downwind leg for runway 36, the plastic odor became "stronger." He turned off "all aircraft-powered electrical equipment," including the radio and transponder, but not the master switch. He abbreviated the traffic pattern by turning onto the left base early for runway 36. He then observed "wisps of smoke" emerging from behind the instrument panel on the passenger side, forward of where the glove box opening was located. There was no liner behind the glove box opening, and he could see behind the box. It became obvious to him that he had a "potential electric fire developing." At the halfway point of the base leg for runway 36, flames about "4 to 6 inches" in diameter were visible through the glove box opening. He then turned towards the runway threshold, increased power to increase airspeed, and turned final for runway 36. The flames got larger, and "dense black smoke" filled the cockpit, making it difficult for the pilot to breathe and see. The pilot increased the flaps to 40 degrees, added full power to the engine, put the airplane into a nose-down "hard" left slip to lose altitude and put the "flames on the negative pressure side of the aircraft." The smoke increased in volume, and it was becoming "increasingly difficult and irritating to breath." He opened the pilot's side window, which enabled him to breath easier, and cleared some of the smoke from the area above the windshield making it easier for him to see outside the airplane. Over the runway threshold he fully reduced engine power, raised the airplane's nose, the airplane "ballooned" and touched down at a high speed. At touchdown the airplane "swerved" right and exited the paved runway onto an adjacent grass runway. He was able to maneuver the airplane back onto the paved runway and he applied full brakes once on the pavement. As the airplane slowed he maneuvered it onto a grass parking ramp, came to a full stop, turned off the master switch, and exited the airplane through the left door. Examination of the airplane by an FAA inspector revealed that the insulation on the electric wiring from the master relay on the firewall was burned along the entire wire run on the engine firewall to the point where it passed through the firewall on the pilot's side of the airplane. The wire had no in line circuit protection other than the airplane's master switch, and in this case the pilot did not turn the master switch off until the airplane stopped.