On May 17, 1995, at 0630 Pacific daylight time, a Fairchild SA227-AC, N2727B, operating as Horizon Air flight 2342, experienced a runaway trim while cruising at 19,000 feet, en route to Spokane, Washington. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed for the flight that was conducted under 14CFR135. The airplane was not damaged and neither the airline transport pilot, commercial pilot or 15 passengers were injured. The flight had departed from Portland, Oregon, on May 17, 1995, at 0600. The flight landed at Spokane, without further incident, at 0720. The flight crew reported that while cruising at 19,000 feet, there was an intermittent sonalert for the trim in motion. At this time the trim indicator and the aircraft trim were not affected. The captain stated that some minutes later, and just prior to descent, the elevator trim began to move without any input. The captain deactivated the trim on his control yoke, however, the runaway continued. The control was then switched to the co-pilot's position, however, the co-pilot's control would not deactivate the trim. The left side essential bus was then selected off, which deactivated the trim. The airplane then landed without further incident. The captain reported that the trim runaway and DC bus fail checklists were complied with. Maintenance personnel at Horizon Air reported that the captain's side trim power wire was found shorted directly to the left hand essential bus, and the jumper wire at the captain's yoke trim (ground side) switch was found shorted to the yoke. These wires were replaced and the systems operation checked ok.