On February 20, 1997, at 0645 central standard time, a McDonnell Douglas DC-9-15, N93S, operated as Northwest Airlines Flight 219, from Minneapolis, Minnesota, to Kansas City, Missouri, experienced an electrical fire during cruise. None of the four crewmembers nor the 32 passengers were injured. The airplane sustained minor damage. The 14 CFR Part 121 flight diverted to the Des Moines International Airport, Des Moines, Iowa, without further incident. The flight had departed from Minneapolis, Minnesota, at 0556. Visual meteorological conditions prevailed and an IFR flight plan was filed. The pilot reported that takeoff and climb were "normal." The flight crew leveled the airplane at flight level 310. They noted the "instrument lights flicker twice followed by a loud pop. The first officer noticed flames coming from the jumpseat area just above the locking device. Smoke and fumes filled the cockpit." The crew donned their oxygen masks and turned off both generators and the battery switch. "The flames went out and the smoke did not get any worse." They flew with a flashlight for one minute and then turned the emergency power switch on. A flight attendant reported that the smoke was limited to the cockpit and first class galley with only minor smoke in the main cabin. The flight crew declared an emergency and diverted to Des Moines. The smoke cleared during the descent. The flight crew flew an ILS approach with ASR backup. Fire and rescue crews met the airplane at the runway and escorted it to the gate. Passengers deplaned via the jetway. Examination of the airplane revealed soot tracks from the aft, interior side of the left main equipment rack near the left forward cabin door. The interior of the rack was soot covered. The cross tie relay, four other relays installed in the equipment bay below the cross tie relay, and associated wire bundles were charred. The most severe, black soot patterns were located near the cross tie relay. Visual examination of all charred and soot covered wire bundles revealed no evidence of chaffing, heat concentration, or prefire insulation failures. The cross tie relay, fuse links, generator controllers, and four charred relays were retained for subsequent examination and the airplane was released to Northwest Airlines. Retained components were examined on February 26, 1997 at the Northwest Airlines maintenance facilities, Minneapolis, Minnesota. Both generator controls functioned normally during a standard bench test. Examination of the other four relays revealed superficial exterior charring. The fuse links on the cross tie relay were charred and melted. The left A phase and the right C phase links measured continuity with an ohm meter. The other four links were melted and open. The cross tie relay, part number 914F567-4 exhibited severe charring and burning around the arc box assembly. Both coils measured open with an ohm meter. The arc box contained charred debris and a silver track was melted between all three phases on the right side of the relay contactor. The inter-phase barriers were fragmented. The viton seals were installed. The relay was latched in the closed position. Northwest Airlines' records indicated the relay had 35,160 hours total time and 7,775 hours since overhaul. Northwest Airlines' allowable time between overhaul (TBO) is 14,000 hours for the relay installed in a vertical or horizontal orientation, based on an OPSPEC Reconciliation from December 2, 1981 after a merger. Westinghouse service bulletin 75-703 revised June 1977 suggests a 14,000 hour TBO for the relay when it is mounted in a vertical orientation as a generator, auxiliary power, or external power relay and a 7,000 hour TBO when it is mounted in a horizontal orientation as a cross tie relay. The service bulletin states "the cross tie relays must be cleaned and overhauled at 7,000 hours maximum." In addition, the service bulletin specifies the addition of viton seals on the inter-phase barriers "for the purpose of preventing migration of the metallic dust through the barriers, thereby establishing a low dielectric path between the phases." McDonnell Douglas All Operator Letter AOL 9-1120B specifies a recommended TBO of 7,000 hours for relay. The letter does not specify vertical or horizontal orientation. Examination of the relay overhaul/component maintenance manual revealed no specification for inspection of the arc box during overhaul. Parties to the investigation were the Federal Aviation Administration, Northwest Airlines, Air Line Pilots Association, Douglas Aircraft Company, and Sundstrand Aerospace.