On October 9, 1997, Limerick 2 experienced a fire in the exhaust stack of the D21 emergency diesel generator (EDG) during an operability test. The unit was operating at 100% power at the time of this event. During the test run, the #4 cylinder lower piston seized, resulting in damage to the piston, connecting rod and cylinder liner. The fire occurred due to the release of unburnt fuel into the EDG exhaust manifold and exhaust stack. The fire burned out when the EDG was secured. A second, smaller, fire occurred on the body of the EDG in the vicinity of the exhaust manifold. This fire was extinguished within two minutes using a portable fire extinguisher. After the failure of the EDG, the licensee performed test runs of the three remaining unit 2 EDGs to ensure operability. Assuming a complete loss of offsite power and the failure of one EDG, the other EDGs have sufficient capacity to operate all necessary equipment in the event of a design basis accident on one unit and an emergency shutdown of the other unit. Justification: Using Part III-2.4.1 of the INES User's Manual , this event is rated at level zero/below scale. The failure of the EDG was discovered as a result of a routine surveillance program. Despite the failure of this EDG, the safety function (cooling the radioactive material) remained above the minimum required b y plant operational limits and conditions. Since there was no initiator, the use of Table I in Part III and the resulting rating of level zero are appropriate.