An ATR 42-300 impacted a ground power unit at the parking spot at London-Gatwick Airport, U.K. The aircraft operated on a flight from Dublin with the a new captain undergoing his first session of line training after a simulator conversion programme. He was Pilot Not Flying (PNF) on this leg. The Pilot Flying (PF) was a senior Training Captain. The taxiing of the aircraft however was accomplished by the PNF in the left seat as the nosewheel steering system hand wheel was on the left side of the flight deck. During the taxi out for departure from Dublin, the take-off configuration test was carried out and it was noted that the No 2 DC generator had failed. The crew stopped the aircraft on the taxiway and attempted to reset the generator. They also checked the aircraft’s Minimum Equipment List (MEL) in order to ascertain whether it was acceptable to operate the aircraft with one DC generator off-line. In the event, the generator was restored, and the aircraft took off at 13:47 hrs. Shortly after departure, the No 2 DC generator again failed. The appropriate checklist was carried out for the DC Gen 2 Fault, which simply involved the switching off of the affected generator. There were no other actions required in the checklist. Apart from crew consideration of possible expeditious en route diversion airfields in the event of a second generator failure, the flight to Gatwick was uneventful, and the aircraft landed on runway 08R at 15:02. The aircraft vacated the runway on No 2 rapid exit turnoff and was cleared to taxi to Stand 3 routing via the Alpha Exit link. The company’s normal operating checklist, and that of the manufacturer, called for the No 1 engine to be shut down as part of the after landing drills, once a one minute cooling period had been observed. Nothing in the abnormal checklist for a DC Gen Fault contradicted this procedure. When the left engine is feathered during taxi on this type of aircraft, there is a tendency for the aircraft to initially swing to the right. The left seat pilot therefore suggested that the feathering action be taken as the aircraft approached the natural right turn after passing through the Alpha Exit. This was accomplished, and the left propeller was feathered. In order to enable the engineers to carry out an accurate engine oil contents check during the turnaround, the left engine was kept running with the propeller feathered. The usual procedure prior to this check requires a further period of 20 seconds before engine shutdown. In the event, a period of 35 seconds elapsed prior to engine shutdown. The apron surface was damp. The toe brakes were checked for effectiveness by the left seat pilot, prior to turning onto the stand centreline of Stand 3, and he proceeded under marshalling guidance towards the appropriate stop position. On receiving the stop signal from the ground crew, the left seat pilot applied the toe brakes, but these were ineffective. He notified the other pilot of the problem, and proceeded to apply reverse thrust to the right engine. The commander also applied his toe brakes, to no effect. The PF then reached across and pulled the emergency/parking brake handle, located on the left side of the power lever quadrant, but this action, along with the initiation of reverse thrust, occurred too late to prevent the right propeller from striking the ground power unit (GPU) which had been pre-positioned to the front right side of the normal parking position in order to allow expeditious connection to the aircraft’s ground power socket. The collision occurred some 30 seconds after the shutdown of the left engine. The two ground crew, who were in position to meet the aircraft, observed that a collision was imminent and began to run clear of the area. Debris from the impact struck one of the crew men and narrowly missed the other. Fortunately, neither sustained injuries from the debris.