MIL-HDBK-274A
4.4.2.2.1 Predominant fault mode. Since the predominant fault mode concerns the external power supply connector, one cannot rely on the neutral connection to the aircraft to provide a fault return circuit to activate the overload trip. For that reason, it is necessary to bond the aircraft to the external electrical power source, for example, the MEPP or FLEDS DBAs. In the case of deck edge power, this has already been done when the aircraft is grounded to the ship's deck. Usually, the power required to activate the overload circuit is very high, well in excess of 100 amperes. It is possible that the total resistance in the bond circuit will be high enough to restrict the current flow and thus prevent activation of the overload circuit. In this case, the bond strap between aircraft and the MEPP might become hot and cause the wire to overheat. A "smoking" ground or bond wire is often the first indication of an electrical circuit problem and requires that external electrical power be shut off immediately. In the case of the engine-driven MEPP, the aircraft is grounded to a static ground. This ground also serves to ground the MEPP via the external bond wire and, under normal circumstances, the AC neutral wire. It is then not necessary to independently ground the engine-driven MEPP.
4.4.2.2.2 Engine-driven MEPP problem. The engine-driven MEPP provides a different problem. The MEPP engine is connected to the 220/440 VAC, 60 Hz main system. A 60 Hz supply cable includes a safety ground wire connected directly to the MEPP chassis. When an aircraft is connected to an engine-driven MEPP, it is not uncommon for 115 VAC, 60 Hz powered equipment to be used on or about the aircraft. It then becomes essential that should a fault occur in any of the 60 Hz powered equipment, including the MEPP engine, that the aircraft be grounded to a low resistance point of less than 25 ohms which is connected to the 60 Hz power source ground system.
4.4.2.2.3 Aircraft serviced in hangar. For aircraft being serviced inside a hangar this presents no problem, as all aircraft are routinely connected to low resistance points connected to the 60 Hz power ground. For aircraft parked on an apron and fed by an engine-driven generator, it may be difficult to locate a power ground of less than 25 ohms, but every effort should be
made to meet this requirement. It should be noted that 25 ohms is not a low enough resistance to provide adequate short circuit protection at all times against power ground faults, when the airframe may be at a 115 VAC potential. The 25-ohm value was chosen as a practical,
obtainable value to provide reasonable safety and an adequate airframe system ground.
A 25-ohm ground resistance will not provide enough fault current to trip a 50-amp circuit breaker, nor will it cause a grounding cable to burn; in fact, there may be no indication of a power system fault. The wire may get too warm to touch if a ground fault exists.
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