MIL-HDBK-274 Electrical Grounding For Aircraft SafetyThis handbook has been prepared to provide U.S. Navy and Marine Corps aircraft maintenance and operations personnel with the background and technical details of the different types of aircraft grounds and how they are applied. In addition, this handbook provides background information for aircraft grounding, static electricity theory and how it affects aircraft, and techniques used for measurement of grounding points. This handbook is intended for use by all U.S. Navy and Marine Corps aircraft maintenance and operations personnel for the purposes of ensuring that aircraft are properly and safely grounded. This handbook is for guidance only and cannot be cited as a requirement.

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MIL-HDBK-274A

a. MMG-1. This is an electric motor-driven MEPP, normally used inside hangars. The electric motor input is 220/440 VAC, 3-phase, 60 Hz. The input power includes a safety ground line which is connected to the MEPP chassis. The generator output neutral is also connected to the MEPP chassis. Both engine-driven and motor-driven MEPPS are bonded to the aircraft, independent of the power cable neutral or negative. Neither MEPP is independently grounded.

b. Flight line electrical distribution system (FLEDS). This system is installed on shore station aprons. AC power is provided to the system by a MEPP located at a central location. The AC neutral line is connected to ground at the point of entry to the FLED and at the remote ends. Distribution boxes assemblies (DBA) are provided at intervals throughout the system, each containing circuit breakers, overload relays, and service cables to supply two aircraft. The distribution cables are protected from taxiing aircraft by a metal ramp.

c. Deck edge power. This is a similar system to the FLEDS except for being installed aboard an aircraft carrier. The 115 VAC, 400 Hz generator is driven by an electric motor fed from the 220/440 VAC, 3-phase, 60 Hz, three-wire delta system. Aboard ship, the

3-phase, 60 Hz system is not grounded.

4.4.2.1.3 Deck Edge Power Connector. The 115 VAC, 3-phase, 400-Hz power is fed to the aircraft via a standard connector. This connector (SAE-AS25486) has six connections, A, B, C, and N for the 3-phase AC, and E and F for control purposes only. The manner in which E and F are connected varies from aircraft to aircraft. The 28 VDC supply is fed to the aircraft by a separate 3-pin connector. The connector is designed in accordance with SAE-AS25487 or

SAE-AS25488. In each case, two pins are for 28 VDC positive and negative, with the third pin for aircraft control purposes.

4.4.2.2 Ground fault. Prior to mating the power connectors to the aircraft, be sure that the aircraft is connected to a power ground. The purpose of this is to provide protection against a ground power fault for all personnel working on or about the aircraft. Occasionally, a fault may occur in the electrical supply system, probably at the supply connector, that results in the neutral line becoming disconnected from the aircraft and a phase line contacting the aircraft structure. In this condition, the aircraft skin is said to become "hot," with about 115V AC potential with respect to ground. If the aircraft is not connected to ground, a person standing on the ground

who touches the aircraft could receive a lethal shock. It is accepted that electrical voltage in excess of 45 VDC or 30 VAC can give lethal shocks to humans. This is based on a body resistance of 1,500 ohms and lethal current levels of 30 mA DC or 20 mA AC.