scope:

Covered in this handbook are polyphase electrical systems used by the US Army that either may be powered continuously by an engine-driven-generator set or powered by a backup engine-driven-generator set, which supplies power when the primary supply is disabled. The typical polyphase system used by the Army has certain characteristics:

1. Alternating current (AC)

2. Power capacity up to 750 kVA

3. Polyphase-usually 3 phases

4. Line voltages of 120, 240, 2200, or 2400 Vac between any phase and ground (208, 416, 3800, or 4160 Vac between phases)

5. Tactical systems or backup systems used for strategic equipment, e.g., communication facilities, or hospitals.

This handbook covers electrical system components and interconnections from the power source through the distribution system to those end-items that are connected directly to three-phase systems. The power sources covered are primarily the engine-driven-generator set including the prime mover (internal combustion engine or turbine) and the generators driven by them. In addition, the interface with permanent utility systems is discussed along with transfer switching apparatus to provide information pertinent to those systems that use engine-driven-generator sets as a backup power system. Also discussed are the safety aspects of those end-items that are connected directly to the polyphase system. These components include large motors, strip heaters, and communications equipment. The transformers that convert the polyphase power to one or more single-phase circuits are also discussed as an end-item. Not discussed as end-items are the wiring or loads associated with the single-phase circuits connected to the transformer secondary.

As stated in the par. 1-1, the purpose of this handbook is to provide technical guidance for the design of polyphase systems and components that affects the safety of personnel and equipment. For this reason only design features that affect safety are discussed in this handbook. Those features that affect only system performance and do not impact safety are not discussed since they are covered extensively in the numerous design handbooks on electrical power systems.

MIL-STD-882 (Ref. 1) requires that hazards, especially those involving catastrophic or critical risks, shall be reduced through design selection or reduced by appropriate warning devices.

If the hazard cannot be eliminated or the risk cannot be reduced with warning devices, operational procedures and personnel training must be used to minimize the hazard. Therefore, system operation and maintenance manuals must discuss the proper procedures for safe operation of the system, and operators and maintenance personnel must be trained in those procedures. For this reason operation and maintenance procedures that have safety implications are discussed as well. Procedures discussed are those that have an inherent hazard to personnel at the time the procedure is performed and those procedures that, if improperly performed, may precipitate unsafe conditions that remain after the procedure has been completed. The intent is not to include detailed procedures in this handbook but rather to describe practices that must be incorporated in procedures and the rationale behind them.