MIL-STD-1399-300 Electric Power, Alternating CurrentThis military standard section establishes electrical interface characteristics for shipboard equipment utilizing AC electric power to ensure compatibility between user equipment and the electric power system. Characteristics of the electric power system are defined and tolerances are established, as well as requirements and test methods for ensuring compatibility of shipboard user equipment with the power system. The policies and procedures established by MIL-STD-1399 are mandatory. This section and the basic standard are to be viewed as an integral single document for use in the design and testing of electric power systems and user equipment.

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MIL-STD-1399-3008

5.1.1 Types of power. Types of power are as follows:

5.1.1.1 Type I, 60 Hz power. The ship service electrical power distribution system supplied by the ship's generators is 440 Vrms, 60 Hz, three-phase, ungrounded. Power for the ship's lighting distribution system and other user equipment such as electronic equipment, supplied from the ship service power distribution system through transformers, is 115 Vrms, 60 Hz, three-phase, ungrounded. Single-phase power is available from both the 440

Vrms and the 115 Vrms systems. The ship service power and lighting distribution systems are labeled as Type I. Type I, 230 Vrms, 60 Hz, single or three-phase, grounded or ungrounded power can be made available for NATO load equipment upon special request. See 1.2.1, 1.2.2, and 1.2.3 for special power types.

5.1.1.2 Types II and III, 400 Hz power. The ship service power supplied by 400 Hz motor-generator sets or solid-state converters is 440 Vrms, three-phase, 400 Hz ungrounded. The 400 Hz power is of two kinds, designated as Types II and III. Subject to the approval of a deviation request, the use of Type II is preferred over Type III, but, if more precise characteristics are required, Type III power may be supplied. Type III, 115/200 Vrms, 400 Hz,

three-phase, four-wire, grounded wye power is available for avionics shops and for aircraft servicing.

5.1.2 System grounding. Electric power systems shall be ungrounded, except as specified in 5.1.6.2 (c) and (d) and 5.2.4. Momentary intentional grounding is permitted for the operation of ground detection equipment. High resistance grounding may be installed only with NAVSEA approval. Line-to-ground current of up to 20 amperes may exist in an electric power system as a result of capacitive coupling of cables and equipment filters connected to ground.

5.1.2.1 Ungrounded system. Under ungrounded system conditions, an ungrounded electric power system shall continue to perform normally if one line conductor becomes solidly grounded.

5.1.2.2 Grounded system. A single ground fault from one line to ground will produce high fault current that shall trip protective circuit breakers, isolating the fault.

5.1.2.3 High resistance-grounded system. This system employs an intentional resistance between the electric system neutral and ground. The resistance shunts the system capacitance-to-ground, reducing possible over- voltages-to-ground. The resistance reduces line-to-ground fault current so the electric power system shall continue to perform normally if one line conductor becomes solidly grounded; the ground is indicated by the reduced fault current.

5.1.3 Power interruption. Power interruptions may range from less than 1 msec to several minutes. The interruptions can occur as a result of an equipment casualty, training exercise, or operator error. In order to maintain reliability and continuity during the diverse operating conditions such as anchor, cruising, functional, and emergency conditions, some loads are provided with a limited-break power source where possible. The extent to which this can be done will vary with the ship design, electric plant capacity, and the specific user equipment. In some instances, user systems and equipment are not provided with a limited-break power source because of the need to control the power-up cycle after the interruption. In other instances, the capacity of the ship service or emergency generators may limit the use of limited-break power sources. In those instances, user equipments will be provided two sources of power selectable by means of a manual transfer switch.

5.1.3.1 Limited-break power source. A limited-break power source is accomplished by means of a normal feeder from one ship service switchboard and either an alternate feeder from another ship service switchboard or an emergency switchboard by means of a bus transfer switch. A time delay may be required between the loss of the normal power source and switching to the alternate power source to avoid excessive transient currents caused by residual voltage. In the case of switching from a normal to emergency power source, an additional time delay is experienced to automatically start the emergency generator on standby. The time delays result in a transfer time from one power source to another of 0.008 seconds (solid-state switch) or 0.04 seconds (electromechanical switch) to several minutes.