MIL-STD-704 Aircraft Electric Power CharacteristicsThis standard establishes the requirements and characteristics of aircraft electric power provided at the input terminals of electric utilization equipment. MIL-HDBK-704-1 through-8 defines test methods and procedures for determining airborne utilization equipment compliance with the electric power characteristics requirements defined herein. Electromagnetic interference and voltage spikes are not covered by this standard.

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MIL-STD-704F

5.3.2.3 Emergency operation. The DC steady state voltage in emergency operation shall be between 16 and 29 volts.

5.3.2.4 Electric starting. The DC voltage in electric starting operation shall be between

12 and 29 volts. Electric starting of an auxiliary power unit (other than battery starts) is a normal operating function and is not included under this condition.

5.3.3 270 volts DC system.

5.3.3.1 Normal operation. Normal operation characteristics shall be in accordance with figures 16 and 18 and table IV.

5.3.3.2 Abnormal operation. Overvoltage and undervoltage values shall be within the limits of figure 17.

5.3.3.3 Emergency operation. All power characteristics in emergency operation shall be the same as normal operation.

5.4 Load characteristics.

5.4.1 Grounding. All electrical power input terminals, including AC neutral and DC negative terminals, shall be electrically isolated from case ground. Equipment chassis shall not be used for power returns.

5.4.2 Load unbalance. Load unbalance of three-phase AC equipment shall be in accordance with figure 1. Load unbalance of equipment exceeding 30 kVA shall not exceed

3.33 percent of the total three-phase load.

5.4.3 Power factor. Power factor of AC equipment greater than 500 VA shall be between 0.85 lagging and unity when operating at 50 percent or more of its rated load current in steady state condition. AC equipment shall not have leading power factor when operating at more than 100 VA.

5.4.4 Polarity or phase reversal. Three-phase AC equipment shall not be damaged by reversal of input phase sequence. Single-phase AC equipment shall not be damaged by reversal of line and neutral connections. DC equipment shall not be damaged by reversal of positive and negative connections. Employing a positive physical means to prevent phase or polarity reversal shall also fulfill this requirement.

5.4.5 Multiple input terminals. Equipment having multiple input terminals for connection to more than one power source shall isolate the inputs from each other so that one power source cannot supply power to another. AC inputs shall not be paralleled. DC inputs shall be protected with blocking diodes if they are paralleled.