First released in October of 1959 to replace MIL - E - 7894, MIL-STD-704 delineates the standards for compatibility between aircraft power systems and utilization equipment. Initially, MIL-STD-704 only accounted for 28 Vdc and 115/200 Vac at 400 Hz originating from a three-phase, four-wire source. Over time, various revisions were made to the standard by establishing new requirements; adding voltage ranges; correcting erroneous information; creating new test methods; and releasing change notices.
The latest revision, Revision F (issued in 2004), is the current MIL-STD-704 standard. Below is a review of this standard according to its latest revision.
Review of MIL-STD-704F Because MIL-STD-704 does not include Data Item Descriptions (DIDs) for documenting test and evaluation programs, it is best to refer to MIL-STD-461 for such information.
Additionally, for test result data forms, one should refer to MIL-HDBK-704. Accompanying MIL-STD-704 are seven handbooks, which serve as guidance tools for the test and evaluation process and document preparation process. In the handbook, seven power groups are defined based on power type for utilization equipment:
MIL-HDBK-704-2: Single Phase, 400 Hz, 115Vac
MIL-HDBK-704-3: Three Phase, 400 Hz, 115Vac
MIL-HDBK-704-4: Single Phase, Variable Frequency, 115Vac
MIL-HDBK-704-5: Three Phase, Variable Frequency, 115Vac
MIL-HDBK-704-6: Single Phase, 60 Hz, 115Vac
MIL-HDBK-704-7: Direct Current, 28 Vdc
MIL-HDBK-704-8: Direct Current, 270 Vdc
In addition, MIL-STD-704-1 delineates six operating conditions for electrical systems:
Normal Electrical Power
Abnormal Electrical Power
Emergency Electrical Power
This standard also specifies a True RMS voltmeter, by which many have been confused. A True RMS (Root-Mean-Square) voltmeter is specified in this standard due to non-sinusoidal AC waveforms. When AC waveforms are not sinusoidal, the multiplier used to convert the average-value measurements to RMS is incorrect. A True RMS voltmeter will calculate the equivalent DC heating effect, mitigating any mathematical mistakes. AC/DC Power Requirements MIL-STD-704 requires that operating voltage limits and transient voltage characteristics are measured within 10 cm of utilization equipment power input terminals. Electrical power source measurements should be conducted at the output terminals of the source or at the point-of-regulation for regulated supplies. For external power sources, follow the same guidelines for internal power sources, but increase the lower voltage limit to compensate for external wiring length. See Figures 1 and 2 below for examples of test configurations for AC and DC power.
Figure 1: Example Test Configuration -AC Power
Figure 2: Example Test Configuration - DC Power EMC/EMI While MIL-STD-704 does not address electromagnetic compatibility (EMC), compliance to applicable aircraft EMC requirements is obligatory, with reference to MIL-STD-461 and MIL-STD-464. According to MIL-STD-461, temporary requirements may be imposed by MIL-STD-704. EMC should be considered separately from MIL-STD-704, as it is important to test for EMC with any electric/electronic product. At Rhein Tech Laboratories, we provide Military EMC testing, including susceptibility testing; radiated susceptibility testing; conducted emissions testing; radiated emissions testing; and receiver disturbance and sensitivity testing and measurement. Our Military and Defense Aerospace EMC testing follows the standards below:
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