Review of MIL-STD 704 Requirements

Introduction

This article discusses the power quality test and evaluation with a review of MIL-STD-704 and the relationship with electromagnetic compatibility (EMC). MIL-STD-704 deals with aircraft electric power characteristics, and provides the standard to ensure compatibility between the aircraft power system (including external power) and the utilization equipment.

MIL-STD-704 covers the power generation system by citing the requirements that the power system is to provide at the utilization equipment power terminals. The testing evaluates the utilization equipment for compatibility with the cited performance of the power system. EMC is not covered by the standard but compliance to the applicable aircraft EMC requirements is stated with reference to MIL-STD-461 and MIL-STD-464.

If the information in the reference documents conflicts with MIL-STD-704, then MIL-STD-704 takes precedence. This is also true of MIL-STD-461, so if MIL-STD-704 conflicts with MIL-STD-461, the prevailing authority would have to resolve the conflict with the outcome most likely applying the more conservative requirement for overall system compatibility.

Background

MIL-STD-704 was issued in October 1959, as a standard superseding the engineering document MIL-E-7894 dealing with aircraft electrical power. The initial MIL-STD-704 considered only 28 Vdc and 115/200 Vac at 400 Hz from a three-phase, four-wire, “y” source. The requirements were based on the available technology for power generation and distribution without undue weight and volume demands within the airframe capacity. Revisions have been made over the years to implement new requirements, add voltage ranges, clarify and correct errors, update limits and establish more standardized test methods. In addition to new revisions, change notices were released to make corrections between revision releases.

Revision “B” added an alternate standard voltage of 230/400Vac, three-phase at 400 Hz by including a statement that the existing 115/200 Vac voltage limits apply proportionally to the 230/400 Vac standard. An alternate DC nominal voltage of 270V was introduced into the standard.

Revision “F” issued in March 2004, is the current MIL-STD-704 standard with Change 1 dated December 5, 2016 providing the most recent update. This revision introduced an alternate voltage of 115 Vac/60 Hz single phase for use by commercial-off-the-shelf (COTS) equipment aboard the aircraft. An integral part of MIL-STD-704F is the link to MIL-HDBK-704-1 through MIL-HDBK-704-8 that provides test procedure guidance for various type of aircraft electrical power.

This review will consider the requirements of revision “F”, but if your product is required to comply with a different revision, you will need to review those particular requirements for differences.

Data Item Descriptions (DIDs)

MIL-STD-704 does not specify a DID for documenting the test and evaluation program but the DIDs associated with MIL-STD-461 provide good outlines for test procedures and reports. Integral to the report are the test result data forms contained in the applicable MIL-HDBK-704 that should be used. The approving authority is familiar with those forms, so the review process is simplified, and expected test result data are not omitted.

Handbooks (MIL-HDBK-704-1 through MIL-HDBK-704-8)

The handbooks are cited in the base standard with a statement that the issues of these documents are cited in the solicitation or contract. In the handbook, it is stated that MIL-STD-704 and the handbook are companion documents, and the handbook is for guidance only and cannot be cited as a requirement. This appears to be contradictory, but the handbooks guide the test and evaluation process and guide documentation preparation. The handbooks are thorough, but the need to develop a test procedure is not replaced by the applicable handbook.

MIL-HDBK-704-1: Serves as the introductory document for the MIL-HDBK-704 series providing a listing of the other handbooks along with how the test numbering scheme applies for identifying the detailed test required.

Seven power groups are defined by different handbooks based on the power type for utilization equipment. The power type for utilization equipment is selected based on the specific platform capacity and availability on selected aircraft.

1. MIL-HDBK-704-2: Single phase, 400 Hz, 115Vac
2. MIL-HDBK-704-3: Three phase, 400 Hz, 115Vac
3. MIL-HDBK-704-4: Single phase, variable frequency, 115 Vac
4. MIL-HDBK-704-5: Three phase, variable frequency, 115 Vac
5. MIL-HDBK-704-6: Single phase, 60 Hz, 115 Vac
6. MIL-HDBK-704-7: Direct current, 28 Vdc
7. MIL-HDBK-704-8: Direct current 270 Vdc

MIL-STD-704-1 also defines six distinct operating conditions for the electrical system. The utilization equipment specification must define the performance criteria for each of the operating conditions.

1. Normal electrical power
2. Power transfers
3. Abnormal electrical power
4. Emergency electrical power
5. Engine starting
6. Power failure

Performance criteria examples are included in MIL-HDBK-701-1 to guide the selection of appropriate performance for each of the operating conditions. The exact performance criteria to be applied depend on the item criticality. A few of the performance criteria to be considered include:

1. 100% full performance
2. Reduced performance for selected operating conditions
3. Recovery within X-seconds after short interruption
4. Recovery within Y-seconds after short interruption
5. Recovery without data loss after power is restored
6. Recovery with data loss after power is restored
7. Recovery with operator intervention to restore operation
8. Not recover after power failure

As part of the requirements following requirements discussion, the power type handbook guidance will be included without trying to cover each of the types. Notes from other handbooks will be included in the discussion to identify significant differences. As stated earlier, the test procedure should be prepared to detail the applicable tests and the elements called out in MIL-HDBK-704-1.

AC Power Requirements

The requirements specify the operating voltage limits and transient voltage characteristics are measured within 10 cm of utilization equipment power input terminals. This distance is not at the mains end of a power cable but within 10 cm of the equipment under test (EUT) power connector. Figure 1 provides a generic test configuration that tends to be common to the various tests cited in the handbooks with many of the test equipment items used as necessary for the measurement being accomplished. Electrical power source measurements are made at the output terminals of the source or at the point-of-regulation for regulated supplies.

This review is not going to go over each requirement, because the handbooks guide one through evaluating each of the requirements. The applicable handbook also provides a requirement listing for each of the MIL-STD-704 revisions, identifying the acceptance limits or will refer to the specific location in the standard to find the detailed information. Table 1 provides a test matrix identifying the test number, and which requirement is being evaluated by the requirement. Each of the test methods includes a sketch of the test configuration identifying the test equipment necessary for that evaluation. The test record forms are valuable tools, providing a place to record the data in a very organized manner. The forms should be used for the test report.

Table 1: AC Power Requirements Test Matrix

DC Power Requirements

The requirements specify the operating voltage limits, and transient voltage characteristics are measured within 10 cm of utilization equipment power input terminals. As with the AC power, this distance is not at the mains end of a power cable but within 10 cm of the EUT power connector. Figure 2 provides a generic test configuration that tends to be common to the various tests cited in the handbooks with many of the test equipment items used as necessary for the measurement being accomplished.

Similar to the AC power, this review will not go over each requirement, because the handbooks guide one through evaluating each of the requirements. Table 2 provides a test matrix identifying the test number and which requirement is being evaluated by the requirement. Each of the test methods includes a sketch of the test configuration identifying the test equipment necessary for that evaluation.

Table 2: DC Power Requirements Test Matrix

External Power Requirements

The requirements for external power sources follow the same guidelines as the internal power with the lower voltage limit increased to compensate for external wiring length. The voltage required from the external source is still measured at the utilization equipment terminals. If evaluating the external power source, remember that the unit is treated as if it is the internal source.

Questions received

1. Why is a True RMS voltmeter specified?Two basic types of voltmeters are commonly used: 1) average responding; 2) True RMS (root-mean-square). A sinusoidal AC waveform of X volts (rms) applied to a resistive load will produce heat equal to the DC X-volts. The average-responding meter measures the average value and converts that measurement to RMS by multiplying the average by 1.11 to obtain the RMS. If the waveform is not sinusoidal as with most power signals from switched mode distortion, the multiplier is inappropriate. A True RMS voltmeter will calculate the equivalent DC heating effect and provide that measurement. The error from average-responding meters for non-sinusoidal waveform can be significant.
2. In MIL-STD-704F, paragraph 6.9 indicates that the table embedded in a graph takes precedence over the graph line because the figures get distorted from reproduction. When I present data vs. limit for maximum distortion (Figure 7 of the standard) my limit line takes a strange curve for the VF curve at 500 kHz instead of the straight line in the graph. What should I use?Paragraph 6.9 states tables and equations—not just the table. The table at 500 kHz provides a value of -24.44 but using the equation (-20 dB/decade) in the graph the value is -28.42. The table appears to be incorrect.
3. Only single phase 115 Vac at 60 Hz is covered by the standard. Why is a limit provided for voltage unbalance in Table III?Typo—we realize that a phase unbalance is not applicable to a single-phase power system. However, if an unusual demand for three-phase 60 Hz were to arise, the unbalance limit is provided.
4. What measurement tolerance applies to MIL-STD-704?A very good question—I don’t seem to find that information—readers, if you know where this information is located please let me know!

EMC and MIL-STD-704

MIL-STD-704 does not address EMC other than to state that EMI/EMC and voltage spikes are not covered by the standard. MIL-STD-461 indicates that transient requirements may be imposed by MIL-STD-704, so spike control may not be fully addressed at the equipment level between the two standards. This does not mean to ignore these issues when spikes can disrupt required operation.

CE101 limits for Army aircraft limit consider MIL-STD-704 distortion requirements to avoid conflicting requirements. In addition, the CE101 supports using a 5 µH Line Impedance Stabilization Networks (LISNs) with an appropriate limit adjustment to preserve the MIL-STD-704 power quality without undue limit requirements.

MIL-STD-461 RE102 limits may conflict with the implied MIL-STD-704 limit that could support radiation at levels above the RE102 limit. MIL-STD-461 recommends tailoring the limit based on the aircraft equipment to avoid an overly restrictive power quality concern.

Summary

EMC and MIL-STD-704 should be viewed as separate requirements, but neither can fail to consider the other to assure aircraft functionality and absence of interference.

The addition of the handbooks has helped make qualification testing more straightforward, and the forms contained in the handbooks are valuable to support reporting.

Hopefully you will find this information useful and I welcome questions. I am willing to share an Excel spreadsheet for log-linear and log-log chart interpolation—just request and agree not to place in the public domain.