Design Practices for Military EMC and Environmental Compliance

The reliable operation of complex electronic communications, control and armament systems in extreme environments demands stringent design criteria and careful validation. Severe shock, vibration, heat, humidity and airborne contaminants are common in land, sea and air platforms.

Coupled with dense packaging, high-power radio and radar illumination, Hazards of Electromagnetic Radiation to Ordnance (HERO), and a possible electromagnetic pulse (EMP), the military equipment environmental requirements can be extreme indeed.

In order to expedite equipment availability and reduce cost, the acquisition of commercial-off-the-shelf (COTS) equipment for US military applications is an attractive consideration. But many types of commercial equipment are unlikely to meet all military environmental requirements as manufactured, so some modification or re-design is usually needed. Defining the gap between the commercial equipment’s environmental performance and its military expectations is a first step in determining its potential suitability.

The full cycle of US military product development from environmental assessment, to definition of requirements, to test reports, is carefully spelled out in the relevant military standards or ancillary documents for the applicable physical and electromagnetic environments. These provide the design guidance, along with competent engineering practices, for a cost-effective and robust military product design.

The Electromagnetic Environment

Electromagnetic compatibility (EMC) requires the component, equipment or system to perform its designed functions without causing or suffering unacceptable degradation due to electromagnetic interference to or from other equipment. The starting point for EMC is self-compatibility, where the final product or system does not interfere with its own operation. This is a basic requirement in military EMC standards; for example, in MIL‑STD‑461F clause 4.2.3:

The operational performance of an equipment or subsystem shall not be degraded, nor shall it malfunction, when all of the units or devices in the equipment or subsystem are operating together at their designed levels of efficiency or their design capability.

As we shall see, this is the modest starting point for military EMC, which extends to both lower and higher frequencies than most commercial EMC standards and to both lower emission limits and much higher susceptibility requirements. Test methods generally differ from their commercial counterparts in both setup and detail.

History of Military EMC

EMC problems in commercial applications were first noted worldwide in the 1930s when early broadcast radios were being installed in automobiles. Reception was degraded by ignition noise and electrostatic buildup caused by non-conductive rubber tires.

The first US military specification on EMC also addressed this problem. It was published by the US Army Signal Corps in 1934 as SCL-49, “Electrical Shielding and Radio Power Supply in Vehicles”.  It required shielding of the vehicle ignition system, regulator and generator. With the increased use of mobile military radio communications, SCL-49 became inadequate. In 1942 it was superseded by specification 71-1303, “Vehicular Radio Noise Suppression.”

In the period 1950 – 1965, each major military agency imposed its own EMC specifications. The Air Force used MIL‑I-6181 and MIL‑I-26600; the Navy used MIL‑I-16910; the Army used MIL‑I-11748 and MIL‑E-55301(EL). These specifications limited the levels of conducted and radiated emissions, and they set susceptibility levels which systems and equipment must reject. These specifications also detailed the test configurations and methods for demonstrating compliance.

Unfortunately, over this period of time, the various military EMC standards diverged from each other in test frequency ranges, limits and required test equipment. The differences made it quite expensive for a test lab or manufacturer to be fully equipped to test to all EMC specifications.

In 1960 the US Department of Defense enacted a comprehensive electromagnetic compatibility program that charged the military services to build EMC into all of their communications and electronics equipment. In 1966, EMC personnel of the three military departments jointly drafted standards addressing the overall EMC needs of the Department of Defense. That program resulted in 1967 in military standards 461 (requirements), 462 (methods) and 463 (definitions and acronyms). After revision, MIL‑STD‑461A was issued in August 1968. Subsequent revisions were designated B, C, and D. MIL‑STD‑463 was withdrawn after 1990.

In 1999 the 461D and 462D standards were merged into one document, MIL‑STD‑461E. The current version is MIL‑STD‑461F (2007), and updates to it are in the planning stage. Prior revision levels A-E may still be specified for testing.

USA: Supporting Documentation

The designer of military electronic equipment has an abundance of guidance available for successfully meeting the EMC demands of the intended operating environments.

Standards

Active military standards (Table 1) specify a variety of scopes, environmental sub-categories, limits and test methods clearly and in great detail.

Table 1: Active US military EMC standards for equipment, systems and facilities

The most commonly-used MIL standards are 461 (subsystems and equipment) and 464 (systems), and they apply to ground-based, shipboard and airborne applications. Other government documents may apply to a specific platform or application, and some of these are listed in the standards such as MIL‑STD‑461 and -464.

Handbooks

In addition to the EMC standards listed in Table 1, there are a number of handbooks available that provide procedural, EMC assessment and design guidance for specific military applications. These provide guidance only, and are not to be construed as requirements. A list of relevant handbooks is given in Table 2.

Table 2: Active US military handbooks relating to EMC

Generally, these handbooks are tutorial in nature, clearly written, and with explanations of the underlying physical principles. They provide invaluable assistance to the equipment or systems designer.

Data Item Descriptions

Finally, there are very detailed documentation specifications associated with military EMC standards. In some cases the required documentation is described in separate Data Item Descriptions (DIDs) or Test Operational Procedures (TOPs). These Data Item Descriptions cover EMC design procedures, test and verification procedures, and test reports. Table 3 contains a list of Data Item Descriptions and TOPs and the military standards with which they are associated.

Table 3: EMC Data Item Descriptions and Test Operational Procedures

For example, the Data Item Description DI-EMCS-80199C associated with standard MIL‑STD‑461F is very explicit in the level of detail to be provided regarding equipment design procedures:

3.2.  Design techniques and procedures. The EMICP [Electromagnetic Interference Control Procedures] shall describe the specific design techniques and procedures used to meet each emission and susceptibility requirement, including the following:

1. Spectrum management techniques.
2. EMI mechanical design, including the following:
   1. Type of metals, casting, finishes, and hardware employed in the design.
   2. Construction techniques, such as isolated compartments; filter mounting, isolation of other parts; treatment of openings (ventilation ports, access hatches, windows, metal faces and control shafts), and attenuation characteristics of Radio Frequency (RF) gaskets used on mating surfaces.
   3. Shielding provisions and techniques used for determining shielding effectiveness.
   4. Corrosion control procedures.
   5. Methods of bonding mating surfaces, such as surface preparation and gaskets.
3. Electrical wiring design, including cable types or characteristics, cable routing, cable separation, grounding philosophy, and cable shielding types and termination methods.
4. Electrical and electronic circuit design, including the following:
   1. Filtering techniques, technical reasons for selecting types of filters, and associated filter characteristics, including attenuation and line-to-ground capacitance values of AC and DC power line filters.
   2. Part location and separation for reducing EMI.
   3. Location, shielding, and isolation of critical circuits.

This DID also requires, among other items, analysis (results demonstrating how each applicable requirement is going to be met) and developmental testing (testing to be performed during development such as evaluations of breadboards, prototypes, and engineering models). For the equipment designer, these points to be documented constitute a virtual punch list of EMC design attributes.

MIL‑STD‑461F – EMC for Subsystems and Equipment

This is no doubt the most widely-used standard for US military EMC assessment. Specific test requirements are grouped according to conducted (C) or radiated (R) coupling, and emissions (E) or susceptibility (S). Thus the tests are designated:

• Conducted emissions:    CE—
• Radiated emissions:    RE—
• Conducted susceptibility:    CS—
• Radiated susceptibility:    RS—

The dashes are replaced by the test reference number. Over time, the numerical test designations have transitioned from 01 to 101, 02 to 102, etc., but the prefixes have remained constant. Table 4 indicates the changes in MIL‑STD‑461 test requirements from versions A through E, and Table 5 reflects the present version F requirements.

Click here to view Table 4.

Table 5: MIL-STD-461F requirement changes from versions E to F (2007).

ESD and lightning effects are not included in MIL‑STD‑461F, although they are being discussed for inclusion in the next (G) version which is currently in draft to be released in 2014. ESD and lightning protection are covered in MIL‑STD‑464A, and in the current US standard for commercial aircraft equipment DO-160G, “Environmental Conditions and Test Procedures for Airborne Equipment.” DO-160G contains a number of non-EMC environmental requirements, and equipment qualified to revisions C – F of RTCA DO-160 is often suitable for military aircraft applications. A summary of DO-160G test categories is given in Table 6.

Table 6: EMC and environmental requirements in RTCA DO-160G

The military electronic equipment designer needs to know the types of EMC tests that will be applied to the equipment, the magnitudes or limits of the tests, and the frequency ranges of the tests, in order to design for compliance. The designer also needs to know that, where the equipment will be used in more than one environment, the most stringent requirements apply. Generally of secondary importance to the designer are the test configuration details, which are amply documented in MIL‑STD‑461F. These test details are of course essential to the testing personnel.

What is important to the equipment designer, for the purpose of understanding the limits, are the radiated emissions test distances – which differ from the normal commercial separations of 3m or 10m. MIL‑STD‑461F is almost unique among EMC standards in requiring a 1m distance between the electric field antenna and the test setup boundary (RE102). Only DO-160G and CISPR 25 (Automotive) has a similar radiated emissions test distance. The magnetic field measurement distance in RE101 is 7 cm.

Figure 1: RE102 test setup showing 1m antenna distance, from MIL-STD-461F

Radiated Susceptibility (RS 103) also has a 1m separation distance and typically requires a field strength of 200V/m in contrast to the 3V/m and 10V/m commonly encountered with commercial product standards such as EN61000-4-3. This higher field strength requirement can often be a hurdle for many designers involved with COTS or used to working on products intended for the commercial market.

In addition to the changes noted in Table 5, MIL‑STD‑461F addresses several topics of general applicability:

• The requirement to qualify “Line-Replaceable Modules (LRMs)” is added;
• Restricts the testing of shielded power cables;
• Includes software in the requirement to verify test procedures;
• Frequency step size above 1 GHz has been increased for susceptibility testing.

Simultaneously with the publication of the F version of MIL‑STD‑461 (December 2007), the F version of RTCA DO-160 was published. DO-160F also included, for the first time, the CS106 test that was originally in MIL‑STD‑461 but later deleted only to be restored in the latest version. Since that time DO-160G has been released (December 2010), bringing more clarifications and updates.

RTCA DO-160F and G include the ESD and lightning requirements currently absent from MIL‑STD‑461F, and it includes the environmental requirements which are found in separate MIL documents discussed below. The European Union version of DO-160G is EUROCAE/ED-14G, which is identically worded.

MIL‑STD‑464A – EMC Requirements for Systems

This standard establishes electromagnetic environmental effects (E3), interface requirements and verification criteria for airborne, sea, space, and ground systems, including associated ordnance. MIL‑STD‑464A contains two sections, the main body, and an appendix. The main body of the standard specifies a baseline set of requirements. The appendix portion provides a detailed rationale and guidance so that the baseline requirements can be tailored for a particular application.

Verification is intended to cover all life cycle aspects of the system. This includes (as applicable) normal in-service operation, checkout, storage, transportation, handling, packaging, loading, unloading, launch, and the normal operating procedures associated with each aspect.

The scope of E3, as used in this standard, is very broad: all electromagnetic disciplines, including electromagnetic compatibility; electromagnetic interference; electromagnetic vulnerability; electromagnetic pulse; hazards of electromagnetic radiation to personnel, ordnance, and volatile materials; and natural phenomena effects of lightning and static.

Margin requirements apply to all EMC related tests performed in a 464A verification exercise. The intent is to account for manufacturing variations, aging and maintenance to assure that all equipment, not just test samples, will be compliant in the field over the equipment lifetime. Additional compliance margins to the limits specified in the standard are required for safety-critical, mission-critical and electrically-initiated devices (EIDs) such as electroexplosive devices and fusible links. The additional margins are:

• ≥ 6 dB for safety critical and mission critical system functions;
• ≥ 16.5 dB of maximum no-fire stimulus for safety assurances;
• ≥ 6dB of maximum no-fire stimulus for other purposes.

The worst-case (lowest emission limit or highest susceptibility requirement) for the environments categorized in MIL‑STD‑464A are summarized in Table 7. In many cases the requirements are frequency-dependent, and are much lower than worst-case over much of the frequency range. The standard should be consulted for details and definitions.

Table 7: Summary of MIL-STD-464A requirements. The high field strength susceptibility values occur in radar bands.

MIL‑STD‑1310H – Shipboard Bonding, Grounding and Other Techniques for EMC

This document specifies standard practices in wiring, bonding, grounding and shielding to facilitate achievement of the intra-ship and inter-ship electromagnetic compatibility (EMC), electromagnetic pulse (EMP), bonding, and intermodulation interference (IMI) requirements of MIL‑STD‑464A. It applies to metal and nonmetallic hull ships and is applicable during ship construction, overhaul, alteration, and repair. MIL‑STD‑1310H is not a typical EMC standard, but it provides the methods guidance appropriate to obtaining EMC in the shipboard environment.

This revision of MIL‑STD‑1310 has been expanded to include procedures for Electromagnetic Pulse (EMP) hardening. It also provides procedures and guidance to more easily address MIL‑STD‑464A requirements in relationship to intra- and inter-ship EMC, hull-generated IMI, lifecycle electromagnetic environmental effects (E3) hardness, EMP, and electrical bonding. A separate appendix is included, with procedures to identify whether commercial-off-the-shelf equipment (COTS) or non-developmental items (NDI) meets appropriate safety requirements before use, and to provide direction to bring them into conformance when necessary.

MIL‑STD‑1541A – Space Systems

The requirements covered by this standard apply to launch and space vehicles plus the associated grounds airborne, or spaceborne operational and support elements of the space system. It applies to new and modified or redesigned equipment or systems, and to existing equipment used in new applications.

MIL‑STD‑1541A establishes the electromagnetic compatibility requirements for space systems, including frequency management, and the related requirements for the electrical and electronic equipment used in space systems. It also includes requirements designed to establish an effective ground reference for the installed equipment and designed to inhibit adverse electrostatic effects. Bonding and prevention of electrostatic buildup are covered in detail.

As with MIL‑STD‑464A, this standard imposes additional compliance margin requirements in critical situations:

• Category I: Serious injury or loss of life, damage to property, or major loss or delay of mission capability; 12 dB for qualification; 6 dB for acceptance
• Category II: Degradation of mission capability, including any loss of autonomous operational capability; 6 dB
• Category III: Loss of functions not essential to mission; 0 dB

Intersystem and intrasystem analysis is required by the standard, which also references all emission and susceptibility requirements in MIL‑STD‑461 (as modified by MIL‑STD‑1541A) for the relevant class of equipment. Some of the specific requirements of this standard not covered in MIL‑STD‑461 are summarized in Table 8. Thorough qualification testing is emphasized in the standard.

Table 8: Some requirements in MIL-STD-1541A

MIL‑STD‑1542B – Space System Facilities

This standard is intended for selected space system facilities. The requirements are applicable to all related facilities including, but not limited to, launch complexes, tracking stations, data processing rooms, satellite control centers, checkout stations, spacecraft or booster assembly buildings, and any associated stationary or mobile structures that house electrical and electronic equipment.

MIL‑STD‑1542B addresses in detail the appropriate bonding, shielding, electrical power and ground network for space system facilities. The facility ground network consists of the following electrically interconnected subsystems:

1. The earth electrode subsystem.
2. The lightning protection subsystem.
3. The equipment fault protection subsystem.
4. The signal reference (technical ground) subsystem.

EMC performance for equipment installed in space system facilities is referenced to MIL‑STD‑461. COTS (commercial-off-the-shelf) equipment installed in these facilities shall also meet the requirements of MIL‑STD‑461.

As with the other military EMC standards discussed here, MIL‑STD‑1542B requires electromagnetic self-compatibility of equipment and systems. Clause 4.2 stipulates:

Facility electrical and electronic subsystems and equipment shall be compatible with each other as well as with the technical equipment installed in the facility for support of space system operations.

UK: DefStan Documents

Equipment procured for military purposes by the UK’s Ministry of Defence must meet their defence standards (DefStan). Non-military equipment must meet the essential requirements of the EMC Directive 2004/108/EC. Ministry of Defence EMC standards are listed in Table 9.

Table 9: UK Ministry of Defence EMC standards

Collectively the UK DefStan documents cover the same concerns as UK military standards. Specifically, DefStan 59-411-3 (Part 3) corresponds closely to MIL‑STD‑461F in methods, limits and frequency ranges. For example, Magnetic emissions are measured at 70 cm in both standards, and high-frequency radiated emissions are measured at 1m in both standards. However, there are structural and content differences between the two standards:

• Individual EMC tests in 59-411-3 are denoted DCS—, DCE—, DRE—, DRS— where the “D” denotes “Defence” and is absent from -461 test references.
• DefStan 59-411-3 uses susceptibility criteria A…D, which are familiar to users of commercial IEC and EU EMC standards. Default performance criteria are defined for each susceptibility test in terms of safety-critical or safety-related function, mission-critical function, or non-safety-critical or non-essential function.
• “Man worn” and “man portable” categories and test requirements are specified in detail in DefStan 59-411-3. Testing for man-worn applications requires the use of a non-conductive dummy approximating the shape

Figure 2: Man worn test configuration from DefStan 59-411-3, DCE02

NATO: STANAG documents

The term “STANAG” stands for “Standardization Agreement” among the NATO member countries. There are literally hundreds of active agreements in place, usually drawing from one or more countries’ existing standards. Some of the STANAG agreements relating to EMC are summarized in Table 10.

Table 10: Some NATO STANAGs relating to EMC.

Both environmental considerations and EMC are covered under STANAG 4370. It references several separate documents termed “Allied Environmental Conditions and Test Publication” (AECPT). We will explore the environmental aspects later, but we will look at EMC first.

STANAG 4370 references AECPT-500 (Edition 3, 2009), “Electromagnetic Environmental Effects Test and Verification.” AECPT-500 draws for its tests and methods both from MIL‑STD‑461 and DefStan 59-411, as shown in Table 11. Individual EMC tests in AECPT-500 are denoted NCS—, NCE—, NRE—, NRS— where the “N” denotes “NATO” and is absent from -461 test references.

Table 11: Cross-reference between NATO EMC test references, MIL-STD-461 and DefStan 59-411

AECPT-500 also contains a flow chart to guide the gap analysis between commercial and military EMC requirements, when COTS (commercial-off-the-shelf) or MOTS (military-off-the-shelf) acquisitions are being considered.

This paper was authored by Intertek.  Currently, Intertek sits on more than 70 SAE standards committees to help draft the test and certifications necessary to keep people safe. Find more articles on EMC issues at www.interk.com. For more information on this topic or to find an Intertek EMC testing lab near you contact icenter@intertek.com or 1-800-WORLDLAB.