FEATURE MILITARY & DEFENCE
Military systems and EMC: A code of practice
Pete Dorey, Principal EMC Consultant at TÜV SÜDand TÜV SÜD BABT, explores handling EMC issues in military systems
adding the appropriate protection ‘barriers’ to reduce the coupled RF fields or currents that the equipment could be exposed to, or could emit, to below the levels it was originally required to meet. Many manufacturers now offer suitable RF
shielded racks and enclosures for this purpose, which allow the MOTS/COTS equipment to be housed without modification therefore preserving the validity of its CE marking. Additional filters and transient protection
can also be accommodated within the enclosure. If the equipment itself is modified to achieve EMC, it is considered to have become 'modified off the shelf' equipment and needs to meet the EMC Directive with CE marking as a ‘new apparatus’ in its own right.
W
hen integrating military off-the-shelf (MOTS) and commercial off-the-shelf
(COTS) into defence systems, correctly understanding and performing the EMC assessment requirements is a complex challenge. The UK EMC Defence Standard 59- 411 provides guidance on a risk assessment process for successfully achieving electromagnetic compatibility (EMC), and it contains four key steps:
1. Defining the Electromagnetic Environment Once this environment definition is known, the adequacy of the MOTS/COTS EMC performance can be evaluated. The target electromagnetic environment is usually specified in the User Requirement Document (URD) or System Requirement Document (SRD), and is likely to be one of the default electromagnetic environments within Def Stan 59-411 (e.g. a ship below-decks environment). However, it may also be specified for a unique purpose, to ensure compatibility with specified systems, or tailored to a specific environment. 2. Evaluation of EMC Compliance All non-exempt MOTS/COTS equipment must carry the CE marking to European Directives, but its presence alone is not enough to identify levels of EMC performance to which the equipment must comply. This must be identified from the manufacturer’s EU Declaration of Conformity, test report, certificate or specifications. It is best practice that a copy of the EMC test report or certificate is obtained to confirm the limits applied during testing, in order to ensure the equipment’s suitability for military use. However, obtaining the evidence of EMC compliance is often a major challenge. Using the guidance in Def Stan 59-411, a ‘gap
26 MAY 2016 | ELECTRONICS
analysis’ process can be used to determine whether the MOTS/COTS EMC compliance evidence is more or less stringent than the Def Stan 59-411 test limit. Any shortfalls identified also helps to specify the degree of additional protection that is required, such as shielding or filter attenuation. During this process, it is imperative that you identify the test methods and account for them as part of the comparison. This can be a time consuming, costly and complex exercise, so the use of specifically designed gap analysis tools is recommended. 3. Functional Criticality The risks identified during the gap analysis process must now be compared to the criticality of the equipment and platform environment impact in which the COTS/MOTS equipment will be used. If any unacceptable risks are identified, they
must then be mitigated. For example, if the equipment has a critical function itself, adequate immunity is required. Secondly, if the equipment is co-located with other sensitive critical equipment, adequate emission control is required. Once the functional criticality process has determined the unacceptable risks, they must then be mitigated. 4. Mitigation of Unacceptable Risk There are two options: 1. Retest the MOTS/COTS equipment to determine compliance with Def Stan 59-411. This is technically a good approach, as any additional protection can be properly specified and over-protection will be avoided. However, the disadvantage is the cost of the additional required testing. 2. Remedial re-design can be achieved by
Figure 1:
When designing MOTS and COTS into defence systems, correctly understanding and performing the EMC assessment
requirements is a complex but vital challenge for mission critical applications
EMC REGULATIONS Something that could be overlooked is that the current UK EMC regulations (SI 2006 No.3418), implementing the EU EMC Directive (currently 2004/108/EC due to change to 2014/30/EU in April), do not have a specific exemption for defence equipment, whereas the previous regulations did, under Article 346 of the Treaty of the European Community. Member states can exclude defence equipment
from the scope of European Union procurement regulations and Directives pursuant to Article 346 (ex.296) of the Treaty on the Functioning of the European Union on the basis of the protection of national security interests. This is not a decision for the manufacturer. Applying the EMC Directive to defence
equipment requires careful management to avoid unnecessary duplication of compliance- testing - once for Def Stan 59-411, and once for CE marking, which incurs additional costs. However, testing can be minimised by using
the gap analysis process to establish within the technical documentation the equivalence between Def Stan 59-411 and the MOTS/ COTS standards. Gap analysis is not a time consuming and costly exercise if the appropriate tools and expertise is available. Therefore a partner that offers gap analysis
as well as testing could offer a low risk, and ultimately low cost solution to successfully deal with defence EMC requirements. Taking such a step will ensure that the resulting products are legally placed on the market and acceptable for delivery, particularly for high-value projects.
TÜV SÜD
www.tuv-sud.co.uk T: 01489 558100
/ ELECTRONICS
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