The science of compliance

Andrew Lawson, EMC chief engineer at a global product testing and certification organisation, TÜV SÜD discusses the electromagnetic compatibility (EMC) compliance of avionics equipment.

EMC affects all electrical and electronic systems in use including those in avionics. The problems of interference range from nuisance, such as poor quality TV/radio reception to unreliability, safety issues and failure.

Example sources of electromagnetic interference (EMI), known as emissions, are both natural, such as static and lightning, and man-made intentional transmission and unintentional interference, such as electronics, electric power, communications, tools, machines and appliances and ignition systems.

Avionics platforms and installations have high power intentional transmitting sources and other sources of interference. They must therefore be designed to ensure EMC with the natural and man-made electric, magnetic and electromagnetic environments in which they are to be deployed. They must also offer compatibility with natural environment sources.

Manufacturers need to ensure their products meet safety legislation for the products and their use in safety related applications, and EMC standards contribute to ensuring equipment functional safety. However, EMC testing alone does not provide safety assurance as it would not be practical to test for all conditions. This means it does not consider foreseeable faults, misuse, ageing, production tolerances, environmental factors, full range of EM phenomena and levels over lifecycle, and testing uncertainty. Therefore, the focus should be on design and risk analysis to identify how EMC susceptibility would cause the product to fail, the consequences and any design mitigations.

Unfortunately, all avionics electromagnetic phenomena are not covered in a single standard. RTCA DO-160 is an avionics environmental standard and therefore includes a wide range of non-EMC phenomena, as well as EMC test methods and limits for equipment. RTCA DO-357 is the user guide to DO-160. DO-357 provides additional background information for the associated test procedures and requirements of DO-160. It also includes the rationale for requirements, guidance in applying the requirements, commentary, possible troubleshooting techniques and lessons learnt from laboratory experience.

DO-160 Section 1 - Purpose & Applicability

RTCA DO-160 Section 1 requires the equipment performance standards to be specified, which define the minimum functional performance of different categories of equipment. The manufacturer can either adopt an RTCA or EuroCAEMOPS or specify their own equipment specification. These equipment performance standards are required during susceptibility testing to ensure that the equipment continues to function correctly.

DO-160 Section 2 -Definition of Terms -General

RTCA DO-160 Section 2 requires the severity of limits, known as ‘categories’, to be decided. DO-160 lists categories for each test. Note that the category reference is not the same in all tests. Due to the large number of possible categories, they are not listed here but reference should be made to the DO-160 standard and the DO-357 User Guide. The category information can be entered onto the Environmental Qualification Form in DO-160 Appendix A.

DO-160 Section 3 –Conditions of Tests

RTCA DO-160 Section 3 provides the general test requirements: Connection & Orientation of Equipment; Order of Tests, Multiple Test Articles; Combining Tests; Environmental Conditions; Test Equipment; Multiple Unit Equipment; EUT Configuration for Susceptibility Tests.

This describes how to set up the equipment and the test facility and test equipment requirements. Since these apply to all the environmental tests, some will not be applicable to EMC e.g., combining test conditions.

DO-160 Tests

Section 15 to 25 specify the EMC test requirements: 15. Magnetic Effect; 16. Power Input; 17. Voltage Spike; 18. Audio Frequency Conducted Susceptibility – Power Inputs; 19. Induced Signal Susceptibility; 20. RF Susceptibility (Radiated & Conducted); 21. Emission of RF Energy; 22. Lightning Induced Transient Susceptibility; 23. Lightning Direct Effects; 25. Electrostatic Discharge (ESD).

Some tests are subdivided into multiple sub tests which correspond to separate tests in other standards. When writing test plans it is easier to consider these sub-tests as tests in their own right as they have their own test method and limit. Section 19 is a good example as it comprises five different tests.

EMC General Test Requirements

A screened room must be used for both RF emission and susceptibility measurements to provide a low level of ambient noise (emissions) and to prevent radiated interference infringing the Wireless Telegraphy Act (susceptibility). Some low frequency and conducted tests do not require a screened room.

A basic screened room has the disadvantage that it is a metal box and will resonate at certain frequencies, increasing the measurement uncertainty (40dB has been quoted). To improve the performance of the screened room, an RF absorber is attached to the walls to provide absorption of the RF energy to prevent reflections and resonances.

Key issues in selecting a facility for test are: Adequate size for EUT. Antenna to be 0.3m minimum from absorber; Door size for EUT access; Semi anechoic lined (to meets Def Stan 59-411 Part 3 requirements); Penetration panels for test antenna/sensor and drive equipment; Ambient levels 6dB below limit; Power filtered (levels 6dB below limit); Remote monitoring for susceptibility; Cleared of all unnecessary items and personnel; Other services (exhaust extraction, compressed air, water, etc).

The choice of test facility depends on a range of factors such as accreditation, price, capability, location, and good reputation. Choosing an accredited facility is not usually mandatory but reduces the risk to the client who will otherwise have to satisfy themselves that a non-accredited facility has performed the testing correctly. UKAS accredited test facilities are given on the UKAS website: https://www.ukas.com/find-an-organisation.

Contents of a Test Plan

A control plan or procedure may be produced at the start of a project and is normally used for large or complex systems. It demonstrates to a purchaser a defined EM strategy and contractual compliance. It provides EMC guidance throughout the project lifecycle. It also defines the EM management organisation, responsibilities, EMC requirements, design approach, test and qualification programme.

The test plan or procedure provides all the information to enable a test facility to perform the tests. Generally, there is one test plan per equipment/system. The test report is produced by the test facility as a record of the tests carried out. This demonstrates how the equipment complies with the test standard against the requirements of the control plan/test plan, and hence contractual requirements. Guidance on the content of these documents is provided in DO-160, which requires the test categories and minimum performance standards (specification) to be documented on an Environmental Qualification Form, examples of which are given in DO-160 Annex A. The cost of testing and timescales can become excessive unless good engineering judgment is applied in the form of a technical rationale for the selection of tests.

Test Plan Generic Content: Description of equipment under test; Power supply, interfaces, cable lengths, size and weight; Modes of operation.

Exercise all functions for emission and susceptibility: Test configuration and layout.

Block diagram and layout on the bench/floor, grounding arrangement: Drive and support equipment; Test requirements and rationale (any tailored requirements); Susceptibility performance criteria and method of monitoring.

Parameters to be monitored, failure criteria; Sweep speed, dwell time and susceptibility modulations; Test procedures and limits; QA, documentation, safety and security.

In order to achieve consistency throughout the phases of EMC testing it is essential to formalise the details of the test plan/procedure for the project. A test plan/procedure should therefore be developed and agreed with the client project manager prior to the commencement of EMC testing and shall be sufficiently detailed to enable any test to be repeated by another approved test house. Without a formalised test plan/procedure the results of the EMC test may vary considerably due to possible variations in the test arrangement, thus obscuring the effects of any modifications during development of the equipment to the production stage.

www.tuvsud.com/uk