When Is EMC Testing Required for Functional Safety?
The simple answer is that it is always required. “You cannot do a functional safety evaluation without looking at environmental impacts, and electromagnetic phenomena are among the most important environmental impacts to consider,” says Thomas Maier, principal engineer, functional safety, at Underwriters Laboratories (UL).
It is electromagnetic immunity that is critically important to functional safety. “All of the immunity phenomena that are known and specified in the standards need to be considered,” continues Maier. “What is important for functional safety—what is required by the major standard IEC 61508—is that testing go beyond normal levels, that higher levels of electromagnetic immunity be considered to decrease the probability that electromagnetic phenomena could cause loss of the safety function.”
Emissions are also included as well as immunity; but, in the United States, the Federal Communications Commission (FCC) typically deals with electromagnetic emissions. A few areas of functional safety (e.g., the elevator industry) are required to look at emissions and immunity, because a system can itself generate emissions that could exceed tested immunity levels. But this is the exception.
“One should be clear in making a distinction between emission requirements and immunity requirements,” says Maier. “Functional safety is overwhelmingly about immunity, and being protected against any electromagnetic emissions that could be expected in a certain environment.”
Why Is EMC Important?
EMC is critical because it is a major common cause of failure. If you design a functionally safe product, it usually has a level of redundancy: two channels that perform the safety function. This is how fault tolerance is achieved. Electromagnetic impacts could destroy or disturb both channels at exactly the same time, meaning the loss of the safety function. That’s a “common cause.”
“If you have a control signal that is involved in some kind of safety-critical control function, that signal is supposed to look a particular way,” says Anura Fernando, research engineer, predictive modeling and risk analysis, at UL. “Electromagnetic interference can distort the signal so that it looks very different, and the system then responds in an unexpected way.”
Two IEC standards specifically address EMC requirements in relation to functional safety: the technical specifications IEC/TS 61000-1-2 and IEC 61326-3-1. These are referred to in the second edition of the principal functional safety standard, IEC 61508.
IEC/TS 61000-1-2 covers electromagnetic compatibility (EMC), Part 1-2: General – Methodology for the achievement of functional safety of electrical and electronic systems, including equipment with regard to electromagnetic phenomena. This standard is not only about the EMC test; it is also about appropriate design. The standard states that it is not feasible to demonstrate electromagnetic immunity by test only, and provides techniques and measurements for EMC design appropriate to functional safety. It recommends testing against higher levels than would be expected in the system environment, even when the probability of electromagnetic interference is very low.
IEC 61326-3-1 is primarily a test standard. It provides a clear test plan for EMC, including:
- Configuration of Equipment Under Test (EUT)
º Selection of most susceptible configurations and installations (in accordance with specified use),
with rationale
º Selection of Input/Output (I/O) ports (in case of I/O ports of same type and function, only one
may be needed), with rationale
- Operation Conditions During Test
º Selection of representative “worst-case” operation modes
· Not necessarily all functionality aspects of a safety function must be tested
º EUT software during test shall be such as to enable simulation of selected operation modes
· If to-be-released software is not available
IEC 61326-3-1 also provides specific performance criterion that take into account functional safety elements. It specifies increased immunity levels as they would apply to industrial environments, as well as parts criteria that are particularly suited to functional safety applications.
Meeting the Challenge
The challenge of EMC is isolating phenomena, and UL is ideally positioned—with deep domain expertise and state-of-the-art facilities—to assist manufacturers in performing the tests necessary to secure a UL Functional Safety Listing Mark or Functional Safety Recognized Component Mark.
“Among our staff, we have decades of experience in EMC testing, much accrued through working with manufacturers having challenges in developing EMI resistant product design,” says Fernando.
UL has EMC chambers throughout the United States (Northbrook, IL; Melville, NY; Research Triangle Park, NC; Novi, MI; Fremont CA), Europe (Italy, United Kingdom), and Asia (Japan, India, New Zealand), making it easy for manufacturers to conduct EMC testing at a UL facility near them. Or UL can witness EMC testing at a company’s own or third-party facilities.
“For anyone struggling with EMC testing or design issues, UL is a flexible and proven resource to help address those issues,” concludes Fernando.
For more information on EMC testing, UL’s worldwide capabilities, and how we can help you go through the EMC design and test processes, please contact:
Kevin Connelly
631-546-2691
[email protected]
Or go to the web: http://www.ul.com/global/eng/pages/offerings/services/emc/emctesting
