At the recent PSES Symposium, I attended a couple of interesting workshops on EMC and Functional Safety. One was called “Workshop on EMC & Functional Safety” presented by Keith Armstrong, Bill Radasky and Jacques Delaballe. The other was a paper presentation called “Why Conventional EMC Testing is Insufficient for Functional Safety” presented by Keith Armstrong.
For readers who are new to the idea of Functional Safety, this field deals with the ability of a product or system to function in it’s intended use environment, or in any foreseeable use environments, while reliably providing the protection required by the users. Here’s the formal definition taken from IEC 61508-4:1998:
part of the overall safety relating to the EUC and the EUC control system which depends on the correct functioning of the E/E/PE safety-related systems, other technology safety-related systems and external risk reduction facilities
equipment under control (EUC)
equipment, machinery, apparatus or plant used for manufacturing, process, transportation, medical or other activities
NOTE – The EUC control system is separate and distinct from the EUC.
Table 1: (E/E/PE) electrical / electronic / programmable electronic
Reliability requirements are found in two key standards, ISO 13849 and IEC 61508. These two standards overlap to some degree, and do not define reliability categories in the same way, which frequently leads to confusion. In addition there is a Machinery Sector Specific standard based on IEC 61508, called IEC 62061, Safety of machinery – Functional safety of safety-related electrical, electronic and programmable electronic control systems. These three standards make reference to EM effects on systems but do not provide guidance on how to assess these phenomena. This is where IEC TS 61000-1-2 comes into play.
All three experts are members of IEC TC 77 and are directly engaged in writing the second edition of IEC TS 61000-1-2 (more info on this at the bottom of this post). This IEC Technical Specification deals with electromagnetic (EM) effects on equipment that result in functional safety problems, like failures in guarding circuits, or failures in some of the new programmable safety systems. This is becoming an increasingly important issue as programmable controls migrate into the traditionally hardwired safety world. In fact, Keith pointed out that EM effects are present even in many of our “tried and true” circuits, but the failures have been incorrectly attributed to other phenomena because most electrical engineers have not been used to thinking about these phenomena, especially in 24Vdc relay-based control circuits.
In the workshop, the presenters discussed a typical product life cycle, then went on to explore the typical environments that a product may be exposed to, including the EM and physical environments. They went on to discuss the need for an EMC-related Risk Assessment and then finished up by looking at Electromagnetic Safety Planning. The whole workshop took the entire second day of the Symposium.
A key point in the workshop is that conventional EMC testing cannot practically prove that systems are safe. This is due to the structure of the EMC tests that are normally undertaken, including the use of fixed modulation frequencies during immunity testing, failure to assess intermodulation effects and many other issues. In addition, EMC testing does not and cannot test for aging effects on performance, wear & tear and other use-related conditions. The presenters discussed a number of ways that these problems could be addressed and ways that testing could be extended in selective ways to attack predicted vulnerabilities. EMC testing does not consider the reliability requirements of the tested product (i.e. IEC 61508-1 SIL-3 or SIL-4).
On the following morning, Keith Armstrong presented his paper. In this paper, Mr. Armstrong went into considerable detail on the shortcomings of conventional EMC testing when it comes to Functional Safety. He suggested some approaches that could be used by manufacturers to address these issues in safety critical applications.
The workshop presentations and Mr. Armstong’s paper can be purchased through IEEE Xplore for those that did not attend the Symposium.
The IET has published a new book, available for free from their web site, entitled Electromagnetic Compatibility for Functional Safety. This guide will be reviewed in a future post, so keep reading!
Keith Armstrong, Bill Radasky and Jacques Delaballe are members of IEC Technical Committee 77, writing IEC TS 61000-1-2 Ed 2.0, ELECTROMAGNETIC COMPATIBILITY (EMC) – PART 1-2: GENERAL – METHODOLOGY FOR THE ACHIEVEMENT OF THE FUNCTIONAL SAFETY OF ELECTRICAL AND ELECTRONIC EQUIPMENT WITH REGARD TO ELECTROMAGNETIC PHENOMENA. Edition 2 of this standard should be published by Mar-2009 according to the IEC.