Machinery Safety 101

Why Conventional EMC Testing is Insufficient for Functional Safety

This article was updated in May 2020. Author’s note: While this article is now 12 years old, the topic of EMC and Functional Safety has only become more important. IEC/TC 77’s move to change IEC/TS 61000-1-2 from a Technical Specification to an International Standard is evidence of this. Considering the impact of electromagnetic phenomena on machinery and other technical systems’ functional safety is an essential and often neglected part of machinery safety engineering. I encourage readers to understand this aspect of design better and take steps to implement additional EMC testing to validate their designs’ robustness. – DN

At the 2008 IEEE PSES Symposium On Product Compliance Engineering, I attended a couple of interesting EMC and Functional Safety workshops. One was called “Workshop on EMC & Functional Safety” [1], 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” [2], presented by Keith Armstrong.

For readers who are new to functional safety, this field deals with a product or system’s ability to function in its intended use environment or any foreseeable use environments while reliably providing the protection required by the users. Here are the relevant formal definitions:

3.1.9
functional safety
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

3.2.3
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

IEC 61508-4:1998 [3]

Reliability requirements are found in two key standards, ISO 13849 [4], [5], and IEC 61508 [6]. These two standards overlap to some degree and do not define reliability categories in the same way, which frequently leads to confusion. IEC 62061, Safety of machinery – Functional safety of safety-related electrical, electronic and programmable electronic control systems [7] is the machinery sector-specific standard based on IEC 61508. These three standards refer to EM effects on systems but do not guide the reader on how best to assess these phenomena’ effects on their own designs. This is where IEC/TS 61000-1-2 [9] comes into play.

All three presenters are expert 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, Mr. Armstrong pointed out that EM effects are present in many of our “tried and true” circuits. The failures have been incorrectly attributed to other phenomena because most electrical engineers have not been used to thinking about these phenomena, especially in 24 Vdc relay-based control circuits.

In the workshop, the presenters discussed a typical product life cycle and then explored the typical environments that a product may be exposed to, including the EM and physical environments. They discussed 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 typically undertaken, including fixed modulation frequencies during immunity testing, failure to assess intermodulation effects and many other issues. EMC testing does not test for ageing effects on performance, wear & tear and other use-related conditions. The presenters discussed several ways that these problems could be addressed and how testing could be extended in particular ways to attack predicted vulnerabilities. Conventional EMC testing does not consider the tested product’s reliability requirements (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 regarding functional safety. He suggested some approaches that manufacturers could use 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 book entitled Electromagnetic Compatibility for Functional Safety. Unfortunately, this book is no longer available from the IET; however, here is another source.

Keith Armstrong, Bill Radasky and Jacques Delaballe are members of IEC Technical Committee 77. Since the original publication of this article in 2008, IEC/TS 61000-1-2 Ed 2.0 has been withdrawn and converted to an international standard, published as IEC 61000-1-2 [9].

IEC TC 77 Electromagnetic compatibility

Keith Armstrong is Principal Consultant at Cherry Clough Consultants in Brocton, UK.

Bill Radasky works with Metatech Corporation from his office in Goleta, California.

Jacques Delaballe works for Schneider Electric Industries SAS in Grenoble, France.

References

[1] 2008. Workshop On EMC & Functional Safety.

[2] K. Armstrong, “Why Conventional EMC Testing is Insufficient for Functional Safety,” in IEEE PSES Symposium On Product Compliance Engineering, 2008.

[3] Functional safety of electrical/electronic/programmable electronic safety-related systems – Part 4: Definitions and abbreviations, IEC 61508-4. International Electrotechnical Commission (IEC), 1998.

[4] Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design, ISO 13849-1. International Organization for Standardization (ISO), 2006.

[5] Safety of machinery — Safety-related parts of control systems — Part 2: Validation, ISO 13849-2. International Organization for Standardization (ISO), 2003.

[6] Functional safety of electrical/electronic/programmable electronic safety-related systems, IEC 61508, seven parts. International Electrotechnical Commission (IEC), 1998/2000.

[7] Safety of machinery – Functional safety of safety-related electrical, electronic and programmable electronic control systems, IEC 62061. International Electrotechnical Commission (IEC), 2005.

[8] 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, 2nd Ed., IEC/TS 61000-1-2. International Electrotechnical Commission (IEC), 2008.

[9] 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, 1st Ed., IEC 61000-1-2. International Electrotechnical Commission (IEC), 2008.

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