Machinery Safety 101

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:

–
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
–
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.

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.

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://​www​.com​pli​an​cein​sight​.ca) in Kitchener, Ontario, and is Lead Author and Managing Editor of the Machinery Safety 101 blog.

Doug’s work includes teach­ing machin­ery risk assess­ment tech­niques pri­vately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as pro­vid­ing tech­ni­cal ser­vices and train­ing pro­grams to clients related to risk assess­ment, indus­trial machin­ery safety, safety-​​related con­trol sys­tem inte­gra­tion and reli­a­bil­ity, laser safety and reg­u­la­tory conformity.

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