Machine Safety: What are fault exclusions?

Where did the term “fault exclusions” derive? Can you simply choose any fault to exclude? Who would ever exclude a fault while trying to provide employees with appropriate safe machine guarding?
By JB Titus March 8, 2013

Has anyone heard about “fault exclusions”? Where did this term emerge from? Can you simply choose any fault to exclude? Who in their right mind would exclude a fault while trying to provide employees with appropriate safe machine guarding?

Okay, now that I have your attention, let’s try to provide some background and understanding for this term.

Does a definition exist? I’ve examined several ANSI, NFPA, IEC and ISO standards. I found several uses of the term “fault exclusion” in IEC 62061, ISO 13849-1 and ISO 13849-2, however, I could not find the term in the definitions section of these standards. Additionally, I could not find the term used in NFPA 79, RIA 15.06, ANSI/PMMI B155.1 or any of the current ANSI B11 standards except one – ANSI B11.TR6 – 2010, Safety Control Systems for Machine Tools. Here’s the definition:

3.22 fault exclusion: The elimination from consideration of a specific identified failure within the Safety-Related Parts of the Control System because its probability is low relative to the systems‘ required performance, through design, selection of components, or implementation of additional measures.

Furthermore, in Clause 4 of the General Design Considerations section you’ll find the following additional clarifications for use:

4.6.2 Fault Exclusion

During the analysis, certain faults may be uncovered that cannot be detected during operation without undue economic costs. Further, the probability that these faults might occur may be extremely small, by using mitigating design, construction and installation. Under these conditions, the faults may be excluded from further consideration. This includes recommended maintenance procedures.

Fault exclusion can be based on but not limited to:

·  the low probability of occurrence of some faults;

·  tried and true (good) engineering safety practices;

·  application specific technical requirements for the specific hazard.

Detail justification shall be given in the technical documentation for any excluded faults.

ANSI B11.TR6 is a technical report and not a standard with normative requirements that “shall” be followed. ANSI standards are frequently referenced by OSHA via the General Duty Clause and therefore become law via enforcement by OSHA. As a technical report TR6 provides “guidance in understanding and implementing.”

Why doesn’t this term appear in any of our domestic normative standards? Since it’s used in several international standards why hasn’t it been defined and clarified? Can anyone help the rest of us with the background for “fault exclusion”? Will there be a trend for this term rolled into everyday life for safety professionals, end users and manufacturers?

J.B. Titus, CFSE

Have you encountered any of these issues? Add your comments or thoughts to the discussion by submitting your ideas, experiences, and challenges in the comments section below.

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