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Machine Safety: Which safety circuit design is required?

When considering safety circuit designs and compliance requirements, look at B level versus C level safety standards and their approval dates. See levels of machine safety standards graphic.

July 04, 2014

Machine safety standards are in levels: A) Basic safety standards apply to industry, B) Group safety standards apply to a defined group or type of machinery, and C) Specialists standards apply to one specific kind of machine, such as mechanical stamping,First of all, is everyone familiar with “B” level and “C” level machine safety standards? Secondly, is everyone familiar with the importance of approval dates of safety standards, particularly when contrasting called out compliance requirements?

Machine safety standards: 3 levels

Let’s begin by looking at the graphic showing example machine safety standards in their respective A, B, and C levels: 

A. Basic safety standards apply to industry.

B. Group safety standards apply to a defined group or type of machinery.

C. Specialists standards apply to one specific kind of machine, such as mechanical stamping, injection molding, converting, etc. 

By looking at the example standards per level, it is easy to understand what is meant by a “B” level versus a “C” level machine safety standard. 

Referenced dates matter

Additionally, when a standard is published, it is also designated via the date it was reviewed and approved. Internal to this process, a standard’s working committee will usually review all content within a standard and either update or not update any specific requirement or miscellaneous information. This can include any normative or informative other referenced standards. These other standards may or may not include a specifically dated version. 

For example, the current NFPA 79 edition is dated 2012. During the update cycle we considered whether to update the Annex J Informational References for ISO 13849-1, 1990 to the more current 2006 edition. The working committee considered this change and voted to keep the 1990 edition reference for ISO 13849-1. This vote was significant because the 2006 edition would have brought into play the newer approach for the SRP/CS (Safety Related Parts of the Control System)—specifically, things like functional safety, performance levels, mean time to fail dangerous, diagnostics coverage, B10 values, and more. This is a major change.

Quite often, when a “B” level standard makes a significant change (like NFPA 79, 2002 adopting safety PLC technology), “C” level standards may or may not be reconciled and/or in sync regarding how safety circuits “shall” be designed. So, the question is asked: Which safety circuit design is required? I understand that in Europe under the Machinery Directive only current editions of standards are “listed” as required for compliance. In the U.S. OSHA is the law and consensus standards are an additional level of compliance requirements. Therefore, and in my experience, most “best-in-class” companies in the U.S. default to a current safety circuit design regardless of “B” versus “C” level standards. How has this question been answered in your experiences?

Add your comments, thoughts, questions, or suggestions about level of machine standards or related topics by submitting your observations, ideas, experiences, and challenges in the comments section below. under Machine Safety has related machine safety explanations. See specific links below.

Related articles:

Machine Safety: what is the value of ISO 13849-1 for U.S. domestic compliance?

How to use ISO functional safety standards

Machine safety standard merger: One global machine safety standard

Machine Safety: Domestic U.S. versus international standards

EHS Today: The Buzz About ISO 13849-1: The Good, the Bad and the Ugly (and a Possible Alternate Solution) by Mike Carlson

Contact: for “Solutions for Machine Safety.”

For more than 30 years, J.B. Titus has advised a wide range of clients on machine functional safety solutions, including Johnson & Johnson, Siemens, General Motors, Disney, Rockwell Automation, Bridgestone Firestone, and Samsung Heavy Industries. He holds a Bachelor of Business Administration degree from Oklahoma University in industrial management and an MBA from Case Western Reserve University in marketing and finance. He is a professional member of the American Society of Safety Engineers and is OSHA-certified in machine guarding. Titus is also TUV-certified as a Functional Safety Expert and serves on several American National Standards Institute, National Fire Protection Association, and National Electrical Manufacturers Association national safety and health standards committees. Reach him at jb(at) and via