Machine Safety: ISO 13849-1 status, interlock switch safety

Upon reviewing a machine that has two interlock switches mounted to an access door, can you say that the machine is safe? Most industry experts today would say, "Probably not." Here's an explanation of why and the relevant standards.

By JB Titus November 22, 2014

Upon reviewing a machine with two interlock switches mounted to an access door, can you say that the machine is safe? Most industry experts today would say, "Probably not." Why not, and what are the relevant standards?

ISO 13849-1 (Safety of Machinery – Safety Related Parts of Control Systems – Part 1: General Principles for Design) was established in 2006, replacing EN 954 (European Standard – Safety of Machinery) to create an improved international standard for the safety of machinery. It focuses on the less complex control systems with a lot of emphasis on safety devices (like in this example) for functional safety. This machine safety standard also works with another international standard, IEC 62061 (Safety of machinery – Functional safety of safety-related electrical, electronic and programmable electronic control systems), which provides the safeguarding requirements for complex control systems (for very large, complex control systems).

Where might ISO 13849-1 be going? A working committee has been formed and is beginning a three- or four-year effort to merge 13849-1 and 62061 into one international standard. In my opinion, and as I travel the world, my frequent discussions with industry colleagues reveal a very broad awareness of ISO 13849-1. But, this is where the discussion often ends. In many countries, there still is no requirement for adoption of and/or compliance with ISO 13849-1. I have yet to discover any single reason to drive compliance for this very important international standard. Some of the various reasons I have found include:

  • Regulatory requirement
  • Company policy
  • Purchase order requirement
  • Competitive requirement
  • Best-in-class culture
  • Product liability

 … to mention a few.

My hope is that the working committee will address some of the application requirements to reduce the built-in competencies needed for executing its requirements. In my opinion, releasing 13849-1 in 2006 raised the competency bar for an application safety standard. Many medium- and small-sized companies around the globe lack these competencies. Since the medium- and small-sized companies represent approximately 80% of all global industry, it would be very helpful to reduce this added competency bar for greater adoption and compliance.

While that may be admirable and simplify future compliance, that alone doesn’t answer the question about the safety of using two interlock switches on a machine access door. Most safety experts agree that using two standard interlock switches or a safety-rated interlock switch (with dual contacts) satisfies the redundancy requirement for Cat 3 or 4 or PL c, d, or e rated hazards. Furthermore, a safety-rated switch or standard switches must be connected to a safety rated logic solver, such as a safety relay or a safety PLC. Simply applying two-door interlock switches connected to general control may not be in compliance with a relevant safety standard or mitigate the hazard identified in a risk assessment.

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