Not Again – E-Stops Do Not Equal Machine Safety
How many times does this issue come up? An e-stop device is not a safety device! Therefore, it’s not a part of the safety related parts of a control system (SRP/CS). Then, why is an e-stop device so often confused as being part of the safety design for a machine?
In my opinion, we should first look at the term, Functional Safety. “Functional Safety” was introduced to industry by IEC 61508: 1998. IEC 61508, Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems, defines it as the following. IEC 61508 Definition:
- Safety is the freedom from unacceptable risk of physical injury or of damage to the health of people, either directly, or indirectly as a result of damage to property or the
- Functional Safety is part of the overall safety that depends on a system or equipment operating correctly in response to its inputs.
In a functional safety system there are electrical and electronic components like; safety relays, interlock switches, safety contactors, safety PLC’s and drives, safety motion scanners and light curtains, and safety net works. All of these technologies are then designed to form a safety system and are dedicated to perform specific safety functions. These safety functions are usually determined as required to mitigate certain hazards identified by the risk assessment process.
Not mentioned above is the e-stop device. I believe that’s because e-stops have a broader range of application, must be manually activated, and are not dedicated to perform a safety function. The colors, red push buttons with a yellow background, may be confusing. But, they are not required and applied to always perform safety-related functions. According to NFPA 79; 2007, Electrical Standard for Industrial Machinery, e-stops are required for “final removal of power to the machine actuators” to achieve either a Category 0 or Category 1 stop (Clause 9.2.2 Stop Functions and Clause 22.214.171.124.1 Emergency Stop). Many times the e-stop may be activated due to non safety-related functions, such as; material feed issues, impending machine jams, maintenance issues, or machine set-up requirements.
Does this help clarify the often misunderstood e-stop versus safety device issue?
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