E-Stops Aren’t Safety Devices

While safety experts say machine safety e-stops are not safety devices, folks in industry pretty much all seem to think e-stops are safety devices because of their function. Ask yourself these three questions.

J.B. Titus

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Machine safety e-stops are not safety devices! That’s at least what the safety experts all say and what the machine safety standards say. However, the folks in industry pretty much all seem to think e-stops are safety devices because of their function.

1. Are you confusing stopping categories and risk categories?

2. Do you wire your e-stops to a standard (non Safety) PLC or controller in order to comply with the mandatory category 0 or 1 stop?

3. And, does your identified hazard require redundant circuits with continuous self checking and automatic diagnostics?

Think twice is my advice! Yes, safety standards such as NFPA 79, clause 9.2.5.4.1.3 requires that an e-stop "shall function as either Category 0 or a Category 1 stop" as determined by the risk assessment. These are stopping functions with categories as defined by NFPA 79, clause 9.2.2. This requirement does not define that e-stops are safety devices!

Furthermore, your required risk analysis may require mitigation of a risk or hazard identified as Cat. 3 or 4. These are categories of risk. If you're wiring direct to a standard PLC your e-stop may only be at best achieving Cat. 2 or less so my advice is to check your risk analysis thoroughly. Then, double check your physical application, design, and the safety ratings on all components including sensors, logic, and actuators. In short, make sure the entire circuit is completely compliant to the level of hazard (Category) and the level of stopping function with its associated category as determined by the risk assessment.

Yes, it’s true that one of the functions of an e-stop could be safety related and therefore a safety function. However, a safety device will always be a safety device and will only be a safety device and will only be intended to accomplish a safety function.   

In my opinion - to be compliant - don’t confuse hazard Categories with stopping functions/categories!

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