E-Stops and Your Compliance
STOP! Are you confusing stopping categories and risk categories? 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? 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 22.214.171.124.1.3 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.
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 it’s associated category as determined by the risk assessment.
To be compliant - don’t confuse hazard Categories with stopping functions/categories!
Posted by J.B. Titus on October 27, 2009