Machine Safety: Are the stopping categories confusing?

My e-stops are validated to achieve a Category 0 stop function. Therefore, of the three stopping categories, we’re stopping machine motion the quickest way possible. Right? (Standards are cited below. Are they clear enough?)


My e-stops are validated to achieve a Category 0 stop function. Therefore, of the three stopping Categories, we’re stopping machine motion the quickest way possible. Right?

Machine safety photo courtesy of JB TITUS & ASSOCIATES

(Photo courtesy of JB TITUS & ASSOCIATES)


Machine experts often talk about the machine stopping Categories in comparison to the hazard Categories. This is because the hazard Categories are from lowest to highest – B, 1, 2, 3 and 4.


In contrast, the machine stopping Categories are from highest to lowest – 0, 1 and 2. This is one of the first areas of confusion around machine safety. The most severe hazard Category is Cat 4 whereas the most significant stopping Category is Cat 0 as described by machine safety standards.


However, is there additional confusion when you drill into what the standards actually say about the three stopping Categories?


In my experience, yes! First of all let’s look at what one of the most prevalent standards says in describing the stopping Categories:


NFPA 79 – 2012, Electrical Standard for Industrial Machinery

Clause 9.2.2 Stop Functions. Stop functions shall operate by de-energizing that relevant circuit and shall override related start functions. The reset of the stop functions shall not initiate any hazardous conditions. The three categories of stop functions shall be as follows:

(1) Category 0 is an uncontrolled stop by immediately removing power to the machine actuators.

(2) Category 1 is a controlled stop with power to the machine actuators available to achieve the stop then remove power when the stop is achieved.

(3) Category 2 is a controlled stop with power left available to the machine actuators.


Pretty straight forward, huh? Well, we’ve already said that stopping Cat 0 is determined as the most significant. But, many users say it appears that per definition it’s an uncontrolled stop where all power is removed from all actuators. This means that on certain kinds of machinery (such as direct drive) the machine movement will coast to a stop and rotating energy (such as a fly wheel) will coast to a stop.


If “most significant” equals the “shortest time” duration does a Cat 0 stop achieve the shortest time?


All safety standards require that an e-stop function shall achieve either a Cat 0 or Cat 1 stop as determined by the risk assessment. So, a Cat 1 stop is a controlled stop with power to machine actuators is maintained to achieve the stop. Then, power is removed from the actuators after the stop of machine motion is achieved. Okay, folks around machines say that this approach is designed to keep power to machine actuators in order to apply full friction braking, reverse current braking, etc. until all machine motion has stopped and then remove all power.


 If “most significant” equals the “shortest time” duration does a Cat 1 stop achieve the shortest time?


What is your opinion? Are the standards clear enough on these definitions? It appears that a Cat 0 stop which removes all power can not apply any braking to shorten the stopping of all movement. Lacking any further definition by the standards, is a Cat 0 stop the most significant or is a Cat 1 stop the most significant? Which of these two stops causes the most stress on the machine? A Cat 2 stop is not considered in this confusion argument because it’s considered a normal cycle stop. 


Your comments or suggestion are always welcome so please let us know your thoughts. Submit

your ideas, experiences, and challenges on this subject in the comments section below. If you don't see a comments box, to, then find this blog under Machine Safety: Machine Safety: Are the stopping Categories confusing?


Related reading and articles:

ANSI B11.19-2010, Performance Criteria for Safeguarding

NFPA 79 – 2012, Electrical Standard for Industrial Machinery

E-Stops and Your Compliance

E-Stops Aren’t Safety Devices


Contact: for “Solutions for Machine Safety”.

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