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Stop Categories
The stop categories discussed here are not exclusive to emergency stop functions. They are STOP functions and may be used for normal stopping functions as well as the Emergency Stop function.
Stop categories and functional safety system architecture categories are not the same, and there are significant differences that need to be understood by control system designers.
Stop Category Definitions
The stop categories are broken down into three general groups:
| Category | IEC 60204 – 1 | NFPA 79 | CSA C22.2 No. 301 |
| 0 | stopping by immediate removal of power to the machine actuators (i.e. an uncontrolled stop – see 3.56); | is an uncontrolled stop by immediately removing power to the machine actuators. | stopping by immediate removal of power to the machine actuators (i.e., an uncontrolled stop; |
| 1 | a controlled stop (see 3.11) with power available to the machine actuators to achieve the stop and then removal of power when the stop is achieved; | is a controlled stop with power to the machine actuators available to achieve the stop then remove power when the stop is achieved. | a controlled stop with power available to the machine actuators to achieve the stop and then removal of power when the stop is achieved; |
| 2 | a controlled stop with power left available to the machine actuators. | is a controlled stop with power left available to the machine actuators. | a controlled stop with power left available to the machine actuators. |
Definitions from IEC 60204 – 1:
3.11 controlled stop
stopping of machine motion with electrical power to the machine actuators maintained during the stopping process
3.56 uncontrolled stop
stopping of machine motion by removing electrical power to the machine actuators
NOTE This definition does not imply any particular state of other stopping devices, for example mechanical or hydraulic brakes.
Selecting a Stop Function
How do you decide on what stop category to use? First, a risk assessment is required. Second, a start/stop analysis should be conducted.
Once the risk assessment is complete, ask these questions:
1) Will the machinery stop safely using an uncontrolled stop?
If the machinery does not have a significant amount of inertia, meaning it won’t coast more than a very short time, then a Category 0 stop may be all that is required.
2) If the machinery can coast when power is removed, or if the machinery can be stopped more quickly under control than when power is simply removed, then a Category 1 stop is likely the best choice, even if the power-off coasting time is fairly short.
Vertical axes that may collapse when power is removed will likely need additional mechanical hardware to prevent the tooling from falling during an emergency stop condition. This could be a mechanical brake or other means that will prevent the tooling from falling unexpectedly.
3) If the machinery includes devices that require power to keep them in a safe state, then a Category 2 stop is likely the best choice.
If you choose to use a Category 2 stop, be aware that leaving power on the machinery leaves the user open to hazards related to having power on the machinery. Careful risk assessment is required in these cases especially.
Category 2 stops are not permitted for emergency stop functions, although you may use them for normal stop functions. ISO 13850, IEC 60204 – 1, and NFPA 79 explicitly limit emergency stop functions to Categories 0 and 1. CSA C22.2 No. 301 permits the use of Category 2 stop functions for emergency stopping.
Risk Assessment and Stop/Start Analysis
Risk assessment is critical to the specification of all safety-related functions. While emergency stop is not a safeguard, it is considered to be a ‘complementary protective measure. Understanding the hazards that need to be controlled and the degree of risk related to the hazards is basic design information that will provide specific direction on the stop category required and the degree of control reliability necessary to provide the expected risk reduction.
Stop/Start Analysis is quite simple, originating in ISO 12100. It amounts to considering all of the intended stop/start conditions for the machinery and then including conditions that may result from reasonably foreseeable failure modes of the machinery and foreseeable misuses of the machinery. Create a table with three columns as a starting point, similar to Table 2.
Table 2
Example Start/Stop Analysis
| Description | Start Condition | Stop Condition |
| Lubricant Pump | Lubricant Pump Start Button Pressed | Lubricant Pump Stop Button Pressed |
| Low Lubricant Level in reservoir | ||
| High-pressure drop across lubricant filter | ||
| Main Spindle Motor | Start enabled and Start Button Pressed | Low Lubricant Pressure |
| Stop button pressed | ||
| Feed Advance motor | Feed Advance button pressed | Feed Stop button pressed |
| Feed end of travel limit reached | ||
| Emergency Stop | All motions stop, lubricant pump remains running |