Machine safety: Stop categories

by Jan 30, 2018Machine Safety

Machine safety: Stop categories


A distinction between three concepts related to words “category” and “stop” should be made before detailing the main subject of this blog.

The categories of the safety related parts of control system, stop categories and emergency stop categories are frequently understood with confusion.

Let’s stars with the first concept, (category of a SRP/CS), the category in this context refers to a classification of the safety-related parts of a control system in respect of their resistance to faults and their subsequent behaviour in the fault condition, and which is achieved by the structural arrangement of the parts, fault detection and by their reliability (ISO 13849-1: 2015).

For the 2nd and the third concepts, only stop categories are defined and classified (normal on/and emergency stop), while the emergency stop is nothing more than a function that could be concretized in accordance with a given stop category requirements (Standards IEC 60204-1, IEC 61800-5-2, NFPA 79, CSA C22.2 No. 301).

Stop categories could be sorted according to three sets of specifications:

  • Category 0: corresponds to an uncontrolled stop by an immediate removal of power to the machine actuators. For this category, some additional measures might be necessary to reach the safe state into a determined time interval (ex: brakes). One example of a safety function that corresponds to an uncontrolled stop in accordance with stop category 0 is the safe torque off (STO). For this function, the power, that can cause rotation, is not applied to the motor, the power drive system will not provide energy to the motor which can generate torque.
  • Category 1: corresponds to a controlled stop, by keeping the power available to the machine actuators to achieve the stop and removal of power when the stop is achieved.
  • Category 2: corresponds to a controlled stop with power left available to the machine actuators.

The ISO 13850 standard regarding the emergency stop functions require that only stop categories 0 and 1 could be used for an emergency stop function.

Decision regarding the stop category to use, is mainly based on the equipment application for normal stopping functions and the risk assessment for the emergency stop functions.

Finally, the following function are the most used for safety stop applications:

  • Safely-limited acceleration
  • Safe acceleration range
  • Safely-limited speed
  • Safe speed range
  • Safely-limited torque
  • Safe torque range


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