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Stage 2: Determine the Functional Safety System Requirements

The Functional Safety Lifecycle and YOU!

Stage 2: Determine the Functional Safety System Requirements

Machine safety has become more recognized than ever before.  Employers and equipment manufacturers are tasked with providing equipment that is free from recognized hazards yet remain efficient, productive, & competitively priced.   Revere Electric Supply, in partnership with Rockwell Automation, provides the tools needed to improve compliance, simplify development, reduce design time, and reduce costs. 


In Stage 1: Understanding Hazard & Risk Assessments, you learned the importance of having a proven method of providing documentation showing a machine is safe to operate, or mitigation techniques are required.  For Stage 2, we will focus on developing the functional safety requirements for your newly assessed machine. 


A functional specification uses the data collected from the Risk Assessment to determine the appropriate mitigation techniques used in protecting the personnel in and around the machine. 

Revere Electric Machine Safety

What Considerations Should go into Developing Functional Specifications?

  1. What are the different modes of operation?
  2. What are the tasks and hazards associated with each mode of operation?
  3. What can we do to protect employees from these hazards without impacting their ability to do their job?
  4. What is the overall system structure?
  5. How will the system operate in each mode of operation?


At the end of reviewing the considerations, a Safety Function is developed.  This can be one simple function or it can extend to numerous devices, depending on the number of tasks / hazards found during the risk assessment.


A Safety Function Consists of Three Components, They are as Follows:

  1. Input Device – Considered to be the “Start” of the safety function.  The Input device will send the signal to the logic device that its safe state has been altered, thus an action triggered by the logic device should take place (Light Curtains, Safety Mats, Guard Door Switches, Trapped Key, Non-Contact Switches).
  2. Logic Device – Considered the “brains” of the safety function.  The logic device will read the status of the input device and output device and determine if the system needs to be put into a safe state.  The number of input devices will generally dictate which logic device is appropriate (MSR & GSR Safety Relay, CR30 Configurable Safety Relay, Compact GuardLogix, Control GuardLogix).
  3. Output Device – These are the “action items” of the safety function.  They are the devices that can remove power from the actuator, or prevent rotational energy from being delivered to achieve a safety stop (Safety Control Relays or Contactors, drives and Servos).
  4. Functional Safety Description – A detailed description of how the input, logic, and output devices are designed to work for the specified Safety Function.

For a complete list of Input, Logic, & Output Devices, please visit: https://ab.rockwellautomation.com/Safety.

At the end of the day, Rockwell Automation has assembled numerous Machinery Safety Functions that are applicable to current Input, Logic, & Output devices.  Along with those devices, you can see samples of the functional safety descriptions that highlight the expectations of the safety system in a common environment.  If you are working on current project, please use these as reference to help you move from your Risk Assessment (Stage 1) to Design and Verification (Stage 3).


Rockwell Automation Machinery Safebook 5

Rockwell Automation Pre-Engineered Safety Functions




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