
Rockwell Automation Publication IASIMP-QS005H-EN-P - April 2016
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GuardLogix® Tools
Appendix C
Single-fault Reaction Time (ms)
This value is the worst-case performance of the system when it encounters a single fault or error. This type of error would
not cause the controller to fault or shutdown, but could cause a delay in performance. An example would be if a
communications packet is corrupted and a retry is needed. During this sequence, controller performance for that fault is
affected. As another example, if an error occurred with a communication from an I/O module, the data from the module
is delayed by one RPI period of that module, so any downstream logic, including a safety output, would be delayed by the
RPI period.
Multiple-fault Reaction Time (ms)
This value is the calculated worst-case performance of the system. An example of this type of fault could be a controller
hard fault, this number would be the absolute worst-case delay before a safety output would transition to a safe state.
Which Reaction Time to Use?
Only you can determine which reaction time meets the requirements of your specific safety application. Safety systems
are intended to minimize the risk of a hazard, not eliminate all of the risk. Some safety systems will use the no-fault
reaction time because a risk assessment of the application indicates it is sufficient. In an example light curtain application
where an operator is loading material, the safety system lets the load occur only at a non-hazardous phase of machine
operation. Since the operator knows when to load material, it’s unlikely that they would try to load during a hazardous
phase. The chances of the operator attempting to load at the wrong time, and the system having a fault at that same time,
is considered to be remote. Another consideration is that the possible injury would likely be medically recoverable and
not more severe. The resulting decision was to use no-fault performance.
Other application criteria may result in the use of single-fault performance. The decision on which number to use is up to
the user to decide based on their risk assessment criteria.
Safety Estimator Installation Information
The wizard feature of the Rockwell Automation® Safety Estimator works only in MSExcel 2003 or MSExcel 2007.
• Microsoft Office Professional Edition 2003 software
• Microsoft Office Professional Enterprise Edition 2003 software
• Microsoft Office Excel 2003 (standalone) software
• Microsoft Office Professional Edition 2003 Trial software
• Microsoft Office System Evaluation 2003 Enterprise Edition software
• All suite editions of the 2007 Microsoft Office system
• Microsoft Office Excel 2007 (standalone) software
NOTE: Microsoft Office Standard Edition 2003 software does not support the wizard.