Rockwell Automation Publication 1715-UM001J-EN-P - December 2020 215
Chapter 7 1715 Redundant I/O System in SIL 2 Safety Applications
For example, if the RPI = 120 ms, then consider 480 ms as the minimum
CRTL. The information in the next section helps determine the
maximum setting for the CRTL.
Determining the Appropriate CRTL Value
Use the following method to confirm whether the default value is acceptable or
you must change the CRTL value for your application.
This equation governs the value of CRTL for the I/O connections:
where CRTLeuc is the process safety time for the equipment under control
(euc).
Set Safe State Values for Inputs
You must define the safe state values for inputs if the SIL 2 data verification
checks have determined that the connection is not valid and the Connection
Reaction Time Limit (CRTL) has expired.
The default safe state value for digital inputs is OFF; for analog input modules,
the safe state value is 0. Follow these steps to change the default values.
1. Click the Input States When CRTL Expires tab.
2. Use the pull-down menus to choose values for each input point.
3. Click OK.
EXAMPLE
Consider a system function using one sensor and one actuator
given the following parameters:
• CRTLeuc: 10,000 ms
•Sensor delay: 250 ms
• Time for actuator (an ESD valve) to fully operate: 1750 ms
In this example, the setting of CRTL for the I/O connections is
less than or equal to 3000 ms.
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