SIL 2 and SIL 3 Considerations
The following information is only typical and may not be required for all safety I/O that can be used with safety controllers. For information specific to your I/O product, see the product user manual.
A difference between the safety integrity levels is that single-channel I/O devices are possible for SIL 2, and dual-channel I/O devices are typically required for SIL 3.
From a safety architecture perspective, one channel means that the hardware fault tolerance (HFT) is zero. When the HFT is zero, there are guidelines that state that faults must be detected and the safety function must be taken to a safe state within the process safety time. An exception applies if the diagnostic test rate is 100 times the demand rate. If you use safety I/O modules in single channel SIL 2 applications, consider the following:
- The input or output channel must be configured for Safety Pulse Test
- A process safety time greater than 600 ms (the typical safety I/O pulse test interval) or the demand rate must be less than one demand per minute, such as one per hour
Digital safety input modules support single-channel SIL 2 and dual-channel SIL 3 safety input circuits. Because these modules are rated for both SIL 2 and SIL 3 operation, you can mix SIL 2 and SIL 3 circuits on the same module.
The following figure shows how to wire SIL 2 safety circuits to safety input modules.
IMPORTANT:
The test source must be configured for pulse testing.
Input Wiring
If you have two SIL 2 safety circuits, you can add a second as shown below.
Input Wiring Pairs
A typical SIL 3 wiring diagram is shown below.
SIL 3 Input Wiring
IMPORTANT:
These wiring drawings are examples of possible wiring configurations. Depending on your I/O device and system configuration, other wiring configurations can also be used.
IMPORTANT:
The onboard pulse test outputs (T0…Tx) are typically used with field devices that have mechanical contacts. If a safety device that has electronic outputs is used (to feed safety inputs), they must have the appropriate safety ratings.
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