Definition:
A Safety PLC (programmable logic controller) is a programmable device designed to perform logic operations for Safety Instrumented Functions (SIFs) and certified for specific SIL ratings. This means that both the hardware and software as well as the diagnostic coverage are independently verified. Safety PLCs are a step up from a Safety Module and a big step up from Relay Logic.
A safety PLC is a common logic solver in a SIF. But a logic solver does not have to be a safety PLC.
A safety PLC is a good choice for large and complicated SISs. It is arguably over-complicated for a simple SIS with only a few SIFs.
Note that a safety PLC is somewhat of a loose term. Often people say the logic solver in a SIF is a safety PLC but a safety PLC is arguably more of a PLC carrying a certain certification. Safety PLCs can be used in manufacturing or in machinery safety if the engineering for that system requires it. Some brands of PLCs often visually distinguish safety PLCs from others, such as a certain color. Yellow is a common color for Siemens and Allen Bradley.
Key Points:
- A safety PLC must meet requirements of IEC 61508 Parts 2 and 3.
- Often redundant and fault-tolerant.
- Often it is yellow depending on the brand.
Example:
A Triconex SIL 3 PLC used in emergency shutdown systems would be a safety PLC This could handle multiple SIFs.
See Also: logic solver, smart logic module, watchdog timer
Cited Source:
- IEC 61508-2:2010.
- Siemens SIMATIC controllers
- Inst Toole Online Magazine – What is a Logic Solver?