This article is part of a guided learning series on building our understanding of the Siemens S7-1200 PLC.
Canonical Article: Navigating Siemens S7-1200 PLC model numbers
Learning Companion (Q&A): Learning Companion Navigating Siemens S7-1200 PLC model numbers
You are reading: Question 3
What is the difference between type C and type FC PLC?
Definition of failsafe
The failsafe industrial automation equipment is designed to protect the user. There are several universal properties:
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Elevated testing with high mean time between failure.
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Tiered rating using scales such as a SIL (Safety Integrity Level) and Performance Level (PL) to indicate the level of danger associated with the machine. For instance, a machine capable of causing death will be guarded and rated higher than a machine limited to bruising. The featured PLC is rated for:
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SIL 3 according to IEC 61508
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Performance level PLe according to ISO 13849-1
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Redundancy as described in this article exploring an e-stop connected to a dedicated safety relay
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Protected against short-circuit to supply or return. A pulsed signal is often incorporated.
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Independance from conventional control logic. In the case of a combinational PLC featured in the canonical article, we can assume the PLC included redundant circuit to continue protection even when the main processor experiences a fault or other logic error.
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Simplified programming to reduce the risk of error.
Industrial automation safety equipment is typically yellow or red in color.
Answer
The type C (e.g., CPU1215C) is the standard PLC. The type FC (e.g., (CPU 1215FC) contains the core functionality of the type C but adds a layer of fail-safe technology.
Tech Tip: Originally the C stood for “compact”. We could therefor describe the FC as a compact PLC with an integrated fail-safe option. The fail-safe hardware is identified by the yellow color. A subtle example is shown in Figure 1 where the left and right centers of the PLC contain a yellow band.
Article by Aaron Dahlen, LCDR USCG (Ret.), Application Engineer at DigiKey