When is a PLC a safety PLC? Generally, a safety PLC is a programmable logic controller designed for use in mission-critical or safety-related applications; if a safety PLC fails, it does so without endangering people or processes. Actually, "safety PLC" is a bit of a misnomer. Even if a PLC achieves a specific safety integrity level (SIL) rating, it still can be applied in an unsafe way.
Mark T. Hoske
When is a PLC a safety PLC? Generally, a safety PLC is a programmable logic controller designed for use in mission-critical or safety-related applications; if a safety PLC fails, it does so without endangering people or processes.
Actually, "safety PLC" is a bit of a misnomer. Even if a PLC achieves a specific safety integrity level (SIL) rating, it still can be applied in an unsafe way. Also, PLCs used for standard automation could serve in a safety application, depending on desired SIL rating. Certain PLC programming now can be certified for safety applications, and safety network controllers may eliminate need for a safety PLC in some applications.
When applying any safety technology, people need to understand applicable safety standards, advises Connie Chick, business manager for controllers at GE Fanuc. As more applications integrate automation and safety, SIL ratings, long familiar in process controls, are appearing more readily in machine safety applications. "Safety system design is all about mapping out everything ahead of time. Half of failures can be attributed to design. A better design significantly improves overall safety," she advises.
A safety PLC, says Lyle Masimore, business manager for Rockwell Automation safety business, needs to be designed and built to fail safely with various levels of redundancy, use silicon from separate lots, and be third-party certified to the EIC 61508 standard. Also, it's best if the programming and interface to the safety PLC is the same as for all other PLCs, to avoid confusion or possibilities of errors, he suggests.
"A safety PLC is 'safe,' " explains Roy Tanner, systems marketing manager at ABB, "when it has a certain level of reliability/probability of failure upon demand that can be proven/documented diagnostic coverage that detects various aspects of hardware status, program execution and operating system status." A safety PLC must also perform the "necessary 'fail safe' actions certification from a recognized standards organization (such as TUV, FM) that it was designed in accordance with, and meets, certain criteria as defined in approved international safety standards like IEC 61508 and EN 954-1." It also may include security precautions and/or access management to protect the safety PLC from outside interference, Tanner says. A safety PLC would be appropriate for use in applications that could pose harm to the environment, business, equipment, and especially people, says Tanner, including applications and installations that need to meet IEC 61508 and IEC 61511.
Industries using safety PLCs include oil and gas, petrochemical, refineries, critical chemicals, marine, power plants, incineration, machinery, boiler controls and burner management, and high-pressure applications. Add non-traditional safety applications where loss of control means big dollar losses, or where the application is remote, unstaffed, and expensive to maintain, Tanner explains; a large tank farm, for instance.
Networks, I/O blocks
Various safety network controllers or safety I/O blocks may handle an adequate number of safety inputs and outputs for many applications at thousands of dollars less than safety PLCs, allowing conventional PLCs to handle any integrated control functions.
Also, certain standard controllers have SIL ratings that may be acceptable for many safety applications. In addition, function block programming, certified for use in safety applications, can further streamline time and cost of a safety application.
Mark T. Hoske, Control Engineering MHoske@cfemedia.com
Why use a safety PLC?
Lifecycle costs of owning a safety PLC for some applications may be less, even though list price can be higher compared to a standard PLC. A safety PLC, says Rockwell Automation, can:
Simplify overall design and support, depending on complexity and size of control task;
Provide for re-use of designs and flexibility in many applications;
Reduce time for development, installation, start-up, and support (mean time to repair);
Include robust diagnostics or information tracking capabilities that are easily integrated;
Have a logic-editing environment that's familiar to maintenance personnel; and
Offer networking of I/O devices and diagnostics via human-machine interfaces, reducing installation costs.
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