Using PLCs in safety applications
- Moore Products Co., Spring House, PA (United States)
Programmable logic controller (PLC) technology is being used more frequently in safety applications. When documentation and configuration management issues are considered, the advantages of programmable technology over relay/solid-state technology become very significant. If the failure modes of these solid-state devices can be controlled, future safety systems will likely use programmable controllers. The chances of a dangerous failure can be reduced by using special circuits and application level programming in a conventional PLC. Or alternatively, a safety PLC can be used. Many control engineers are selecting programmable electronic systems for safety protection applications. Advantages include ease of installation, lower false trip rate, math capability and more sophisticated logic capability. In newer generation PLCs, other benefits include IEC1131 standard language capability and self-documenting graphical configuration. With all these advantages, why not? The big problem is that solid-state components can fail in several ways, many of which may create dangerous undetectable failures. The possibility of a dangerous failure is the reason many insurance companies and corporate standards to not allow conventional PLCs to be used in safety applications. Specific problems with conventional PLCs include input circuits that get stuck, output circuits that jam on, I/O addressing errors caused by shorts or opens in the parallel backplane, covert watchdog timer failures, memory corruption or loss, CPU instruction failures and many others. Several of these potential failures have been specifically identified in insurance industry documents. In spite of these problems, with extra resources it is possible to increase the safety of an ordinary PLC. The paper discusses this possibility.
- OSTI ID:
- 253728
- Journal Information:
- Hydrocarbon Processing, Journal Name: Hydrocarbon Processing Journal Issue: 6 Vol. 75; ISSN HYPRAX; ISSN 0018-8190
- Country of Publication:
- United States
- Language:
- English
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