I was told by an end user engineer that he considers all field devices to be simple Type A devices.  In IEC 61508, Type A is defined as a device with well-defined failure modes, well known failure rates, and behavior under fault conditions that can be completely determined.  Type B is everything else like anything with a microcomputer. Back when these definitions were written (late 1980s), the logic solvers were 16 bit microcomputers with simple one loop software operation.  Most field devices were simple, even analog.  The “smart” transmitter was coming on strong in the market but often rejected for safety applications.  So the IEC 61508 committee wrote down requirements including strong engineering procedures they felt would allow complex microcomputer devices to be used in safety protection applications.

As exida has done assessments on hundreds of field devices, I can report that some field devices are Type A – about 10%.  One pressure transmitter had two microcomputers; one of them was a 32 bit with a multi-tasking operating system.  In 2015 field devices had far more computing power than the most powerful safety PLC of 1990.  And those devices need the software engineering discipline and strict engineering processes as defined in IEC 61508.  Yes, most field devices were Type A back in the 1970s, but not now.

Tagged as:     PLC     IEC 61508     Dr. William Goble  

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