The regular PLC was designed about 1970 to make elasticity for process control and to realize facilities for instance sequencing, PID-loops, calculation, etc. The common development towards more complex processes and the flexibility requirement during engineering, as well as during use built up operational, have established PLC as SIS (Safety Instrumented Systems) in the industry. Frequently also non-Safety associated functions have been put in the similar systems, because the great capabilities.
Nowadays extraordinary Safety-PLC does all utilize a choice of configurations of redundant to get better the Safety as well as accessibility.
• The PLC of Single Safety (1oo1), that use a single path of signal from the inputs through a microprocessor to the outputs are inappropriate for the systems of SIS, since any category of software or hardware failure can generate a potentially situation of unsafe, if a switch of output can no longer be de-energized to carry out its function of protection. This situation is not expected to be noticed by the self-diagnostics systems. Consequently a single PLC does not meet the terms with today’s standards of Safety and are measured inappropriate for applications of Safety.
• The PLC of Dual Safety (2oo2) is according to the standard of diagnostics by assessment. Both PLC carries out the similar program and in case of a inconsistency between the results of two processors the outputs will be reenergized. The circuits of I/O need special conditions for assessment or self-test. This makes an enhanced performance of the Safety when measured to a system of 1oo1, at the cost of growth hardware use, that is in charge for the better FTR (False Trip Rate) or lower availability systems of 2oo2.
• The PLC of Triple Safety (2oo3) or systems of TMR, are according to the standard of analytics by agreement or highest voting, making about same Safety as double systems of PLC, with no the requirement to outing at the initial malfunction. A mechanism of voting acts on active changes in sequence to recognize any differences. Nevertheless the main of possible unsafe failures is static and is in charge for on-demand failures. Also the common cause consequences are a cause of concern, because in most the systems of TMR no variety is affected in the software or in the hardware. The higher accessibility (or low FTR), measured up to to systems of 2oo2, is accomplished at the cost of totally triplicate hardware.