Abstract
Abstract Programmable logic controllers (PLCs) are used in many industrial control and/or safety applications. The increased use of quantitative reliability analysis, particularly in the chemical process and nuclear industries, requires additional and more detailed failure rate estimates for analyzing PLCs. This technical note presents failure rates derived from actual PLC performance in chemical process and nuclear power plants. Failure rate estimates range from 0·0082/year to 0·025/year. These values are remarkably consistent when one considers that the data analyzed come from PLCs from different manufacturers, of different size and complexity, and with different control/safety uses. Coverage values, the probability that a PLC with redundant processors will recover from a single processor failure by successfully switching the control function(s) to the other processor, derived in this study range from 98·37% to 99·59%.
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