TABLE OF CONTENTS "There's always an easy solution to every human problem, neat, plausible and wrong."
H. L. Menken
If it were intuitively obvious which system was most appropriate for a particular application, then there would be no need for this book or any of the design standards. The problem is that things are not as intuitively obvious as they may seem. Dual is not always better than simplex, and triple is not always better than dual. Consider the nine choices presented in Table 8-1.
| Sensors | PES Logic | Diagnostic Coverage | Common Cause | Outputs | Test Interval |
|---|---|---|---|---|---|
| Single | Single | 99.9% | N/A | Single | Monthly |
| Dual | Single | 99% | N/A | Dual | Quarterly |
| Triple | Single | 90% | N/A | Dual | Yearly |
| Single | Dual | 99% | 0.1% | Single | Monthly |
| Dual | Dual | 90% | 1% | Dual | Quarterly |
| Triple | Dual | 80% | 10% | Dual | Yearly |
| Single | Triple | 99% | 0.1% | Single | Monthly |
| Dual | Triple | 90% | 1% | Single | Quarterly |
| Triple | Triple | 80% | 10% | Single | Yearly |
Let's assume at this point that all nine different cases shown in the table have software-based logic systems (programmable electronic system PES). Let's not even concern ourselves with relay and solid state systems at this point.
First, remember that safety instrumented systems can fail two ways. They may suffer nuisance trips and shut the plant down when nothing is actually wrong. They may also fail to function when actually required. Could one system...
