Manual proof test effectiveness

Problem A set of non-redundant (lool) safety equipment is used to perform a safety instrumented function in low demand mode. The equipment is to be inspected and fully restored every five years. Therefore the manual proof test interval is five years and the manual proof test effectiveness can be assumed to be 100%. 

In continuous mode, the demand is effectively always present. Dangerous conditions always exist and a dangerous failure of the safety instrumented function will immediately result in an incident. There are no safety benefits that can be claimed for manual proof testing or even automatic on-line diagnostics in a single channel system (Tool). By the time the diagnostics detect the fault and initiate action, it is too late. Therefore, in continuous demand mode probability evaluation cannot take credit for any diagnostics except in redundant systems. 

Manual proof testing is carried out on a monthly basis with an effectiveness of 95%... 

The manual proof test is assumed to be 100% effective and to occur annually (ca. 8000 hrs). One maintenance crew is assumed to be available for each of the three equipment types (PES, Instrumentation, Pneumatics). 

Manual proof testing reduces the probability of failure on demand because it effectively resets the clock on the cumulative probability of a failure. After a proof test has been successfully carried out the cumulative probability figure is very low and begins to rise with time. Hence the average probability between proof tests is lower than the untested and cumulative probability of failure (which would eventually approach 100%). 

The formula given for manual proof testing above must be corrected for the effects of downtime whilst the test is in progress and for the fact that there may be errors in the testing or that the test work may itself leave the system in a less than healthy state. For example the calibration of a sensor may have been wrongly done. Then the equation becomes ... 

Armor Testing. The purpose of testing is to det the effect produced on armor (such as plates, castings, weldments, etc) on impact of various projectiles, such as HE, AP, APC, HEAT, HVAT, etc The tests are described in Ordnance Proof Manual No. 22-10... 

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