Principles, Standards and Implementation

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System Design According to ISO/EN 13849 and SISTEMA

Systematic Faults

We have already discussed quantified safety reliability data in the form of MTTFd and the probability of dangerous failure. However, this is not the whole story. When we referred to those terms we were really thinking about failures that appear to be random in nature. Indeed IEC/EN 62061 specifically refers to the abbreviation of PFH_D as the probability of random hardware failure. But there are some types of failures collectively known as “systematic failure” that can be attributed to errors committed in the design or manufacturing process. The classic example of this is an error in software code. The standard provides measures in Annex G to avoid these errors [and therefore the failures]. These measures include provisions such as the use of suitable materials and manufacturing techniques, reviews, analysis and computer simulation. There are also foreseeable events and characteristics that can occur in the operating environment that could cause failure unless their effect is controlled. Annex G also provides measures for this. For example it is easily foreseeable that there may be occasional losses of power. Therefore the de-energization of components must result in a safe state for the system. These measures may seem to be just common sense, and indeed they are, but they are nevertheless essential. All the rest of the requirements of the standard will be meaningless unless due consideration is given to the control and avoidance of systematic failure. This will also sometimes require the same types of measures used for the control of random hardware failure [in order to achieve the required PFH_D] such as automatic diagnostic test and redundant hardware.