22/01/2012 by TDS.

There appears to be some confusion about what constitutes an element as defined in the 2ed.  The formal definition is somewhat circular; an element being one or more components to implement an element safety function. An element safety function being that part of a safety function implemented by an element.

Exida have proposed (in a position paper) that '...an element should contain all the equipment/devices that are needed to perform a safety function.'  They argue that 'The actuator does not perform a complete function, it needs the solenoid valve, and it needs the valve...If one wants to argue that the actuator is an element, then one would have to take that approach all the way down to the individual components as these cannot fulfill complete functions either.  In that scenario the spring would be an element as well.  This is illogcal.'

I suggest that it is NOT illogical; an element could be construed as any combination of components (and their associated failure rates); if there is a type B component included then the element that includes that component will also be type B, with corresponding implications for HFT. It may be preferable to separate type B components from type A to minimise HFT requirements for some components.

So you could have one element = (barrier + SOV + actuator + valve), but if the SOV is a type B, then you may have to have more than one such element to satisfy HFT e.g., another (barrier + SOV + actuator + valve) in 1oo2 configuration.
Alternatively you could, for example, constitute as three distinct elements: (barrier), (SOV), (actuator + valve). With 1oo1 barrier, 1oo2 SOV, 1oo1 (actuator + valve).

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05/01/2012 by TDS.

In November HazardEx magazine there were a couple of articles explaining about functional safety matters.  Fairly typically they identified a whole bunch of stuff you ought to be doing to be ‘compliant’.  Strong on ‘theory’, woefully short on ‘pragmatism’.  Prompted a comment piece from me which is available on the HazardEx website: http://www.hazardexonthenet.net/  Just click through ‘Comment’ on the sidebar or use this link:

http://www.hazardexonthenet.net/article/46960/Who-s-afraid-of-functional-safety-.aspx?AreaID=7

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