I am just starting out in trying to understand the control system directive and I've reached a conundrum with the calculations that I am doing and I was wondering if you could help me.
I have come to a situation where the values for the system that I have do not fall into the options listed in Annex K of 13849, an example of this would be:
DC of 99%, MTTFd of 21 years
Now with the DC of 99% I can't have any category other than category 4 (according to Annex K of 13849) but my MTTFd does not allow me to achieve the performance level that this entails. My question then is this, with the values that I outlined above do I simply reduce the DC level so that it fits with a Category 3 architecture (from where I could still achieve performance level "e" with the correct values) or is this sat in a limbo value where they either reduce the DC of their components (i.e. changing the wiring of the system) or raise the MTTFd values for the components by buying better components?
Any help is greatly appreciated!