The point is that many Brigades do not use the guidnace in BS5839 Part 1 2002 about numbers of Uwfs, when they should be investigated and whether that is by the engineer or the Rp for the FA. When a system has hundreds of device and the FRS send a letter after 2 incidents then that belittles what they are attempting to do. Also, there is an issue for some FRS with recording the actual address of a premises and the details that officers record\afetr an incident. All false alarms are not all false alarms.
I agree that BS 5893 does 'allow' a certain failure rate, but i can't believe you would be happy to allow UwFS to continue. In some premises i deal with, where that is all they have (no history of fires), it leads to a de-sensitising of the response to an activation.
Acheivable or not, our target has to be zero false alarms, and we will continue to seek reductions even when a premises is 'performing' in the lower zones defined in the BS.
What would you do if the system was aging fast and becoming less reliable, but because there were a large number of heads, and the rate of increase wasn't too high - would you allow this to continue until the number crossed the first threshold?