If you have been recommended to install AFDDs by BS 7671 (which is currently the case) and you did not and an issue arises.  How would you defend your decision? 

well at the brutal level, "because they are not compulsory". Much like labels about insulation colours now, or conduit.

In a more nuanced way, it would depend if the issue arising could be shown that it would have not happened or been less serious, had an AFD been present.

"Sorry the warehouse burnt down but an AFD would not have detected  the faulty heater thermostat anyway.."  is one extreme, and the other, the only one where there may be a case to discuss, "An arc fault of the kind an AFD may well have detected sooner has caused.." or similar.

So to me any design decision sadly does depend very strongly on how well in my engineering judgement I consider the things actually work as advertised.
And purely personally, I think more smoke detectors may be a better use of  money saved by not fitting AFDs, but I can see that it is one of those reputation on the line moments that most sensible folk try and avoid, though in reality we make cost-effort-benefit decisions like that every day. It is always far easier and cheaper to not burn the house down at the planning stage.

Oh, and for what it is worth I think the video in the OP oversimplifies this.  Every 'recommend' is a lot weaker than 'should'...

mike

If you have been recommended to install AFDDs by BS 7671 (which is currently the case) and you did not and an issue arises.  How would you defend your decision? 

well at the brutal level, "because they are not compulsory". Much like labels about insulation colours now, or conduit.

In a more nuanced way, it would depend if the issue arising could be shown that it would have not happened or been less serious, had an AFD been present.

"Sorry the warehouse burnt down but an AFD would not have detected  the faulty heater thermostat anyway.."  is one extreme, and the other, the only one where there may be a case to discuss, "An arc fault of the kind an AFD may well have detected sooner has caused.." or similar.

So to me any design decision sadly does depend very strongly on how well in my engineering judgement I consider the things actually work as advertised.
And purely personally, I think more smoke detectors may be a better use of  money saved by not fitting AFDs, but I can see that it is one of those reputation on the line moments that most sensible folk try and avoid, though in reality we make cost-effort-benefit decisions like that every day. It is always far easier and cheaper to not burn the house down at the planning stage.

Oh, and for what it is worth I think the video in the OP oversimplifies this.  Every 'recommend' is a lot weaker than 'should'...

mike