Thoughts on AFDD's being recommended per BS 7671 18th Ed effective 1st Jan 2019?

[Faith]

Hey everyone!~ It just came to my knowledge that AFDD's/AFCI's aka "Arc Fault Detection Devices" have made their way into the UK electrical code as a "recommendation" effective 1st Jan 2019.

Thoughts?

I have no personal experience with them but I recall reading loads of complaints about them from across the pond where if I recall they're mostly mandatory now.

They're supposedly extremely prone to nuisance trips especially with common machinery you'd expect even in a small workshop (think electric saws, drills and the like) but I've also seen stories of them tripping from even more mundane appliances such as vacuum cleaners, cordless phones, treadmills, radios, and et cetera.

AFDD's are supposedly designed to look for a waveform which resembles that of an arc due to shoddy wiring or compromised insulation while not reacting to something from say an electric motor brush.

Sounds like a complete load of crap to me though? Not to mention how bloody expensive these damn things are and how much space they occupy in a consumer unit.

I just cannot imagine how they can possibly test for every single "unintended" vs "intended" scenario.

[German_EE]

See where one of these devices is tested, it doesn't do too well.

[tooki]

The thing is, I think it worked as it was supposed to: it did not react to transient arcs, only to a sustained arc. If they reacted to every transient spark, you’d have constant nuisance tripping.

[Faith]

Yep, his method of testing would likely not be too dissimilar to flipping a light switch.

Thing is at what point do you need to account for so many real world variables that detecting an actual fault becomes so slim to the point where most faults would already trigger either an MCB or RCD.

[ajb]

AFCIs have been required in bedrooms in the US and Canada for about 20 years now.  Most people aren't often operating motorized appliances in the bedroom (most people are missing out ), so I don't know that nuisance trips from motors are that big of a deal there.  Later code revisions have expanded the requirement to other living areas, but that's only been in the last 5-10 years, and not all local jurisdictions have adopted the requirement yet.  So even here most people don't have them in their homes--the median age of owner-occupied homes in the US is something like 37 years, and new requirements like this are only effective for existing installations if renovations are made.

North American AFCIs tend to be the same size and form factor as a standard branch circuit breaker, so retrofitting is fairly simple, other than moving the neutral wire from the bus bar to the breaker.  I think you can also get AFCI receptacles, that can be installed like a GFCI.