The Arc Fault Detection Device… again.

QUOTE:

"Retrofit is always the side of the regs that isn't thought about. Because the regs are applied from the point forward of their publication, it seems to me that the authors and manufacturers think of them as only being for shiny new builds.

That the same technology has to be applied to the understairs cupboard for Mrs Scroggins when her BS3036 board is no longer adequate falls by the wayside. Imagine too the nightmare of being called out for an AFDD trip if you have retrofitted a new AFDD CU onto an older installation. One can test for and apply some detective work to overload/earth leakage faults, but potentially trying to locate a loose connection 'somewhere in the house' could be a nightmare.


Overzealous cable clipping, pinched/pulled cables, a DIY junction box some idiot hid under the floorboards thirty years ago, a connected appliance with internal damage to its flex/connection position... it could keep you running around for hours with fruitless results. If such a device trips intermittently, how likely is it that the fault can be tracked down without decorative damage (pulling up floors, opening walls etc.) On a new build, you know there are no hidden junctions and the wiring has been tested as sound before commissioning, but applying this technology to older installations without the option to be able to fall back to a non-AFDD solution could be very problematic as dead-testing prior to upgrade may not indicate you're going to be left with a nuisance tripping problem afterwards. "

I found the comment that a large surface area was needed for the arc very interesting - that would seem to undermine detection for many (if not most) real-world arc situations (e.g. loose wire in terminal or damaged cable).

 

trying to locate a loose connection 'somewhere in the house' could be a nightmare

Given that one of the more annoying limitations of AFDDs is that they can only cover a single final circuit each (so you can't have an AFDD covering multiple circuits or an entire installation like you can with RCDs) then it should be just one final circuit you're searching.


  - Andy.

Some very pertinent points made in this video, especially relating to the unintended consequences of the  possibility of AFDDs becoming mandatory. Despite my Presbyterian upbringing, I still chuckled at the rather raw humour.

I always find it easiest to relate to concrete experiences.


I had an arc fault in my lathe. It would always switch on, but sometimes it died after a few revolutions. Sometimes I thought that I could hear a wee bit of sizzling. The problem was that when the voltage dropped, the relay and contractor also cut out so it was difficult to locate the point of trouble. After quite a bit of persistence, I found a slightly singed loose connection block. I tightened it up and job's a good 'un.


What I wonder is whether it would have tripped an AFDD. If an AFDD had tripped every time that I fired up the lathe, the cause would have been obvious to the extent that the machine was a problem, but it wouldn't have helped me locate it within the machine.


So as WHJ says, finding a loose connection somewhere under the floorboards is going to be a nightmare. Mind you, with new builds and maintenance-free JBs, it shouldn't be a problem. So in fact perhaps their usefulness lies in detecting loose connections in appliances rather than the fixed wiring.

The problem is, as he says in the video, is that these devices won't pick anything up past an isolating transformer, nor anything downstream of any kind of electronic power supply, and it seems they are intended only to protect the fixed wiring part of an installation.

Love the humour - reminds me of the working environment in which I spent 26 years many moons ago. I love it even more if it upsets the woke brigade too!

it seems they are intended only to protect the fixed wiring part of an installation.

I'm not sure about that - there's plenty that can go wrong in flexes, extension leads and the general "mains" side of appliances - which in theory an AFDD should be able to detect

(although I'm increasingly suspecting that the current crop might not in practice).

   - Andy.

The point though Andy, is do these "connections" cause fires? In the normal case, they do not (Chris's lathe wiring did not think about catching fire) because the dissipated energy is very low. To start a fire needs a significant amount of energy, even if the materials nearby are very flammable. The only case where this is not true is vapour fires from something like petrol. which has a very low flashpoint (ie. required energy) to ignite. From all of these videos, one can see that the dissipated energy is very small and that mechanical movement is also a requirement to start an arc. These two things together are very rarely present in fixed wiring. It is not possible to start an arc unless there is a source of ions (charged particles which can carry a current) and these come from vapourised metal or conductive liquids (or carbon rods), and these sources are very unusual in fixed wiring. Those with an arc welder may like to try an experiment. Set it to the lowest possible current setting, say 30A or less. Chose a clean piece of bare metal rod as an electrode and a clean surface to strike an arc. Can you get anything to happen? Answer: just some sparks. If you hold the rod a fraction of a milimetre from the surface does an arc start: no? If the rod touches the surface does an arc start: no, you simply get a short circuit? If you use a carbon rod in place of the metal one can you start an arc: yes. Does this simulate a real fixed wiring situation: no. That is why the standard uses a carbon rod, it is the only realistic way to start a low current arc, by providing an ion source. The standard test is fatally flawed! That is why there are no fire improvement statistics from America, there is no reduction in fires.


The science behind these devices is simply not there, and it is easily shown that these devices are being sold to detect a problem which does not exist. The situation in which they may trip are NOT likely to start a fire, and those which might start a fire do not exist in a fixed installation. Existing protection measures, the CPD and an RCD provide protection against most faults very well, and AFDDs would not help where they fail. I challenge anyone to show any mechanism which allows an AFDD to be useful in a real installation which is not already involved in a fire or other extreme situation such as flooding. The manufacturers are strangely quiet, except saying that the software is still being "improved". Unfortunately, there is nothing much that these devices could improve. I see no reason at all why a single device should not protect a whole installation either, except that detecting an arc on a circuit with a stable 60A load would be even more difficult. The only situation which might be improved is a high current loose connection like loose tails connections, but we have already dealt with that alleged problem!

I must say I am pleased to see this video, even if the rapping is not to my taste,  it rasies some very valid points and the experiments are far more revealing than the official literature, and last but not least because at the end it also shows how to use the draft for public comment and may encourage more folk to actually comment.

This is a more erudite consideration as posted by Dutch of the Elm in a concurrent thread. Well worth taking 10 to read.
https://www.researchgate.net/publication/345910304_Arc_Fault_Detection_Devices_AFDDs_-_Are_they_worth_it

I wonder if anybody has done an experiment with an AFDD using a loose connexion? It would be easy enough - just poke a bit of cable, preferably stranded such as 1.5 mm² singles, into a wee choc-block and don't tighten the screw.


If an AFDD tripped, then all well and good. If not, it would be interesting to see whether an ordinary (nylon?) choc-block would melt. In that event, in a class I device, presumably ADS would operate.