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The Arc Fault Detection Device… again.

whjohnson
Some humourous but valid observations on AFDDs, the state of the industry, the wiring regs and future amendment requirements.

He also attempts to build a AFFD tester which electrically tests rather than relying upon the mechanical action of the test button on the device itself.

https://www.youtube.com/watch?v=e0ElFaKc_e8

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    Does anyone know whether there's a simple relationship between current and power emitted by an arc? I don't know what the effective resistance of a plasma's arc is, but perhaps at below about 2.5A not enough heat can be generated at the site to ignite anything?
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    wallywombat:

    Does anyone know whether there's a simple relationship between current and power emitted by an arc? I don't know what the effective resistance of a plasma's arc is, but perhaps at below about 2.5A not enough heat can be generated at the site to ignite anything?


    I have been thinking about that. I have also been thinking about the spark which ignites a cigarette lighter. Surely it must depend upon the presence (or absence) of any fuel and the temperature attained. (Fire tringle - fuel, oxygen, heat.) That's why I come back to the PVC choc-block. If it melts, then ADS should take over. If it doesn't melt, how can it get hot enough to ignite?


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    From what has been recorded by crews attending real fires involving electrical items, many instances of ignition arise from resistive heating (i.e. poor connections with high current devices - showers for instance) or direct contact of flammable material with a heat source (tumble driers with 1-2kw+ heaters that for whatever reason set alight fluff for instance). I provide below some real extracts from the additional information field on the IRS (Incident Recording System) records for fires in Scotland listed as cause being electrical supply or appliances. These give a flavour of what crews find. I have not selected them to show up resistive or conductive ignition alone, as the first entry shows. I remain sceptical about the benefits of AFDDs given that typical fires in electrical wiring or appliances in domestic properties are unlikely to be prevented by these devices. The dozen extracts only give a flavour of what in practice are hundreds of such incidents each year in Scotland alone. As you will see, some of those incidents caused severe fires - devastating for the occupants.

    Edit: @davezawadi (David Stone)‍, I have posted a variant based on this post with more suitable intro comments to the BSi DPC for 421.1.7


    "sparking in main fuse panel in tenement stair, remained at location until scottish power arrived.  Indicated a loose joint that was moving and sparking in cabinet. No fire service action required or equipment used."


    "2 storey semi-detached house. Fire damage - 5per cent to ground floor wet room, 10per cent to upstairs bedroom, 25per cent to roof. 75per cent smoke damage throughout. Faulty extractor fan"


    "insulation foam within tumble drier burnt due to overheating, fire contained to within the tumblr drier"

    "Fire in Electricity Meter and wiring. Large cupboard within hallway. Owners attempted to extinguish with Dry Powder Extinguisher."


    "damage confined to scorching behind electrical socket.  extinguished on arrival."


    "Tumble dryer destroyed. Extinguished on arrival. Motor burnt out"


    "we where mobilised to a smell of burning, on inspection and liaising with the occupant we found that there was an electrical fault with the washing machine. We removed it to the outside for safety precautions. No equipment was used at this incident."


    "mid terraced 2 storey dwelling, damage 90 percent by fire. electrical fault in tumble drier"


    "Small fire in electrical transformer within washing machine"


    "Kitchen smoke damaged. Electrical fault with fridge"


    "2 storey terraced building approx 10m x 8m. Electric shower destroyed by fire, 25 per cent fire damage to bath, 80 per cent smoke damage and 20 per cent heat damage to bathroom."


    "Fuse box wires in airing cupboard and clothes damaged by fire."


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    wallywombat:

    Does anyone know whether there's a simple relationship between current and power emitted by an arc? I don't know what the effective resistance of a plasma's arc is, but perhaps at below about 2.5A not enough heat can be generated at the site to ignite anything?


    There isn't a simple relationship sadly. As the arc current rises, the density of conductive ions rises - if you like there is  more ionised plasma (charged smashed up bits of atom) floating about, so the arc resistance falls - giving a negative dynamic resistance, meaning  the voltage dropped falls as the current rises, which is not stable.  To strike an arc without contact you need a very close approach, and a high enough voltage to jump the gap initially. Then you need enough current to be available without the voltage dropping too much to allow a decent volume of plasma to build up.  With no current limit the ball of hot ionised gas grows without limit, but in reality there is always a current limit at some point, even if it is the substation fuse !


    In the case of the 2.5 amp load there is an open arc with approx 100 ohms in series, so as the arc resistance falls its share of the voltage falls, and when the voltage drops low enough the arc will go out. Then the voltage rises and there is a  re-strike usually this repeats and leads to the periodic buzz in time with the peaks of the mains waveform.

    regards Mike


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    You have described exactly why it is so difficult to get an arc Mike, unless there is enough hot plasma after the first half cycle the arc is extinguished immediately. This is the situation we see in all the tests on youtube, and why there is this insistence on carbon electrodes or charred cables. In my view, AFDDs do not do that which they are named unless triggered by very artificial conditions. In other words, a solution looking for a non-problem. There is no evidence that they prevent real fires, and even simulated conditions performance does not match loads or installations in the UK. The cost of the devices is unrealistic compared with the fire reduction possible and therefore they should not be a requirement in installations.  I cannot understand how we got to this situation with BS7671, unless there is deliberate deception involved somewhere in the standards chain, or that everyone involved along the chain has been silenced or is incompetent. Changes without evidence and manufacturing tests that need special conditions clearly indicate a serious problem somewhere, so the question is "WHY did no one ever question the devices properly", they certainly have in the UK.
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    David,


    Bold but well said.  As an IET member (for now), this single issue has tainted my view of what is meant to be my institution.
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    Hello David.

    Well said.  Your little paragraph nicely sums up the disquiet that runs through this thread.  Something is amiss here.

    Regards,

               Colin.
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    I think the fact that if you do a google search for AFDD fire reduction statistics, nothing much comes up apart from the manufacturers of these products. And they are just how many fires were electrical based and that "our products would reduce this number". Not really the black and white numbers that are needed from an unbiased 3rd party to allow anybody, professional or public, to make a decision about how effective these items really are. If they are so good why aren't there more statistics out there that actually prove this?
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    Do we put them in the same bin as voltage conditioners in domestc properties?

    Snake oil!


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    ebee:

    Do we put them in the same bin as voltage conditioners in domestc properties?

    Snake oil!




    That's harsh! ?


    They do actually detect potentially dangerous arcs. The real question is do such arcs exist in practice? The consensus seems to be no. Take the much-vaunted burnt out shower switch. If a terminal is not properly secured, is the over-heating due to arcing or due to only a couple of strands carrying the full current?


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