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When is a spark an arc? OR - When is an arc a spark?

whjohnson
Just watched some chap on the E5 youtube channel visiting Eaton in Austria. Eaton AFDDs have been something of a subject of ridicule in youtubeland, with various respected electrical content providers demonstrating through various real-world means that they don't function. So, off this chap goes to Eaton's HQ in Vienna where they provide him with a aluminium case full of test kit, complete with the Eaton logo and fitted out with various Eaton devices inside.

One of the devices is the Eaton AFFDD which has famously failed to operate on numerous youtube video presentations.

Of course, it trips when tested with their own test kit. After all, no point in trying to sell something which isn't really needed unless you can demonstrate that it actually works, so Eaton helpfully provides the 'right' arc signature so that the device can trip on command in front of all those cynical doubters.


Apparently, all those heath robinson youtubers have been getting it wrong because they have unhelpfully been simulating real world arcing events which these devices won't actually pick up. You see, according to the 'experts' you need an arc instead of a spark to trip the device! What the hell is the difference?


Oh how I laughed! Is this how far they'll go to flog you some old tat you don't really need?

Just how many different arcs and sparks are there out there? Has anyone told David Attenborough of all these new species to explore?


Feel free to jump in!
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  • 0
    In normal high voltage parlance a spark is pre-avalanche, and is based on simple breakdown, say ~1mm/kV for sub 10kV sort of voltages.

    Once current starts to flow however,  the gap rapidly fills with ionised material - that is atoms from the air, or metal torn from the electrodes, but with more or less electrons than would make them neutral, and they move towards one electrode or the other. At high current densities, these ions collide with more atoms, and create more ions, and the amount of plasma that can be created by this avalance  reaction is only really limited by the power available to tear electrons off atoms, and this means that if enough current is flowing, the spark can be drawn into an arc that is many times longer, and is not directly related to the initial voltage. A classic must be a welder, to strike the electrodes must practically touch for we only have 50V or so available, but once struck the arc length, and also the weld depth, is set by the available current.

    A similar thing occurs with spark gaps, or even the small neon tubes, in low current glow, the voltage is almost fixed, but as current rises, the arc forms, and the voltage falls dramatically . At 10A or more, the neon that is 90V and orange at a few mA, will only drop only about 5V and glows blue white. Such tests must be limited to a very short duration - typically  hundreds of microseconds at most, total energy is limited with capacitor discharge circuits., as the arc is hot, and the total energy will cause the envelope to shatter.


    For AFDs however, I am not sure these definitions stand up.
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  • 0
    In normal high voltage parlance a spark is pre-avalanche, and is based on simple breakdown, say ~1mm/kV for sub 10kV sort of voltages.

    Once current starts to flow however,  the gap rapidly fills with ionised material - that is atoms from the air, or metal torn from the electrodes, but with more or less electrons than would make them neutral, and they move towards one electrode or the other. At high current densities, these ions collide with more atoms, and create more ions, and the amount of plasma that can be created by this avalance  reaction is only really limited by the power available to tear electrons off atoms, and this means that if enough current is flowing, the spark can be drawn into an arc that is many times longer, and is not directly related to the initial voltage. A classic must be a welder, to strike the electrodes must practically touch for we only have 50V or so available, but once struck the arc length, and also the weld depth, is set by the available current.

    A similar thing occurs with spark gaps, or even the small neon tubes, in low current glow, the voltage is almost fixed, but as current rises, the arc forms, and the voltage falls dramatically . At 10A or more, the neon that is 90V and orange at a few mA, will only drop only about 5V and glows blue white. Such tests must be limited to a very short duration - typically  hundreds of microseconds at most, total energy is limited with capacitor discharge circuits., as the arc is hot, and the total energy will cause the envelope to shatter.


    For AFDs however, I am not sure these definitions stand up.
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