The Arc Fault Detection Device… again.

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

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