Spark Ignition Question

Not familiar with the spec you cite, but here are some of the technical details.

The basics. In order to ignite a fire, you need fuel, oxygen, and an ignition source. Nothing new there, but if you are talking about an explosive atmosphere, there is one more requirement, and that is a stoichiometric ratio of explosive gas to oxygen/air. If the mixture is too lean or too rich, even the right spark won't cause ignition.

But assuming you have a stoichiometric ratio, then the accepted spark energy level is 0.2 mJ (millijoule). Note this is energy, not voltage, and not current. In order to get a chemical reaction going, you need to provide the activation energy necessary to trigger the reaction.

A safety factor of 10, or 10 dB, is usually applied because of the catastrophic failure mechanism, in say a wet wing due to a lightning strike to the wing. You want no more than 0.02 mJ.

Now it is true that a high voltage might generate a spark, or the interruption of a high current at low voltage can generate a high voltage and resultant arcing (think of the opening of relay contacts while a high current at 12 Vdc is flowing). And if you were drawing a lot of current from a single lithium ion cell, and managed to interrupt that current flow, you could also get a good spark. And a Li-ion battery cell is only 2.2 Volts. So voltage alone is not a determinant.

But fundamentally, the basic criteria is spark energy. There has to be enough energy in the spark to activate the oxidation process.

You can find the 0.2 mJ number in tracts about lightning protection of aircraft. There is a similar type discussion in a 1950s era military technical order, TO 31Z-10-4. This TO discusses the ignition of an explosive atmosphere by a source of transmitted electromagnetic power; specifically, a radar. They come up with a limit of 5 Watts per square centimeter, which is huge, but it isn't the electromagnetic field power density that ignites the explosive atmosphere directly, but rather the fact that if you have two pieces of metal close together in such an intense field, you can get arcing between them, and that arcing ignites the explosive atmosphere.

I have observed the same thing in my microwave oven. I once put a jar of refrigerated peanut butter in to soften it up, but I hadn't realized that the security seal underneath the lid had been only partially removed, with remnants still stuck to the periphery of the top of the jar. Right through the solid purple plastic lid I saw a light show from the arcing and sparking.

Had there been an explosive atmosphere in that oven, I would have been a candidate for a face transplant, or worse.

Dear Grae,

Happy Holidays to you as well. I'm assuming that you are concerned with igniting a flammable mixture. It is much more complicated than just current and voltage. The type of gas and amount of energy stored in the circuit by capacitance, inductance etc. are also very important.

This is covered under the subject of "intrinsic safety". If you Google this, you will find what you need.

Here is an example: http://www.mtl-inst.com/appnotes/an9003/why_is.htm

Tad

Thanks guys. Actually, both of you nailed it as we are talking about intrinsic safety and we're well aware of the the gas mixtures, LELs and so on. But I think the answer we were looking for was the point that EMC made about the energy content of the spark. We've seen some pretty bright sparks coming off lithium batteries with lots of current, but apparently they weren't sufficiently energetic to ignite the surrounding gas mixture.

Good call, thanks

Grae

As others will concur, voltage is the key. Look up Jacob's Ladder and Tesla !