While Dr Engle could have expanded on Pashen's law, vaious atmospheres & metals, etc, the fundamental concept of a 'sustained arc' vs. a 'spark'remains available.



http://en.wikipedia.org/wiki/Spark_gap

A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms, ionizing the gas and drastically reducing its electrical resistance. An electric current then flows until the path of ionized gas is broken or the current reduces below a minimum value called the 'holding current'. This usually happens when the voltage drops, but in some cases occurs when the heated gas rises, stretching out and then breaking the filament of ionized gas


We're all familiar with Jacob's ladder, note the volage values>>

http://www.repairfaq.org/sam/jacobs.htm#jlhdj


Perhaps even more everyday is our trade being in the presence of a 'spark' , i.e.- old switches , small to large disconnects

i would venture commutator motors provide a series of sparks, instead of a sustianed arc , even though we all can note the nusiance trips from them

in any version of Pashchen's law , we do not see a sustained arc event @ nominal household voltages


part of the problem Dr Engle laments is the definitional difference bettwen these phenomenon >>


The proposed changes to UL 1699 should have been
acceptable to all STP members. It involved no core changes;

it simply attempted to bring the Standard’s language in line with other UL standards. In particular, while the term “arcing” is defined in UL 1699, the UL terms “series arcing” and “parallel
arcing” are not defined.

Yet the manufacturers and UL on their
web sites claim that the mandated Combination, unlike the Branch/feeder, provides “series arcing” protection of cords.





Further, if UL had not taken it upon themselves to create a standard, instead of testing something to a standard, we wouldn't be discussing this issue at all>>>>




As will be explained in the section discussing UL1699, the fire curve later became the UL1699 40.4 Carbonized path arc clearing time test. This test is the only arc performance test difference between the Branch/feeder and the Combination
AFCI.
After accepting the test, the Task Force admitted its failure to develop a Standard, and turned the task over to UL.
This is not a normal UL responsibility; manufacturers develop Standards, and UL is paid to test and list products to these Standards.






The unfortunate result being>>>>




“Carbonized path arc clearing time test”
This test was discussed earlier, it makes no sense. It was thought that if the technical issues were honestly presented and discussed at this STP meeting, the test would be removed from UL 1699.

Once removed, the Branch/feeder and Combination requirements would be the same (see Fig. 16),
so there would be only a Branch/feeder AFCI. The mandate of NEC 2005 would be moot, as there would be no Combination AFCI.

The author was permitted to speak to the group for about half an hour.

The test was carefully described including the
important fact that it represented nothing more than a carefully prepared parallel fault in a VERY long extension cord.

Available short-circuit fault currents as low as 5A were used, even though UL had earlier determined that the lowest available current in a home was 75A (see Fig. 2).
It was not a series arcing fault and had nothing to do with home electrical safety.

The vote was along “party lines”. NEMA manufacturers, with a few exceptions, and UL voted against removing test.

This block of votes exceeded the 1/3 required to defeat theproposal, so it failed. The “Carbonized path arc clearing time test” would remain in UL 1699.


~S~