For ease of the readership, here is what Dr Engle published in his paper (i can't seem to post the graph, sorry)
~S~
Test 12: Operation Inhibit Apparently UL determined that the arc detection schemes of both Products 1 and 2 utilized the high frequency content of the current waveform.
A continuous low current series arc was created using the “arc simulators”. These simulators use
two opposing electrodes, like those of a carbon arc lamp.
Rather than use two carbon electrodes, Simulator 1 used one carbon (graphite) electrode and one phosphor-bronze electrode.
Simulator 2 used one carbon and one copper
electrode. Both simulators created a continuous low current arcing condition. The current, after each zero crossing, is zero for a few milliseconds until an arc re-strike occurs.
This re-strike transient produces a high frequency “noise” component in the arcing current waveshape.
UL evidently theorized that Products 1 and 2 series arc detection algorithms could be masked by normal continuous series arcing, such as from the brushes of an electric drill.
Also a normal EMI filter used in power strips etc. could filter the high frequency current component, so the AFCI circuit breaker would not see the arcing event.
UL was correct in their assumptions. Products 1 and 2 were tested and failed both masking tests that indicate their technology was based on looking for high frequency noise.
Why didn’t UL use copper-copper electrodes, instead of
the odd combinations of carbon and phosphor-bronze and
carbon-copper?
Unfortunately UL didn’t address this important question, and thus the validity of the use of these “arc-simulators” is questionable.
Further the author believes that UL, by
introducing their use, inadvertently gave credibility to AFCI manufacturers’ claim that their product will respond to a series arcing event.
The use of strange materials like phosphor-bronze and carbon, conductive materials not used in house wiring, might be explained but not justified, as follows:
 Copper-Copper: This combination and copper-steel are the only valid electrode choices. If UL wanted to demonstrate “real world” series arc detection they could have used copper.
UL may have tried copper but found they only got
sparks, not the continuous low current arcing that Products 1 and 2 probably needed to trip.
The reason for a simple spark is explained by a century old law of physics. A person named F. Pashchen in 1889 published a law which sets out what has become known as Paschen's Law.
He determined the relationship between breakdown voltage, the gap between two metal plates, and the pressure.
With air as the gas, the minimum voltage is 327V, as shown in Fig. 5. The peak of a 120VAC sine wave is only 170V, and thus continuous low current arcing is, by a law of physics, not possible with copper-copper.
Thus claims that a Combination AFCI will respond to arcing at a break in a
conductor or a loose connection flies in the face of a law of physics.
Fig. 5 Paschen's Law
Copper and Phosphor-bronze and Carbon-
Copper: UL did not justify the use of such strange electrode pairs.
Practically, the use of carbon made it easy to produce
continuous low current series arcing to test the claims of Product 1 and 2.
Paschen's Law applies only to metal-to-
metal arcing. Unfortunately, it also would seem to invalidate the use of any test results as part of a home electrical fire study.
