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 restrike
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.
