Even if they were 100% effective, it is still unknown how often dangerous arcing takes place, or if its even possible in North America... Overseas testing and documentation has proven some possibility when the actual supply voltage is on the high nominal side of 230 volts, hence one reason behind the mandatory requirements of RCDs and low magnetic trip breakers. The same concern was known to the NEC for decades, hence the GFP requirement on 277/480 volt 1000amp and over services. Utilities also have 100 years of documentation, where real world experience has demonstrated nearly all sustained arcing occurs on secondary networks running
over 120/208Y. In fact, derived from UL's very research papers supported by Paschen's law, 330 volt peak is needed. This would then mean a minimum actual RMS voltage of 235 volts L-G is needed in order to have concern of sustained arcing. Key is sustained arcing, or at least what Paschens and the rest of the scientific community would call arcing. Of course UL's definition is less subjective: "a continuous luminous discharge of electricity
across an insulating medium, usually accompanied by the partial volatilization of the electrodes"
Thus, with less stringent definitions, even a short circuit can be called an arc fault, and that is just what happened:
Found here on page 7:
http://newscience.ul.com/wp-content...ng_Parallel_Arcing_Faults_in_the_Home_Run.pdf
Thus, because a luminous discharge takes place in most short circuits with pitting/volitization between the 2 points of contact, we can call this "arcing". Because the standard says "arcing" for more then 8 half cycles is a potential danger (never been proven to hold applicability to the real world), we can then call nearly all short circuits that do not open an OCPD within 8 half cycles dangerous arcing. In most cases the only thing that will open an OCPD that fast is its magnetic trip function, and because that does not happen every time for a variety of reasons (UL489 does not even require magnetic trip) standard circuit breakers are then said to be incapable of mitigating all short circuit arcing fires, or in today's terms parallel arc faults.
As time went on the above theory began to develop holes. One question then was how does damaged insulation under a staple, with a mm or smaller gap suddenly turn into a conductor when 120 volt can and will never jump such a wide gap. Well, a theory was developed to make is sound plausible... The idea is that lightning strikes, and other induced surges coming in from the service cause a voltage potential of thousands of not tens of thousands of volts to develop across the damaged insulation. This brief surge causes a small arc to jump this gap which then extinguishes as soon as the surge goes away. Over the years the damage from multiple surges will add up where the gap becomes "carbonized". Once carbonized, the gap will conduct 120 volts across it without the help of high voltage spikes resulting in high current arcing. This high current arcing while 75 amps and over (current often limited by the source itself because we basically have a short circuit here lol), will not trip the thermal portion of the breaker fast enough (8 half cycles) to prevent fire, but can trip the magnetic portion of the breaker if pickup is low enough (not always practical to set the mag trip at 75 amps), or arc signature analysis looking for this high current waveform catches it. Thus, we have the modern branch feeder AFCI, looking for this short circuit oops I mean arcing signature.
Of course as time went on, the above still raised question, so the concern was then shifted to also include a new type of arcing called a series arc fault which supposedly manifests from loose or glowing connections. However, arcing is often the end stage of a glowing connection, not the beginning, and glowing connection, along with thee sparking from a cut across a cord, can take place under 5 amps...