In the context of safety switch rating (fuses, circuit breakers, etc.), the concept of breaking capacity refers to the maximum electric current which can be safely interrupted by a tripping safety switch, i.e. interrupted without the formation of an electric arc. The article Breaking capacity on Wikipedia states that

The current breaking capacity corresponds to a certain voltage, so an electrical apparatus may have more than one breaking capacity current, according to the actual operating voltage.

I am curious as to what a breaking capacity vs. operating voltage plot would look like, but I haven't been able to find one. I suspect breaking capacity is negatively correlated with open circuit voltage, but that is just a guess.

For my question, consider the following case:

Let there be a circuit with a switch, where \$A\$ and \$B\$ denote the contactors of the switch. A voltage of \$U_a\$ is the maximum voltage between \$A\$ and \$B\$ such that no electric arc can form between them when the switch is open.

If the switch is originally open, a voltage of \$U_a\$ between \$A\$ and \$B\$ will not generate an arc between \$A\$ and \$B\$.

If the switch is originally closed and a current of \$I_b\$ flows, when suddenly the switch is opened, is the voltage \$U_a\$ now enough to generate an arc between \$A\$ and \$B\$?

If the flow of current reduces the voltage required for electric arc formation in the example above, why does it happen? Does it have to do with momentum of the charge carriers?

I hope my question is clear enough for being answered. If not, please point out where it is unclear.


If the switch is originally closed and a current of \$I_b\$ flows, when suddenly the switch is opened, is the voltage \$U_a\$ now enough to generate an arc between \$A\$ and \$B\$?

Switches can be designed to open very quickly, but the opening time can never be zero. As a consequence, it must be assumed that an arc is formed. The current drawn influences the breaking of the arc as the gap rapidly increases. If the arc has not been extinguished by the time the switch is completely open, it may be sustained.

The current level influences the breaking of the arc in two ways. One is the temperature of the arc. The other is the voltage developed by the inductance to the circuit tending to maintain the flow of current.

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  • \$\begingroup\$ Thank you for your answer. Let us check if I grasped this answer correclty. On the thermal side, the higher the current, the hotter the arc, and therefore the more conductive it is. On the electromagnetic side, the current flow before the tripping of the switch will generate a magnetic field which after the tripping of the switch will push the charge carriers to the dead end of the tripped switch, whereby electrons will accumulate, drive the voltage difference up and with it the capacity to break the insulation. Did I get your explanation right or am I just making stuff up? \$\endgroup\$ – Severo Raz Oct 3 '19 at 11:22
  • \$\begingroup\$ Every circuit, even a straight wire has inductance. The voltage across an inductance is proportional to the current multiplied by the rate of change in current. The polarity is such that the voltage adds to source voltage and thus makes it more difficult to break the arc. I don't remember how that principle is derived from the physics of charge carriers, but I don't think your idea about the accumulation of charge carriers is correct. \$\endgroup\$ – Charles Cowie Oct 3 '19 at 13:36
  • \$\begingroup\$ True, yes maybe my interpretation of how the voltage adds to the source voltage is not the right one, but I think I get what you're trying to get across. Thanks for your explanation! \$\endgroup\$ – Severo Raz Oct 3 '19 at 16:54
  • \$\begingroup\$ Will it arc even if the switch is flooded with oil? \$\endgroup\$ – user2813274 Oct 6 '19 at 11:45
  • \$\begingroup\$ @user2813274: I believe the change in medium from air to oil only reduces the arc and makes it easier to break. I also think the comment should have been posted as a new question. \$\endgroup\$ – Charles Cowie Oct 6 '19 at 13:02

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