2.1 Arc interruption

Interruption of an alternating current arc, subtended between parted electrical contacts, will take place if the means for electrical re-ignition is removed. The gap between the contacts has to change from being an electrical conductor to being an electrical insulator at, ideally, a natural current zero.

There are a number of theories relating to the interruption of electrical current, and most of these are based upon the original theories of Cassie [1] or Slepian [2].

Cassie says:

If the energy lost from the arc column at current zero exceeds the energy input from the external electrical circuit, the electrical current will cease to flow.

Slepian says:

If, after current zero, the dielectric strength of the contact gap increases at a greater rate than the transient voltage, then the circuit breaker will clear.

Slepian's theory is illustrated in Figure 2.1.

Figure 2.1: Slepian's theory of interruption and re-ignition

A successful interruption is shown in Figure 2.1(a) where the rate of increase of dielectric recovery exceeds the rate of increase of the transient recovery voltage stress. Figure 2.1(b) shows a failure to clear as re-ignition occurs at a point where the impressed voltage exceeds the dielectric strength of the gap.

2.2 Interruption mediums and techniques

If we put to one side fault current interruption using high-voltage fuses, interrupting mediums used in medium voltage distribution switchgear today are oil, vacuum and SF ₆ gas. There is a small percentage of units based upon hard gas, where the arc is...