In an AC system the phase difference between the currents of each phase is \(120^\circ\). This means the current of one phase is \(120^\circ\) apart from the current of the adjacent phase. Now, when there is an opening operation of a circuit breaker, all three poles of the circuit breaker open simultaneously. However, due to the \(120^\circ\) phase displacement between phases, the current waveform of one phase reaches its zero crossing before the others.
We know that a circuit breaker interrupts the current during zero crossing. This means, during zero crossing, the medium between the CB contacts will be deionized, and the dielectric strength will be reestablished. Ideally, the current will be finally interrupted at that point. Obviousely, the same will not happen at the same time in other two phases. The current continues through the other two phases by arcing. Since, the circuit breaker is gang operated, and the open contact gap of the CB pole in all three phases is the same.
Let’s consider, in the red phase, the arc is finally extinguished, and the current is interrupted at the zero crossing. As a result, the restriking voltage will appear across the open CB contacts of red phase. However, zero crossing has not yet occurred in the yellow and blue phases, so the arc continues here. At the next instant, the arc will be extinguished in the yellow phase, during zero crossing. Consequently, another restriking voltage will appear across the open CB contacts of yellow phase. However, the arc in the blue phase will still continue, as the current has not yet reached its zero crossing in the blue phase. After the zero crossing of the current in the blue phase, it will also be interrupted. Now, restriking voltage will appear across the open CB contacts of blue phase.
After interrupting all three phase currents, a stable system voltage will be established across the open contacts of all three poles of the circuit breaker.
We observe that there are three overvoltage phenomena occurred. In the pole where the current is interrupted first the restriking voltage apears first. Then, in the second pole, the same thing occurs, means restriking voltage apears acrioss the contacts at current interruption. Finally, in the third phase, a restriking voltage apears across the open breaker contact during current interruption. So, restriking voltages do not occur simultaneously.
It has been found that the restriking voltage (voltage spike) in the first pole which clears the current is the higher that that of the other poles. This highest restriking voltage, divided by the stable phase to ground voltage of the system, gives an important ratio used in analyzing circuit breaker performance. It means the voltage of the first pole that clears the current, divided by the phase to ground voltage after the interruption of the entire current. This ratio is referred to as the first pole to clear factor.