AC contactor varistor function and selection

The characteristic of a varistor is that when the voltage across it is lower than a certain value, it is almost non-conductive, which is equivalent to an open circuit; and when its voltage exceeds a certain value, it breaks down, which is almost equivalent to a short circuit. Therefore, we choose safe operating current of the varistor to be greater than the current of the coil, so that the varistor can recover the open circuit state after the power is turned off. Selecting the breakdown voltage of the varistor to be greater than the energizing voltage of the coil can prevent the varistor from having current when the coil is normally energized.

Therefore, when the customer's PLC output is connected to an AC220V AC contactor, a fuse must be connected in series in the line in front of the varistor so that when the line is overvoltage, the varistor breaks down, causing the fuse to blow. The varistor that should be chosen at this moment is 430V~470V. If it is used for lightning protection, it should be connected between the line and the earth. Also, a fuse must be connected in series. When a lightning strikes, the voltage of the lightning is much higher than the line voltage, which exceeds the threshold of the varistor. The varistor conducts, discharges the earth, and avoids damage to the electrical equipment in the line. At this point, the varistor is selected from 470V to 510V.