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When the circuit of an inductive load is suddenly disconnected, its overvoltage can exceed several times of the power supply voltage. Overvoltage will cause arcing and spark discharge between contacts, thereby damaging contacts such as contactors, relays, and electromagnetic clutches, and shortening the service life of equipment. The varistor has a shunt effect at high voltage, so it can be used to prevent spark discharge at the moment of contact opening, thereby protecting the contacts.