Malfunction of varistor

Common resistors are varistor, which is commonly used for lightning protection. The failure mode of the varistor is mainly a short circuit. If the short circuit time is too long, an explosion, fire, and damage to surrounding components may occur; and an open circuit may also occur. What are the failure protection methods for varistor?

The failure mode of the varistor is usually a short circuit. In order to prevent the power supply from short-circuiting and causing a fire due to the failure of the varistor, a temperature fuse or a thermal detachment mechanism may be connected in series to each varistor. The temperature fuse should have good thermal coupling with the varistor. When the varistor fails (high impedance short circuit), the heat generated by it will blow the temperature fuse, thus separating the failed varistor from the circuit and ensuring the equipment. Security.

When a higher power frequency temporary overvoltage acts on the varistor, the varistor may instantaneously break through the short circuit (low impedance short circuit), and the temperature fuse may not be blown, and may also ignite. To avoid this, an impact-resistant power frequency fuse can be placed in series with each varistor (single power frequency fuses may not blow when aging fails). The varistor can also be used in series with the ceramic gas discharge tube. The ceramic gas discharge tube does not conduct during normal operation, and the varistor has no leakage current, which can greatly prolong the service life; when subjected to a surge, the ceramic gas discharge tube is first hit. Wear, then limit the surge voltage by varistor, the total residual voltage is the sum of the two, slightly increased (tens of volts); after the impact, the current is limited by the varistor, the discharge tube can not maintain conduction The arc is extinguished and returned to the normal working state; when the varistor short-circuit fails, the large-scale power-frequency current flowing through the ceramic gas discharge tube will quickly fail, but the failure mode is mostly open circuit, so it is not easy Causes a fire.