Varistor failure protection

The varistor is a protective component that is currently used in lightning protection and surge protection of the power supply system. Its response speed, flow capacity and protection level are recognized. Since the failure mode of the varistor is mostly short-circuited, a short circuit in the system may cause combustion, so various methods are used to avoid the varistor during use.

A short circuit occurs when it fails. In order to prevent the varistor from causing a short circuit in the power supply and causing a fire, a temperature fuse or a heat detaching 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. There will be no problems.

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 the surge voltage is controlled by the varistor, and the total residual voltage is the sum of the two, slightly increased (tens of volts); after the impact, the discharge tube cannot maintain the conduction due to the current controlled by the varistor 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.