Protection Against Failure Of Zinc Oxide Varistors

Today, a friend of the editor asked: What should we do if zinc oxide varistors fail? Let's explore the protection against zinc oxide varistors failure together today, hoping to be helpful to everyone.

In fact, zinc oxide varistors are currently widely used as protective components for lightning protection and surge protection in power systems. Its response speed, flow capacity, and protection level are widely recognized. Due to the fact that most of the failure modes of varistors are short circuits, a short circuit in the system may cause combustion. Therefore, various methods are used to avoid varistors during use.

Short circuit occurs during failure. To prevent the failure of varistors from causing power short circuits and catching fire, a temperature fuse or thermal release mechanism can be connected in series on 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 melt the temperature fuse, thereby separating the failed varistor from the circuit and ensuring that the equipment will not have problems.

When a high power frequency temporary overvoltage acts on the varistor, it may cause the varistor to instantaneously breakdown and short circuit (low impedance short circuit), and the temperature fuse may not have enough time to fuse, and may even catch fire. To avoid this phenomenon, an impact resistant power frequency fuse can be connected in series to each varistor (using a single power frequency fuse may not blow when aging fails).

Varistors can also be used in series with ceramic gas discharge tubes. During normal operation, the ceramic gas discharge tube is not conductive, and the varistor has no leakage current, which can greatly extend its service life; When subjected to surge impact, the ceramic gas discharge tube first breaks down, and then the surge voltage is controlled by a varistor. The total residual voltage is the sum of the two, slightly increasing (by tens of volts); After the impact, due to the varistor controlling the current, the discharge tube could not maintain conductivity and extinguished the arc, returning to normal working state; When the varistor fails due to a short circuit, the ceramic gas discharge tube will also quickly fail due to a large power frequency current flowing through it. However, its failure mode is mostly open circuit, making it less likely to cause a fire.