In previous applications, varistors connected across power lines have caught fire and threatened other components. In this regard, manufacturers and users jointly conducted a lot of research and analysis work, take the
corresponding countermeasures, well reduce the probability of such accidents, but has not been eliminated, therefore, the use of varistor safety is still a worthy of attention, need to continue to study and solve the topic.
The apparent phenomenon of varistor fire can be divided into two types: aging failure and transient overvoltage failure.
(1) Aging failure, this refers to the resistance of low resistance linearization gradually intensified, leakage current malignant increase and concentrated into the weak point, the weak point material melting, forming about 1K ω after
the short circuit hole, the power supply continues to push a larger current into the short circuit point, forming high heat and fire. Such accidents can usually be avoided by a hot-melt contact in series with a varistor. The hot melt
contact should have a good thermal coupling with the resistance body and will not be disconnected when the maximum impact current flows, but will be disconnected when the temperature exceeds the upper working
temperature of the resistance body. The results show that if the varistor has manufacturing defects, it is easy to have early failure, and the repeated action of electric shock with low intensity will accelerate the aging process and
make the aging failure appear earlier.
(2) Transient overvoltage damage, which refers to the strong transient overvoltage to perforate the resistance body, resulting in greater current and high heat fire. The whole process takes place in such a short time that there is no
time for the hot melt contacts on the resistance body to fuse. In the protection of three-phase power supply, the accident probability of varistors burning out between N-PE lines is high, and most of them belong to this situation. The
corresponding countermeasures focus on no fire after the varistor is damaged. Some varistor application technical data, recommended with the varistor series current fuse (fuse) for protection.
