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1、Safety issues with varistors:

In previous applications, voltage sensitive resistors connected across power lines have caught fire and burned, posing a risk to other components. In this regard, manufacturers and users have jointly conducted a lot of research and analysis work, adopted corresponding countermeasures, and greatly reduced the probability of such accidents, but it has not been completely eliminated. Therefore, the safety of the use of varistors is still a subject worthy of attention and needs to be further studied and solved.
The apparent phenomenon of ignition and combustion in varistors can be broadly divided into two types: aging failure and transient overvoltage damage.

       ① Aging failure refers to the gradual intensification of low resistance linearization of the resistor body, with a malignant increase in leakage current and concentrated flow into weak points. The weak point material melts, forming a short circuit hole of about 1k Ω. After that, the power supply continues to push a larger current into the short circuit point, forming high heat and catching fire. This type of accident can usually be avoided by a hot melt contact connected in series with a varistor. The hot melt joint should have good thermal coupling with the resistor body. It will not disconnect when the maximum impulse current flows through, but will disconnect when the temperature exceeds the upper working temperature of the resistor body. The research results indicate that if there are manufacturing defects in varistors, they are prone to early failure. The multiple effects of weak electrical shocks can also accelerate the aging process, leading to premature aging failure.

       ② Transient overvoltage damage refers to the strong transient overvoltage causing perforation of the resistor body, resulting in greater current and high-temperature ignition. The entire process occurs in a relatively short period of time, to the point where the hot melt contacts set on the resistor are unable to fuse in time. In three-phase power supply protection, the probability of the varistor between N-PE lines burning out and catching fire is relatively high, and most of them belong to this situation. The corresponding countermeasures focus on preventing fire after the varistor is damaged. In some technical materials for the application of varistors, it is recommended to connect a current fuse (fuse) in series with the varistor for protection.

       2、The problem of connecting wires for varistors is that the connecting wires for connecting varistors to the circuit should be thick enough. The recommended size of the connecting wires is as follows: the grounding wire should be 5.5 mm2 or more, and the connecting wires should be as short as possible and in a straight line, because the impulse current will generate additional voltage on the inductance of the connecting wires, which will increase the limiting voltage at both ends of the protected equipment. The flow rate of a varistor is ≤ 600A (600-2500) A (2500-4000) A (4000-20K) A, and the cross-sectional area of the wire is ≥ 0.3 mm2, ≥ 0.5 mm2, ≥ 0.8 mm2, ≥ 2 mm2. For example, if there are 3 cm long wires at each end of the varistor MY, its inductance L is generally 18 nH. If there is an 8/20 impulse current of 10 KA flowing into the varistor, the current acceleration is considered as 10 KA/8 μ s, and the additional voltage UL1 and UL2 on the lead inductance are generally UL1=UL2=L (di/dt)=18 × 10-9 (10 × 103/8 × 10-6)=22.5 V. This increases the limiting voltage by 45V.