The selection of pressure sensitive resistance should be observed.
The varistor should work within its rated parameters, otherwise it may lead to the deterioration of the varistor and even the consequences of the breakdown. The failure mode of varistor is mainly short circuit. If the short circuit time is too long, it will explode, fire and damage the components around it. There may also be an open circuit. Therefore, the following selection must be observed:

1) the working environment of the varistor:
It should be within the scope of the technical conditions:
Ambient temperature: 40C ~ 85 c;
Relative humidity: 40 + 2 degrees, the maximum is 96%;
Atmospheric pressure: up to 8.5KPa.

2) selection of voltage sensitive voltage
Select the power supply according to the power supply voltage. Taking full account of the fluctuation amplitude of the working voltage of the power grid (or circuit), the sufficient allowance should be left when selecting the voltage sensitive voltage of the varistor. The general fluctuation range in China is 30%. In addition, it is also necessary to consider the power supply voltage at both ends of the varistor, which can not exceed the "maximum continuous working voltage" listed in the gauge. For 220V ~ 240V AC power supply lightning arrester, the pressure sensitive resistor of voltage sensitive voltage of 470V ~ 620V should be suitable. The selection of a pressure-sensitive resistor with a higher voltage sensitive voltage can reduce the failure rate and prolong the service life, but the residual pressure increases slightly.

3) the selection of through flow
The nominal discharge current of the varistor should be larger than the surge current required or the maximum surge current that may occur every year. The nominal discharge current should be calculated according to the number of shocks more than 10 times in the rating curve of varistor life cycle, which is about 30% of the maximum impact flow volume (0.3 IP).

4) the selection of clamp voltage
The clamping voltage of the varistor must be less than the maximum voltage (the safe voltage) that the protected component or equipment can bear.

5) parallel voltage sensitive resistance
When a varistor can not meet the requirements of the nominal discharge current, a number of voltage sensitive resistors should be used in parallel. Sometimes, in order to reduce the limit voltage, a number of varistors are also used in parallel even if the nominal discharge current meets the requirements. We must pay special attention to, varistor used in parallel, must be strictly selected consistent parameters (for example: U1mA = 3V, delta alpha = 3) are matched to ensure the uniform current distribution.

6) protection of the failure of the varistor
The failure mode of varistor is usually short circuited. In order to prevent the loss of the varistor, a short circuit of the power source will cause fire, and a temperature controller or a heat release mechanism can be connected to each varistor. The temperature safety tube should have good thermal coupling with varistors. When the varistors fail (high impedance short circuit), the heat generated by them will fuse the temperature safety pipes, so that the failed varistors and the circuit will be separated to ensure the safety of the devices. When the higher power frequency is temporarily over-voltage applied to varistors, it may cause short duration of short circuit (low impedance short circuit). To avoid this phenomenon, a shock resistant power frequency fuse will be connected on each varistor (if the power frequency fuse is used alone, it will not fuse when it is aging or failure). Can also put the varistor and ceramic gas discharge tube connected in series with the normal work of ceramic gas discharge tube is not conductive, varistor without leakage current, can greatly prolong service life; the surge, the first breakdown ceramic gas discharge tube, and then by limiting the varistor surge voltage, the total residual pressure for both and, slightly increased (tens of volts); the impact of the past, as the varistor limiting current, discharge tube can not maintain the conduction to extinguish the arc, restore to normal working condition; when the varistor short-circuit failure, because the ceramic gas discharge tube current flowing through the great power will soon fail, but the failure mode of it most are open, so it is not easy to cause a fire.