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The use principle of Varistor is that after it is connected to the protected equipment, it cannot affect the normal operation of the equipment, and can effectively implement instantaneous overvoltage protection for the equipment. Therefore, in addition to the technical parameters of Varistor, the following issues should be considered in the actual selection:

(1) Selection of voltage sensitivity
Considering the deviation between the actual Varistor voltage and the nominal voltage (1.1~1.2 times of the nominal voltage should be considered), the possible fluctuation range of the power supply voltage in the AC circuit (1.4~1.5 times of the rated voltage should be considered), and the relationship between the peak value of the AC voltage and the effective value (1.4 times should be considered), the Varistor with a voltage sensitive of 2.2~2.5 times of the rated voltage should be selected. In DC circuits, Varistor with a voltage sensitive of 1.8~2 times the rated value of DC voltage are often selected.

⑵ Selection of flow capacity
In principle, the selection should be based on the maximum transient surge current that may be experienced, but achieving this is difficult. In practice, the selection of Varistor is based on the use occasion or the test grade specified in the product test standard.

According to the former, 1kA (8/20 μ S current wave) can be used for the protection of silicon controlled rectifier; 3kA is used for surge absorption in electrical equipment; 5kA is used for overvoltage absorption of lightning strikes and electronic equipment; 10kA is used for protection against lightning strikes. According to the latter, the commonly used composite wave (1.2/50 generated when the generator is open circuit output) μ Voltage wave of s; 8/20 generated during short-circuit output μ Current wave of s; The internal resistance of the generator is 2 Ω to assess the equipment's ability to withstand lightning surge interference online. During the 4kV test, the maximum current absorbed by the protector can reach 2kA; For the 6kV test, the maximum absorbed current is 3kA. However, in actual selection, the flow capacity of the selected Varistor should also be appropriately increased. Because Varistor with large current passing capacity should have relatively small residual voltage drop when absorbing surge current of the same size; At the same time, there is also a large protection margin for the selected Varistor.

⑶ Inherent parasitic capacitance
Varistor has an inherent capacitance problem. Its value ranges from hundreds to thousands of pF according to the difference of external dimensions and nominal voltage. The inherent capacitance of Varistor determines that it is not suitable for use in high-frequency occasions, or it will affect the normal operation of the system; Suitable for use in power frequency systems, such as for protection of power incoming lines, protection of thyristor rectifiers, etc.

The instantaneous power of Varistor is relatively large, but the average continuous power is very small, so it cannot work in the conduction state for a long time.https://www.kingtronics.com/Metal-Oxide-Varistor/XKT-Metal-Oxide-Varistor.html 

Kingtronics Kt 05D to 20D Metal Oxide Varistor Features
Operating Temperature Range: -40℃~+105℃
Body Diameters Range From 5D to 20D
Wide Operating Voltage from 18V to 1800V
Full compliance with RoHS and REACH

Kingtronics Kt 05D to 20D Metal Oxide Varistor Applications
Industrial equipment
Home appliance
Industrial equipment
Lighting products
Telecommunication or telephone system

Kingtronics would like to share our another strong production line: XKT Metal Oxide Varistors.
Radial leaded varistor devices be use in electrical surge protectors which protect against overvoltage transients. it can protect rectification diodes, SCRs, power transistors, semiconductor devices, etc.
High Quality with excellent price and lead time support, welcome your inquiry. 
Product datasheet: https://www.kingtronics.com/pdf/XKT-Metal-Oxide-Varistor.pdf
Kingtronics Kt 05D to 20D Metal Oxide Varistor product video: https://www.youtube.com/watch?v=JlZ5yg4GsBU

•       Operating Temperature Range: -40℃~+105℃
•       Body Diameters Range From 5D to 20D
•       Wide Operating Voltage from 18V to 1800V
•       Full compliance with RoSH and REACH

Applications: Industrial equipment, Home appliance, Industrial equipment, Lighting products, Telecommunication or telephone system

Metal oxide Varistors ( MOV, Metal Oxide Varistor ) are voltage dependent resistors with symmetrical voltage-current characteristics which are designed to protect all kinds of electronic devices or elements from switching pulse or induced lightening surges. Its non linear characteristic with broad application range, mass production feasibility are gradually being used in various level of electric and electronic device from low to high voltages level protections ( AC11V- 1000 V as available ranges ).

Kingtronics offers high quality XKT 05D to 20D Metal Oxide Varistor at incredibly aggressive prices, following are our product data, welcome your inquiry!

Product datasheet: https://www.kingtronics.com/pdf/XKT-Metal-Oxide-Varistor.pdf

**Features
Operating Temperature Range: -40℃~+105℃
Body Diameters Range From 5D to 20D
Wide Operating Voltage from 18V to 1800V
Full compliance with RoSH and REACH

**Applications：
Industrial equipment
Home appliance
Industrial equipment
Lighting products
Telecommunication or telephone system