What are the basic parameters of SPD varistor?

What are the basic parameters of SPD varistor

The varistor is a voltage limiting protection device. Using the nonlinear characteristics of the varistor, overvoltage occurs between the two poles of the varistor, which can clamp the voltage at a relatively fixed voltage value, thus realizing the protection of the subsequent circuit. So, what are the basic parameters of SPD varistor

SPD varistor: What are the basic parameters of the varistor

1. Nominal varistor voltage (V): refers to the voltage value at both ends of the varistor when the pulse current with specified duration (generally 1mA duration is generally less than 400mS) passes through the varistor

2. Voltage ratio: refers to the ratio of the voltage generated when the current of the varistor is 1mA to the voltage generated when the current of the varistor is 0.1mA

3. Maximum limit voltage (V): the peak voltage at both ends of the varistor under the maximum pulse peak current Ip that the varistor can withstand and the specified waveform

4. Residual voltage ratio: when the current through the varistor is a certain value, the voltage generated at both ends is called the residual voltage of the current value. The residual voltage ratio is the ratio of residual voltage to nominal voltage

5. Current carrying capacity (kA): current carrying capacity, also known as current, refers to the maximum pulse (peak) current value allowed to pass through the varistor under the specified conditions (specified time interval and number of times, applied standard pulse current)

6. Leakage current (mA): leakage current, also known as waiting current, refers to the current flowing through the varistor at the specified temperature and maximum DC voltage

7. Voltage-temperature coefficient: refers to the change rate of the nominal voltage of the varistor within the specified temperature range (temperature is 20~70 degrees), that is, the relative change of the voltage at both ends of the varistor when the temperature changes by 1 while the current through the varistor remains unchanged

8. Current temperature coefficient: refers to the relative change of current flowing through the varistor when the voltage at both ends of the varistor remains unchanged and the temperature changes by 1

9. Voltage nonlinear coefficient: refers to the ratio of static resistance value and dynamic resistance value of the varistor under the action of a given applied voltage

10. Insulation resistance: refers to the resistance between the lead (pin) of the varistor and the insulation surface of the resistance

11. Static capacitance (PF): refers to the inherent capacitance of the rheostat itself

12. Rated power: the maximum power that can make the change of voltage sensitive voltage less than 10% when working for 1000 hours at a specific ambient temperature of 85 ° C

13. Maximum impulse current (8/20us): the maximum impulse current that can shock the varistor once or twice (with an interval of 5 minutes) with a specific impulse current (8/20us waveform) to make the voltage change of the varistor within 10%

What are the main reasons for SPD surge protective device failure modes

1. Temporary overvoltage, switching overvoltage in low-voltage system, long-term voltage fluctuation caused by poor quality of power grid system and frequent lightning strikes in power supply system are the external causes of surge protection device failure

2. Inadequate design of pulse life, long-term charging life of ZnO varistor and power frequency continuous current breaking capacity of gas discharge tube is the internal cause of SPD failure