# Calculation of varistor parameters for AC line protection

There is a task to choose a varistor to protect the AC circuit, controlling several coils of magnetic starters.

Characteristics:

1. Mains voltage 230-240V AC
2. Frequency: 50Hz
3. Starting current of coils of magnetic starters: 3A max

For these purposes, I want to use the LA series varistors from LittleFuse.

Model: V275LA20AP

Characteristircs:

1. Vm (AC): 275V
2. Vm (DC): 369V
3. Varistor voltage: 387V (min) - 473V (max)
4. Clamping voltage: 710V

Questions:

1. Vm (AC) - Is this a normal mains voltage?
2. Do I need to find the peak voltage amplitude to select Vm (AC)?

For example:

Mains voltage: 230V

I must multiply this value by √2 and add 10% margin. 230 * 1.41 = 324.3V + 10% = 347.3V

• What precisely are you trying to protect and what are you trying to protect it from? – Andy aka Nov 14 '19 at 13:28
• Solid state relays and magnetic starter coils for surge protection. I need to understand the value of Vm (AC), whether 275V is suitable for a 230V network, is it necessary to take into account the maximum amplitude of the AC network here. – Delta Nov 14 '19 at 13:36

The V275LA20AP varistor is specified for up to 275 volts RMS and the data sheets ties this in nicely when multiplying 275 volts by approximately $$\\sqrt2\$$ to get 287 volts. This is the minimum voltage that would cause 1 mA to flow into the device. But, you could get a device with the same marking that only draws 1 mA at a voltage of 473 volts (22% higher).