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


  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


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


  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

  • 1
    \$\begingroup\$ What precisely are you trying to protect and what are you trying to protect it from? \$\endgroup\$
    – Andy aka
    Nov 14, 2019 at 13:28
  • \$\begingroup\$ 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. \$\endgroup\$
    – Delta
    Nov 14, 2019 at 13:36

1 Answer 1


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).

This makes me conclude that the clamping voltage of 710 volts (50 amp surge) might in some cases be 22% higher at maybe 868 volts. You need to consider this if your protection is to be effective.

But, is a 50 amp surge good enough? In terms of standard lightning protection specifications, EN 61000-4-5 testing can inject hundreds if not thousands of amps. This makes me ask you if you think you are designing using the right component.

  • \$\begingroup\$ Andy aka, thank you for the answer! Using these varistors, I want to protect the contacts of a semiconductor relay from the effects of a magnetic contactor. I think a current of 50A is enough. I studied additional information about varistors and realized that it is worth changing course to TMOV, which protects the varistor from fire during prolonged overvoltage. \$\endgroup\$
    – Delta
    Nov 15, 2019 at 5:40
  • \$\begingroup\$ But I still did not understand whether the option with 275 volts is suitable for protecting a network with a voltage of 230V. Indeed, there may be surges in the network above 300 volts, which the measuring devices, most likely, will not have time to notice. \$\endgroup\$
    – Delta
    Nov 15, 2019 at 5:40
  • \$\begingroup\$ Your latter point is the problem. You are trying to protect certain equipment from specific spikes but, in doing so, you make the MOV chosen vulnerable to regular lightning surges if they arise. You have to consider the bigger picture. An MOV of 275 volts is fine for AC voltages up to that voltage but that isn’t the bigger picture. \$\endgroup\$
    – Andy aka
    Nov 15, 2019 at 9:08

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