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I am using a MOV for relay contact protection. I am switching a pump of current 12A @230V using relay. But I faced contact arcing problem so I added a MOV. I followed this guide for MOV selection as I am using their relays. But I found some strange effect of adding MOV. The pump won't start if MOV is added across relay contacts. Without MOV pump starts. I am using MOV of Elpro with rating of 440/20.

In this way I am connecting MOV with relay contacts

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    \$\begingroup\$ Could you add a schematic? \$\endgroup\$
    – Joren Vaes
    Commented May 3, 2017 at 7:02
  • \$\begingroup\$ @JorenVaes The relay is panel mounted so I connected the MOV directly across relay contacts \$\endgroup\$
    – MrAlpha_SU
    Commented May 3, 2017 at 7:29
  • \$\begingroup\$ @MrunalAhirrao Then post a schematic of that. \$\endgroup\$
    – winny
    Commented May 3, 2017 at 8:39
  • \$\begingroup\$ @winny added please check. \$\endgroup\$
    – MrAlpha_SU
    Commented May 3, 2017 at 8:46
  • \$\begingroup\$ Much better. Chek the impedance over the MOV. Even better, put a large resistor in series with it and do a bench test of MOV+resistor against a variable voltage and plot MOV voltage against input voltage to find the knee. I suspect a shorted MOV or too low knee voltage. Also, where did it arc? \$\endgroup\$
    – winny
    Commented May 3, 2017 at 8:55

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You don't give much information about the transil nor the MOV, but it sounds like the stored energy in cable inductance is just too high for the MOV and transil to absorb. If there appeares to be a short when you insert your MOV, the most likely reason is that the MOV is shorted. They do fail into short when overpowered so it makes sense.

If you have selected the voltage rating on the MOV to an appropriate value, you just need to increase the energy rating or use several in parallel. The first option is better.

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