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In some application notes for MOVs I see example usage in power supply design where each input line (N, L) from the mains are protected with a MOV to a ground voltage reference (as well as a differential mode MOV protection between the input lines)

What I don't understand is what is the 0V "ground" reference these application notes are referring to?

It is both regulatory and practically impossible to use the PE (=FG) since even the smallest MOV current could trigger the earth protection circuit breaker.

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  • \$\begingroup\$ a common-mode MOV protection between the input lines I'd say that a MOV between live and neutral is differential mode protection while the MOVs between Line-Earth and between Live-Earth are for common mode protection. \$\endgroup\$ Jan 12 at 10:17
  • \$\begingroup\$ The regulations have also changed during the years, and at some point there was a time when equipment with a pluggable mains cord was not considered reliably earthed so it was not compliant to have a MOV connected to earth, and at present a simple MOV must in practice have a GDT in series to prevent leakage (along with thermal protection). \$\endgroup\$
    – Justme
    Jan 12 at 11:09
  • \$\begingroup\$ @Bimpelrekkie, You are of course correct. I wrote too quickly without thinking. Any common mode charge would raise both MOV terminals and hence have no effect. \$\endgroup\$
    – Johan
    Jan 12 at 11:13
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What I don't understand is what is the 0V "ground" reference these application notes are referring to?

Well, despite you thinking this: -

It is both regulatory and practically impossible to use the PE (=FG) since even the smallest MOV current could trigger the earth protection circuit breaker.

The answer is still actually the "protective earth" (PE). If a surge causes a live-neutral imbalance current, then it may trip an RCD (UK) or GFCI (US). That cannot be avoided other than by designing the power supply AC interface to not require a protective earth point and hence it's impossible to utilize surge protectors connected to PE.

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  • \$\begingroup\$ Thanks. This is kind of what I suspected. But even in normal operation you would have a small current flowing through the MOV which I believe would be enough to cause inbalance and to trip breaker. \$\endgroup\$
    – Johan
    Jan 12 at 11:11
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    \$\begingroup\$ I don't believe that the small current you talk about will be anywhere close to the normal 30 mA trip limit for an RCD or GFCI. \$\endgroup\$
    – Andy aka
    Jan 12 at 11:19
  • \$\begingroup\$ You are right (I thought the trip current was lower). The leak would be in the u-range which is well below the range of the trip limit. But even then my understanding of regulation (in EU) is that a device can not have, by design, even that small leak current in normal operation towards PE. \$\endgroup\$
    – Johan
    Jan 12 at 12:18
  • \$\begingroup\$ Then your "understanding" needs to be upgraded!! \$\endgroup\$
    – Andy aka
    Jan 12 at 12:20

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