0
\$\begingroup\$

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.

\$\endgroup\$
3
  • \$\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\$ Commented Jan 12, 2021 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
    Commented Jan 12, 2021 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
    Commented Jan 12, 2021 at 11:13

1 Answer 1

1
\$\begingroup\$

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.

\$\endgroup\$
4
  • \$\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
    Commented Jan 12, 2021 at 11:11
  • 1
    \$\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
    Commented Jan 12, 2021 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
    Commented Jan 12, 2021 at 12:18
  • \$\begingroup\$ Then your "understanding" needs to be upgraded!! \$\endgroup\$
    – Andy aka
    Commented Jan 12, 2021 at 12:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.