From this Reference Design, I have the following inquiry:

Usually in three phase applications I see varistors connected between phase to phase, or between phase to ground. In the reference design, the common point of the varistors RV1...RV4 is not connected anywhere. I would expect it to be ground, but they intentionally left it floating. How are varistors protecting in this case?


Varistors, in this case MOV's, are installed based on modes of protection. 10 mode protection for 3-phase devices covers phase to neutral (3), phase to ground (3), Phase to phase (3), and neutral to ground (1). This is for stand-alone surge suppressors which can get expensive if parallel MOV's are used, so devices with built-in MOV's are not likely to protect all possible modes.

Ac input is 3-phase plus neutral for the power supply shown.

56 Vac...891 Vac input from Three-phase Mains

The AC input voltage is very high, so 460 VAC MOV's are used but not directly phase to phase. They share a common node which has a 460 VAC MOV to neutral. The MOV's are in series so their soft clamp voltage is 460 * 2 * 1.5, or about 1,380 VAC. This is plenty of safe headroom yet will clamp on input voltage spikes.

Neutral is the lowest voltage connection, and normally neutral is Earth grounded at the service entrance panel. The MOV's clamp to neutral (in series) so any AC input over 1,380 volts is at least soft-clamped. This supply uses 3 phases in case 1 or 2 phases fail. The isolated output is only 12 VDC at 125 mA, so 1 working phase will keep the power supply working.

Notice the isolated grounds, and that there is NO Earth ground on the AC side. The very low voltage on neutral (<= 10 VAC) makes it a virtual ground. The ground symbol refers to a common ground for all parts on the AC side of the power supply. This will have 1/2 the DC supply voltage on it as a negative value (compared to Earth ground), so touching any part of the AC supply while power is on could give a bad shock.

NOTE: The voltage on neutral is dynamic but ranges from effectively zero VAC to normally not over 10 VAC. Some surge protection devices will issue a alarm if neutral is greater then 30 VAC above Earth ground. Neutral is never to be high enough to be a shock hazard. In some cases double neutral wires help lower the neutral voltage at point of use.

  • \$\begingroup\$ Thank you. One more question, let's say that I don't have neutral connection directly for the power supply, so I don't have diodes D5,D16. Would the MOVs still work? \$\endgroup\$ – user115094 Jul 15 '20 at 1:58
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    \$\begingroup\$ NO. They WILL clamp phase to phase only. Neutral is your closest thing to Earth ground and if you take it away you loose the ability to clamp over voltage referenced to Earth ground. The power supply already has dangerous voltages, and the MOV's protect it. \$\endgroup\$ – user105652 Jul 15 '20 at 3:12
  • \$\begingroup\$ Ok, I was asking because in a similar design I only have V1, V2, V3, i.e, I don't have neutral connection. In this case only phase to phase MOVs would be used, but at that voltage they can be really big. \$\endgroup\$ – user115094 Jul 15 '20 at 3:24
  • \$\begingroup\$ For safety reasons you can Earth ground the neutral connection. Use a local grounding rod or tie into a local ground wire of the same gauge (or larger) that feeds your power. Hopefully these power feeds are fused or use breakers, and the AC voltage is not over the Canadian voltage of 600 VAC. The 891 VAC mentioned is an upper limit, beyond which damage will occur. \$\endgroup\$ – user105652 Jul 15 '20 at 3:29
  • \$\begingroup\$ Do NOT ground the internal circuits of the power supply. This would force the positive supply rail to be twice as high above Earth ground. \$\endgroup\$ – user105652 Jul 15 '20 at 3:48

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