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.