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Varistors in three phase application
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Red_Micro:
Usually in three phase applications I see varistors connected between phase to phase, or between phase to ground. In the picture below from a power supply reference design, the common node of the varistors is not connected anywhere. I would expect it to be ground, but they intentionally left the common point of the varistors floating. How are varistors protecting in this case?
trobbins:
Look at it from the perspective of a phase-to-phase, and a phase-to-neutral overvoltage condition. A MOV will only start to conduct once the instantaneous voltage across it exceeds the 1mA Vdc threshold (which has a significant tolerance). So if a phase-to-phase voltage spikes above circa 1.2kV then the connected MOV's will start to conduct to suppress a further rise in voltage. Similarly for a phase-neutral voltage spike. It would likely be rare for more than one conduction path to occur - in which case the voltage of the floating star node would be forced to change somewhat.
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