MOVs between L-E and N-E

[japlytic]

In some UPS units (as well as surge protectors), I have seen MOVs between Live and Earth, and between Neutral and Earth, not to mention the one between Live and Neutral. Will MOVs between Live and Earth, and between Neutral and Earth provide extra protection?

[linuxguru]

Re: MOVs between L-E and N-E

They may provide additional protection, but are you sure they're not the usual EMI/RFI filter ceramic discs? They look very similar to MOVs, often with the same blue or yellow epoxy coating.

[japlytic]

Re: MOVs between L-E and N-E

A MOV is typically designated on a PCB by MOV, ZNR, TNR or Z. There were 3 such designations on a UPS PCB, which corresponded to MOVs.

[larrymoencurly]

Re: MOVs between L-E and N-E

Apparently there are more surges between L-E and N-E than between L-N, so I guess that the extra MOVs help.

[AK0R]

Re: MOVs between L-E and N-E

In American 3-wire installations (L-N-E), the Neutral and Earth are connected at the breaker box. However, the amount of current that flows in the hot wire (Line) also flows in the return wire (Neutral). Depending on the amount of current flow, the distance to the breaker box, and the wire gage, the Neutral can actually be a couple of volts above ground (Earth) due to voltage drop (I*R). No current should flow in the ground (Earth) lead; if it does, it means that the Neutral is open, or the Neutral and ground are shorted at the device. Also, the length of wiring between the breaker box and the outlet acts as an antenna, and is particularly susceptible to induced voltage (i.e., lightning strikes nearby). Therefore, the L-E MOV functions as a backup to the L-N MOV, and the N-E MOV provides the same protection as the L-E MOV on the neutral line. For the same reason, there are shunt capacitors on both L-E and N-E, so that any stray/induced voltages are removed from both lines, and the power supply line is balanced.

Are European installations similar (willawake, Rainbow, cods69)?

[Rainbow]

Re: MOVs between L-E and N-E

See http://fr.wikipedia.org/wiki/Neutre_...ectricit%C3%A9)
(you can ignore the text (as I did because I don't know french), just see the pictures)
Here in Slovakia, TN-C and TN-S are most widely used. TN-S is used in most modern buildings, TN-C in older.

[japlytic]

Re: MOVs between L-E and N-E

With extreme care, voltage measurements between Live/Neutral and Live/Earth should be almost the same. Measuring between Neutral and Earth should measure no more than 0.5V (depending on how big the building is), because Neutral and Earth are tied together at the switchboard before any RCD/GFCI/ELCB (safety switch) device. If there is a significant voltage between Neutral and Earth, call a licensed electrician as soon as possible to remedy this fault.

I think one of the reasons why MOVs are between Live and Earth, as well as between Neutral and Earth because there may be buildings with the earth wire not tied to the neutral wire in the switchboard.