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[MitchellShnier]

I have an XPCR active coupler on a standard residential two-phase fuse panel (so the third phase connection is not used), and during trouble-shooting a problem, I (assume I) disconnected only the neutral before the phase power -- and after a few seconds there was a loud POP from the box. Upon disconnecting and opening the XPCR, I found that both transient suppressors (part number 07D221K -- they're blue, and look like 7 mm diameter flat disk capacitors) had exploded and also blown the fuse wire (really thin wire intended to be an inexpensive fuse).

Perhaps what happened is that by disconnecting the neutral, the "center voltage" (the part of the circuit that would normally be connected to neutral) drifted so that one side had the full 230 volts, and the other had 0 volts. These transient suppressors are for 220v (that's what the "221" of the part number means) maximum, so they started conducting, which blew the fuse wire and both MOVs.

Fortunately, this is exactly what is supposed to happen -- they protected the rest of the circuit. The MOVs only cost a $1 each, and I used one strand of some stranded hook-up wire to replace the blown fuse wiress, and the XPCR is back to working properly (it turned out that the problem was "Interfering Power Transformer", at http://www.x10community.com/forums/index.php?topic=16998.0).

So, I now have a note beside the XPCR in my fuse box: NEUTRAL MUST BE CONNECTED IF POWER CONNECTED.

[Brian H]

Thank you for sharing your findings.
I believe the XPCR installation sheet indicates that both lines must be turned on at the same time.
I agree you probably had a higher voltage on one line; as the load was unbalanced when the neutral was floated.

[JeffVolp]

Without a neutral connection to the XPCR, the internal "ground" will move up and down, depending on current flow.  The transformerless power supply normally charges on one half of the cycle.  Without a neutral reference, the internal ground would be pulled almost to the peak voltage while that capacitor charges.  That would exceed the breakdown voltage on the opposite phase MOV, causing it to overheat and fail.  The failure mode on a MOV is normally a low resistance so it will continue to protect its related circuitry.  Once the first MOV went to low resistance, most of the 240V appeared across the second MOV, causing it to overheat and fail.  The combined low resistance of both MOVs in series across the 240V then caused the fuse to pop.

A similar issue can occur if the neutral is left open on the XTB-IIR because that unit is powered off one phase so it can function on just a 120V circuit.  In spite of the warning on the instruction sheet, this has occurred in about 1% of the installations.  While changing the opposite MOV to 270V would prevent the problem, I am hesitant to do that because it reduces protection against line transients.

Jeff