Regular GFI and downstream duplex recept in bathroom.

Christmas candle - 7 1/2 W - in the window - ungrounded plug type with in-line switch plugged into the GFI.

Wife has hair dryer with GFI cord termination plugged into the duplex.

Turning on or off the candle with the inline switch trips the GFI on the hair dryer - but never the wall mounted one. Push the reset button, and all's well.

I don't understand..... Explanation?

Grov

Hair Dryer GFI also detects Arc Faults?

I've measured some receptacle GFCI's tripping at 6mA and others at 4.2mA, which tend to nuisance trip more with motor loads and magnetic-ballasted lights. Inrush current is a common factor between such induction loads, and 1800W+ hair dryers.

Since, voltage drop increases with amperage, with high inrush current the GFCI may momentary detect more I²R losses across the (Hot with switch leg) than neutral without switch-leg, due to different impedance.

GFCI's may not always mix well (nuisance trip) with switch legs and high inrush currents. If the GFCI can't be replaced, try feeding the light (switch-leg wiring) from line side of GFCI.

Does anyone know if Residual Current Devices (RCD) and GFCI's trip at similar mA settings, or what the different standards are?

NO

Thanks for comment.

Let's see if I can draw a wiring diagram....

wall mounted GFCI---------duplex---GFCI cord terminated hair dryer
|
|--- in-line switch -----7 1/2w lamp

Well, almost drawn correctly. The cord terminated GFCI and the cord with the in-line switch are both plugged into the duplex - so the cord to the small lamp is "upstream" from the hair dryer GFCI.

The GFCI device and the duplex recept are in the same wall case.

I had thought about inrush and inductive load, but in this scenario, the hair dryer is off, and switching on or off the small lamp trips the GFCI on the end of the hair dryer cord.

Grov

This is a very interesting problem! Does this happen if, while switched on, you unplug the candle and plug it back in? Or if you unscrew the light bulb? Does it happen for any other cord-and-plug loads, or just this candle?

With the hair dryer switched off, there should be no current possible at all through the CT in the GFCI. I suspect high-frequency arcing in the switch might be triggering some of the circuitry in the GFCI. If this is the case, it would only happen when you turn the switch, and not via any other way of turning the light on/off.

The hair dryer plug is usually not really a GFCI, it is an immersion detector. There is a ring in the barrel of the blower exhaust that is connected via the 3d wire to the plug and it looks for current on that wire. (at least in the ones I have taken apart).
I have no clue as to why a 7w lamp would affect that. Do you have another dryer you can try in the same outlet?

The light switch is on the cord. Then, arcing noise or device leakage also makes sense to me, if its repeatable in other duplexes (rooms).

If its not repeatable elsewhere, could loading a bootleg or mixed neutral/ground wire on bath duplex cause the hair dryer's safety to detect a change in potential?