Update: I consulted with other electrical engineers. Here is a plausible mechanism. Again refer to the circuit diagram of this GFCI:
Look at the board layout. There is a long trace from where C3 is located to the SCR gate. It is suspected that transient capacitive coupling (within the circuit and not from outside) to the SCR gate is the cause.
Suggestion is to try putting C3 directly at the SCR gate, and a 100 ohm carbon comp resistor in place of the trace. This creates a low pass filter at the gate input and also reduces the capacitive coupling to the gate.
The inductive kick at either turn on or off may be the source of the transient.
The series resistor ln combination with the shunt capacitor to the gate is to reduce transient voltage at the gate.
Does anyone own any Leviton GFCIs (those models without any automatic monitoring self test)? I found out the shaded pole motors from Waterpik can constantly trip on them (50% of the time), but not other brands of GFCI (which I just tried). Someone in the following url has similar experiences with his Leviton GFCI and even a second new Leviton he bought.
So it may not be problems with the normal working of a GFCI but particular implementation of the Leviton GFCI. Anyone got a clue what is that mechanism that can cause shaded pole motors to trip on the Leviton GFCI?
The youtube video describes the shaded pole motor in the Waterpik in details. https://www.youtube.com/watch?v=PmNcRsxSovs
To recap. Even if the plug is 2-prong (without any grounding and it's put on glass surface). The Waterpik motor can trip any Leviton 5mA GFCI (I tested 4 Leviton GFCI outlets) even when isolated and not wet (even if rotor removed). The trips occur either during startup or turning off. Not when it's already on.
In other motor/GFCI combination. What is the reasons why it could trip during startup or turning off. It's not wiring problem. There are many reports of this everywhere. Have you experienced or read about it? What is the mechanism for the constant tripping? (supposed it's not wiring problem which I'm sure it is not in the waterpik motor and Leviton?
You may have heard of new refrigerators tripping on new GFCI. It could if the GFCI is a Leviton brand.. which may not be compatible with refrigerators. I have new refrigerator that also trips every few hours on the Leviton GFCIs. I'm testing it with other GFCI now. Please share your experiences about this too.