Electrical – Why does this night light cause the GFI to trip

TL;DR: the dimmers aren't switching off completely: they're allowing some current to leak through, which is why you're seeing a voltage across the CFL. A different make of bulb may behave better with the leakage current that you're getting. Or perhaps a different brand of fan (if you haven't installed them all already).

I do know that operating CFLs in those sort of conditions will shorten their lives considerably, so you might actually be cheaper for you to use incandescents instead (a quick calculation says about 12 kWh per year for a 60 W bulb).

Read on for the technical explanation...

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This is a circuit diagram of the innards of your fans:

The voltage across the bulbs, V_b is determined by the formula:

V_b = V_in * R_bulb / (R_dimmer + R_bulb)

where:

• V_in is the mains voltage (120Vac or 240Vac depending on country).
• R_bulb is the resistance across the bulb or bulbs.
• R_dimmer is the resistance across the dimmer.

The dimmer is a solid-state electronic circuit, so it has a very high effective resistance -- 10s of megohms is not unreasonable. Ditto for the control circuitry in the CFL. An incandescent bulb is a simple piece of resistive wire; a 60 W / 120 V bulb will have a resistance of 240 ohms.

Now, suppose the dimmer has a resistance of 50 MOhms and the CFL has a resistance of 10 MOhms; plugging the numbers into the equation above gives you 20 V across the bulb. OTOH, the voltage across a 60 W incandescent bulb will be about 600 microVolts, nowhere near enough to make the bulb glow.

If you have two bulbs in the light fixture, the resistance, R, of the two in parallel is given by:

R = R₁*R₂/(R₁ + R₂)

So if you have a CFL and an incandescent installed, the effective resistance is going to be very close to that of the incandescent alone:

R = 10,000,000 * 240 / (10,000,000 + 240) = 239.99 Ohms

Again, not enough to turn on either bulb.

With two incandescent bulbs, the effective resistance is half that of a single incandescent, so you have half the voltage across them.

The flickering you see with two CFLs is because the light you see is basically a high-voltage spark through the tube. The CFL contains circuitry to amplify the incoming voltage up to the point where the spark can occur. Under normal circumstances, the input voltage is enough to cause this spark 100 or 120 times per second (depending on mains frequency), which is far too frequent for the human eye to notice. With the reduced input voltage, it takes longer to reach the required voltage, so you notice the flicker. No two bulbs will be exactly identical, so they'll flicker at different rates and take different times to recover between discharges.

Not an uncommon problem. I have had a lot of problems with GFI's the last few years myself. There are a few likely causes:

1. A fault still exists and will not allow the GFI to reset.
2. The GFI tripped due to an overload and the differential circuit was damaged, thus the outlet is now toast. This seems to be a common complaint with standard 15 amp GFIC's. they don't take current spikes well. In your case 1500 watts on 120VAC is pulling around 12.5 amps, which is getting close to max capacity. Sometimes the initial current draw is much higher as the resistive load needs to heat up a bit before it reaches its operating impedance. A "cold" load can offer a much lower impedence and cause a short duration, but large spike of current to flow at turn on. Very common on motor loads, but not uncommon with filament heating or lighting.
3. Sometimes if you turn off the power to this GFI at the breaker, then push the test and reset buttons, then turn the power back on again, the GFI will default to the tripped condition, but will reset.
4. Disconnect any wires connected to the "Load" side of the GFI. Unplug all loads from the GFI itself, and give it a try.
5. If none of the above items work for you, you will probably need to replace the GFI. Find one with a 20 amp rating instead of 15 amps, assuming that 15 amp is what you have now.