GFCI trips only when a bulb is in the fixture

[lep]

An interesting problem developed (in this new, 3 year old garage), that I need to fix while I am doing new wiring elsewhere.

On the stairway going upstairs, there is a 3-way light switch at the bottom and a 3-way light switch at the top. Whenever the light is turned on and a 60W bulb is in the fixture (on the ceiling joist) the GFCI of an outlet downstairs trips. If I reset the GFCI, remove the bulb, and turn it on the light, the GFCI does not trip.

To debug what is going on, I removed the bulb from the fixture. Using the switch at the top of the stairs, I applied a voltage meter to the common and neutral going directly to the fixture and it showed 120V. Thus, the problem seems to be caused by the cheap "new construction" blue plastic fixture that holds the lightbulb(?). (also, I used several new bulbs -- so it's not the bulb).

Question is, could it indeed be the fixture causing the short? I wonder if even though 120V goes to the bulb, is the resistance or current off for the GFCI? I have an ohmmeter that I have not yet used, but first things first -- why is the GFCI tripping only when a bulb is in the fixture?

[Speedy Petey]

First of all, you do NOT have a short. A short circuit trips a breaker. You have a ground fault. These trip GFIs.

The reason it trips with a lamp in place is that there is no current flow without the lamp in place. Once the GFI senses current it will trip if there is an imbalance.

Check all your wiring and connections on this circuit.

[lep]

Thanks for the tip -- will check the wiring.

[HayZee518]

Why are you using a gfci on a lighting circuit?

[suprheat]

speedy is correct...a short trips the breaker. someone obviously used the ground wire for a neutral. You will read normal voltage this way but the GFI is doing what it's made to do.