GFCI Grounding Issue

Question

I'm trying to replace a two prong outlet in my garage with a GFCI.

The receptacle line is fed from a light switch on the wall, and the load runs to an outdoor(one bulb) lamp post. The general procedure is described here except I'm replacing a two prong outlet and there is only one outgoing wire(to the lamp post). Replacing receptacle with GFCI outlet which will be tapped for outdoor pole lantern

The incoming and outgoing wires are both three wires(black, white, uninsulated(bare copper) ground).

I'm feeding the load through the gfci rather than pigtailing. However, when I connect the incoming and outgoing ground to the gfci and flip the breaker the gfci trips. When I disconnect the ground the gfci lights up properly. If I connect the incoming ground only it also lights up.

Any ideas on this? Keep in mind the old outlet was not a three prong(I can't recall the exact way the ground were wired previously) so it had no grounding for anything that was connected. I read through all related topics and no one describes the exact scenario.

Answer 1

Sounds like you have a ground fault. Remove the bulb and disconnect the wiring from the GFCI. Measure the resistance between all of the wires. It should be infinite (open circuit). Any other value indicates a fault between the wires.

Answer 2

You have installed a GFCI downline of a switch. That's a problem for a lot of GFCIs.

Even worse, if your GFCI+receptacle device is in the same junction box as the switch, it's very easy to make a particular dumb mistake that'll cause this everytime. The way you avoid that mistake is you unhook everything in the box, untie all wirenut bundles for instance, identify the 2 wires coming out of 1 Romex cable that are the supply from the panel, attach those to the LINE terminals, and tape over the LINE terminals so nothing else can be added to them. Then everything else gets hung off the LOAD terminals. This forces you not to make the mistake.

Then if you get a GFCI trip, you have to divide and conquer. Go to the very next thing and disconnect its supply side. See if the GFCI still trips. If it doesn't, reconnect its supply side and remove its downline side. Eventually you will find that adding one thing (device or wire) to the circuit trips the GFCI. That thing has a ground fault.