As you probably know, a low voltage lighting transformer converts 120V to 12V (on the secondary side). A faraday shield isolates the 12V side from the 120V side. Secondary lighting cables are split with one leg attached to a common tap while the other leg is attached to a voltage tap.

My questions are about grounding:

1. Is the 12V common tap connected to the same ground as the 120-volt receptacle (a GFCI)?

2. If one contact of the lighting fixture socket is grounded to the mounting surface (such as a metal gutter) that is also grounded (through a grounding wire), is it important that the common leg of the fixture wire be the leg that contacts the gutter?

The reason I ask is because an electrician attached a fixture to such a gutter and he read 12V across the gutter and it tripped the GFCI. When he reversed the fixture leads, it solved the problem.

Thanks.

 

The answer to both 1 and 2 is "yes."

Sometimes, it is difficult to tell which wire is which simply by looking.

Also, "fixtures" are the wet & drippy things the plumbers install. We electricians put in "luminaires."