Another GFCI question

[bassbone52]

I recently installed some exterior soffit lights. I ran power to a new switch for the lights from a garage circuit protected by an GFCI receptacle upstream from the new switch. I had a length of Romex connected to the load side of the switch. The circuit was energized but the switch was open. I cut the Romex and the GFCI tripped. Why did this happen?

 

[mm08822]

Most likely another load was connected after the gfci recept. When you cut the cable, momentarily you created a path between neutral and ground.

In doing so, that load's neutral current split and some traveled on the ground wire instead of all on the neutral.

Gfci sensed that difference between hot and neutral current and tripped.

 

[rlitman]

...The circuit was energized but the switch was open...

The switch opens the hot wire, but does not open the neutral. GFCI senses the current from neutral to ground. All it takes is 5mA for it to trip.

 

[bassbone52]

Excellent explanation. Thanks to both of you. In fact, there is a garage refrigerator between the GFCI and the new switch.