Originally Posted By: nap
Be careful, if that modified shredder ever causes a fire, and the inspectors figure out it was modified, you may be in for some home insurance surprise.
So I've been told, many times.
Problem is, I can't find a single documented case where a non-UL listed device led to an insurance problem (which is a much worse case than a modified UL-listed device, which describes the shredder in the example). Nor does the US NEC require UL-listed devices exclusively. Nor can I find a clause in my insurance policy that specifies only approved, unmodified electrical appliances can be connected to the household AC outlets. Note that the UL is not a government regulatory agency.
Until I do, I consider it an urban myth.
Back to the OP's problem.
The GFCI receptacle, which if the first receptacle on a line, and is properly wired (you can screw this up) should protect all downstream outlets, remember, looks for Ground Faults.
The most common Ground Fault (and one that will trip a breaker or blow a fuse at the panel, no GFCI required) is when the hot (black) AC wire contacts either the ground wire or a grounded item ... outlet, cover, or internally in an appliance.
GFCI devices take that quite a bit further. They will of course trip under the above conditions. However they will also trip when there is resistance on the ground line, perhaps due to corrosion or a loose ground connection. They should trip at 20~30 mA (which is not very much current differential) and depending on the duration, can trip when detecting as little as 6 mA differential current.
By "differential" we are referring to how GFCIs work ... they measure what should be on the line and what actually is on the line, and if the difference is above a certain threshold, they trip.
You can easily check the operation of a GFCI by putting an incandescent lamp (turned on) on a downward outlet and pressing the "test" button. The lamp should go out. If it does, the GFCI unit is working as it should, and GFCI unit itself is not the problem.