I was installing a GFI in an old home where there was an ungrounded metal box. But, I discovered that the box actually touched a metal vent pipe that ran through the wall space. I was curious to see if this would act as an “unintentional” ground.
I tested the new GFI receptacle with a GFI test plug, and the lights indicated a ground, and the proper feed of a hot and a neutral. But even so, the GFI will not trip when I depress the test button on the GFI test plug.
I’m just trying to understand this. I realize that this receptacle needs to be rewired with a ground to the panel, or to adjacent grounded box.
As installed, the GFI will trip when depressing the test button on the GFI itself, but not when depressing the test button on the test plug. Even though the test indicator shows that all is well. What gives?
Thanks, as always.
DIA
Replies
GFI testers are NOT a valid test of a working (or non-working) GFCI. According to OSHA, "GFCI testers which are not an integral part of the GFCI shall not be used a as means of determining compliance..because such testers often do not produce accurate results. Only the circuit testing which is manufactured as part of the GFCI shall be used to determine whether or not the device will function as intended." (Technical Note 91-1, How to Test the Operation of a GFCI)
The NE-2 neon bulbs that are used in those testers require very little current to light them. In fact I can take one of the two led testers and put one lead on the hot and old the other one in my hand and it will light.
Not as full brightness as when it is across hot/netural, but it does light.
So the connection is probably high enough resistance that will allow the light to turn on.
But with the high resistance the test button will not draw enough current from hot to ground to trip the GFCI (5 ma).
Bill,
Thanks for the input. But why has it been advised on the FHB site before to replace ungrounded 2 prong receptacle (say knob & tube wiring) with a GFI for safety reasons? How can a GFI afford safety if it isn't grounded? Can it still trip?
I'm puzzled?!
Thanks,
DIA
Yes it can still trip.The GFCI has a sense coil through which both the neutral and hot leads feed through. It is arranged so that the currents cancel. So where the load is 1 amp or 15 amps if the hot and neutral are the same the output of the sense coil is zero. If it is different by more than 5 ma it trips.There so if you are between the hot lead and some grounded object then leakage current can flow and the GFCI will trip to protect you.In fact in most cases where you need protection you won't be in contact with the EGC (ground wire) for that circuit. But rather some other ground such as kitchen or vanity sink, the earth when outside, etc.