GFI receptacles and knob and tube

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iand74

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I installed a gfi receptacle on a knob and tube circuit today.

The test button tripped it but my tester button did not. There is not one set of wires so I double checked for polarity and swapped the plug with a different one.

I also tried my tester button with a gfi newer wiring/ with a ground and the tester button worked

I'm guessing the tester button doesn't work w/o a ground but I never noticed this before.

Any thoughts?

Thanks.
 
You are correct -- the plugin GFCI tester will not operate without a grounding conductor. It places a resister from Hot to Grounding when the button is pushed. No grounding conductor no test current. The GFCI device test button bypasses a small current around the detection circuit between the Hot and Neutral conductor which creates a large enough current imbalance to cause it to trip. So a GFCI outlet device can be tested without a grounding conductor using its test button.
 
I have always been told that the test button is the approved test. I use my tester to indicate a potential and advise that the receptacle might need to be checked if my tester does not trip. I have never refused one that tripped with the intergral test button.
 
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