The best place to test is at the test buttons on the breakers or receptacles…Always.
Again. where is the hazard?
The OSHA report listed above describes why the 3 light tester should be avoided due to its limitations and inaccuracy. It again repeats what has been said several times. The built-in test button is the only recognized method.
Repeat: ** The built-in test button is the only recognized method.**
Repeat: The built-in test button is the only recognized method.
Also a side note that GFI protection on a construction site also requires open neutral protection.
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Same for AFCI.
I saw the note about the fault on the egc in that report but don’t know why, especially given the short duration of the fault. Someone would have to be in contact with a metallic surface like a junction box with an open ground upstream at the same time someone tested the gfi. This would be different than testinga gfi on a 2 wire system since the connection to the egc would be made, but the path is incomplete.
You should not just test it by the well known 3 light tester and should always test it with the test button. Does that help anyone understand why?
Someone upstream of the receptacle being tested could get shocked.
Unlikely, but it could happen.
I don’t explain why to any of my clients. I just test it at the receptacle button.
I’ve never had a client ask why I don’t test the GFCI with my little three light tester. 
I test the exterior outlets and all bathroom outlets with my little 3 light tester.
If they trip a GFCI somewhere. Great.
If they don’t I write it up.
You can say over and over the test button is the only recognized test.
So what?
I will continue to use my three light tester to verify GFCI protected circuits.
It would be crazy to only trip GFCIs located and then try to determine which down stream outlets are also protected.
Again. What is the hazard?
Do you know what level of current presents a hazard?
I am asking the hard questions so all learn not just you.
It’s not what I say, it’s what the manufacturer says.
If doing it your way works for you and your clients, then great. I will continue to do it the way the manufacturer recommends.
I see no harm in that
Your point?
I understand that and none said you should not.
But practically speaking do you find all GFCI test buttons.
Trip them and then go around checking for dead outlets at all normally protected locations?
If so how long does that take?
It takes 1/2 amp across the human heart to die. Voltage is pressure, amperage is flow. It doesn’t have to be 120VAC to be hazardous. DC amperage can be dangerous as well, if there’s enough voltage (pressure).
Any level is “hazardous”. I hate being shocked.
These questions are not hard for me, but I’m always willing to learn.
I don’t see how someone downstream could get hit.
The graphic is just incidental to my comment.
I use my tester mostly and somtimes the button.
I check every single outlet in the house. It’s beyond the SOP, I know.
I’ve never timed it. I just do it.
wrong!
They would have to be upstream, Roy.
Please explain. 
Splitting hairs. :roll: Hazardous to what? Plants? Paint? A watch?
Any level is hazardous.