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Old 01-26-2010, 09:58 PM   #1
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gfci outlet testing question


I installed a couple of gfci's tonight. Each one was installed as the first recepticle on a run of several outlets. One tests fine, the other doesn't. Both test buttons work fine and both trip when using a three prong tester, but when testing outlets down the line, one trips normally using the trip function on the three prog tester and the other one does not. On the one that does not, the gfci does not trip and although the tester shows correct wiring when plugged in, while the trip button is pressed, it indicated hot/neurtal reversed. I've made sure both the outlet and the gfci are wirined correctly (hot and neutral on correct side and line/load correct). They are different brand gfci's but i would still expect the same behaviour. The one that does not trip down the line is a new $13 leviton from lowes.
thanks, JC

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Old 01-26-2010, 10:10 PM   #2
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gfci outlet testing question


I imagine you got the line and load terminals reversed on the GFCI device. My bet is that your downstream receptacles are wired off the line side instead of the load side. Disregard the way your tester lights when you push the button if the GFCI doesn't trip...Mine does the same thing. It is wired wrong or the GFCI device is bad (it does happen).

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Old 01-27-2010, 03:20 PM   #3
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gfci outlet testing question


...OR the outlet that is being tested with the GFCI tester is not grounded.

You see, a GFCI does not need a ground to work properly, nor does the device's test button need a ground to perform the test. However, a three-prong tester DOES require that a ground be present or used externally in order to trip the GFCI device it is plugged into (it needs a ground to leak current "to" so the GFCI device circuitry detects the imbalance).
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Old 01-27-2010, 04:18 PM   #4
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gfci outlet testing question


You got it, the ground wire going to the second recepticle has a break or bad connection. I figured it out with a continuity tester and when I attahced a bypass wire to act as ground, the tester worked. So The second outlet is still protected without the ground? (no equipment ground but GFCI protecteted?)?????? thanks in advance.
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Old 01-27-2010, 09:13 PM   #5
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Originally Posted by 205bhamjim View Post
So The second outlet is still protected without the ground? (no equipment ground but GFCI protecteted?)?????? thanks in advance.
It is still protected for a ground fault.

For simplicity sake, lets view a receptacle as having current:
> entering the receptacle device on the hot wire
> going to the contact points (the "holes" where your plug contacts to get the electricity)
> out through your plug's hot wire to your lightbulb
> back through your plug's neutral wire into the contact points of the receptacle
> leaving the receptacle via the neutral wire to return to the point of supply.

The GFCI circuitry monitors the current entering from the supply on the hot wire versus leaving on the neutral wire to return to the supply - they should be exactly the same (this is why you must supply the receptacle using the LINE SIDE terminals). If the current is not balanced, then some amount of current went "somewhere else" unexpectedly, usually the result of a fault to ground. This value is between 4-6mA for a GFCI device to trip.

Now the tests:
1) The test button on the unit creates an imbalance by shorting a tiny amount around one part of the measuring circuitry, simulating a "missing" amount of current, a.k.a. a ground fault. It doesn't do this by shorting/leaking this tiny bit to ground, but instead it sends it around one side's measurement circuitry (probably goes around the neutral, but I'm not entirely sure). This requires only the hot and neutral wires to be present.

2) Your three-prong GFCI tester can't duplicate the same path as the device test button because it does not have access to the internal circuitry. Instead, your tester uses it's ground pin to leak a tiny amount to the receptacle's ground conductor. If the receptacle is grounded, the current will flow and trip the GFCI because it has created an imbalance between the current entering via the hot vs. leaving via the neutral, and if the circuitry detected it, it will trip.

3) If your receptacle isn't grounded, your tester cannot leak any current when you press the button in order to trip the GFCI. In this case, you must use a tester or another setup with the capability of connecting to some other suitable ground point, such as a nearby grounded device.
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Last edited by williswires; 01-27-2010 at 09:23 PM.
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