3 prong outlet tester

Status
Not open for further replies.
Dsg319

Dsg319

Senior Member
Location
West Virginia
Occupation
Wv Master “lectrician”
While I very seldom use one to test any outlets, polarity, etc. I know they cannot detect “bootleg ground”. At least the standard run of the mill ones.

Another thing I have found to be a problem is them to show having a effective ground, when in reality the actual egc (in this case beat up loose emt conduit) has a overall resistance of around 20ohms from EGC to neutral. We obviously know this will never clear a fault on a 20ampere circuit unless GFCI protected

Has anyone else ever noticed this?
 
Yes I have. We had some benches at work with integral power receptacles. The EGC path was poor in these, and someone asked if using those 3 light testers would find the bad ones instead of using a bonding meter. I said no, those lights can light up with a very poor path.

I use them when buying a new house to quickly check for miswired circuits and missing grounds. I do open a few to make sure they didn't hijack the neutral for ground. I also use them when replacing a receptacle to make sure it is dead and to check my work when done to make sure everything is there at least superficially and that I turned it back on.
 
Dsg319 said:
Another thing I have found to be a problem is them to show having a effective ground, when in reality the actual egc (in this case beat up loose emt conduit) has a overall resistance of around 20ohms from EGC to neutral.
Click to expand...
An outlet tester is just a "quick and dirty" device for showing common faults with wiring and receptacles. Like not being able to show bootleg grounds, it has no provision for measuring ground resistance because those are beyond the scope of an inexpensive product like that.

-Hal
 
ActionDave said:
I don't think it's so obvious. There's a big difference between fault current and checking Ohms with digital meter.
Click to expand...
I understand where you are coming from with an ac circuit with impedance rather than just dc resistance measured with a meter.

But shouldn’t the reading still be relatively close?

Also tested the theory with a direct ground fault at the outlet, you could weld with it but a big negative on clearing the fault, all while the outlet tester still showed an EGC.
 
Dsg319 said:
I understand where you are coming from with an ac circuit with impedance rather than just dc resistance measured with a meter.
Click to expand...

I don't understand where he is coming from. If you measure 20 ohms with a meter that's what it is. Nothing to do with impedance. 20 ohms likely will not clear a fault, period. But there are problems with that method. Keep in mind that simply measuring the resistance between the neutral and ground at a receptacle isn't the right way to determine ground condition, particularly if the circuit is energized.

So AGAIN, an outlet tester is just a cheap device to give a go/no go indication of a receptacle's condition. You can't expect it to do complex testing.

-Hal
 
I understand it’s to just show a quick go/no go of receptacle condition.

But am I correct in saying it falsely shows an egc, just because there’s a poor path (high resistance) back to the service neutral to illuminate the small light in the tester.

If measuring resistance from egc to neutral isn’t the right way to determine the total loop resistance of the effective ground fault path, than what is? (Wanting to learn)

Understanding it can change with conditions of appliances connected with parallel paths back to the source.
 
Last edited:
Dsg319 said:
If measuring resistance from egc to neutral isn’t the right way to determine the total loop resistance of the effective ground fault path, than what is? (Wanting to learn)
Click to expand...

The probes of your meter need to be attached to each end of what you want to measure. In this example, one to the ground terminal of the receptacle and the other to the ground bar in the panel.

In your example, when you measure 20 ohms N-G, you really don’t know if the problem is in the EGC or the neutral wire (or a combination of both). Although if you had that much resistance in the neutral connection, you’d have issues trying to power anything substantial from that receptacle.
 
retirede said:
The probes of your meter need to be attached to each end of what you want to measure. In this example, one to the ground terminal of the receptacle and the other to the ground bar in the panel.

In your example, when you measure 20 ohms N-G, you really don’t know if the problem is in the EGC or the neutral wire (or a combination of both). Although if you had that much resistance in the neutral connection, you’d have issues trying to power anything substantial from that receptacle.
Click to expand...
Extactly. I am basically just using the neutral as an extended test lead. If we’re much of that resistance in the neutral that would be in series with the receptacle load and the appliance would not operate correctly likely due to lower than needed voltage.
 
Dsg319 said:
Extactly. I am basically just using the neutral as an extended test lead. If we’re much of that resistance in the neutral that would be in series with the receptacle load and the appliance would not operate correctly likely due to lower than needed voltage.
Click to expand...

I guess my point is that if you measure zero G-N, you’re good. If it shows any significant resistance, further investigation is necessary.
 
Dsg319 said:
I am basically just using the neutral as an extended test lead.
Click to expand...
And not a good one. Consider a potential on the ground or a voltage drop on the neutral conductor, both of which can be present even with the circuit under test off. Any voltage presented to the test leads will result in a false resistance reading, even zero.

You also don't know which might have the abnormal resistance, the neutral conductor or the ground. Could be both as far as you could tell.

Dsg319 said:
If measuring resistance from egc to neutral isn’t the right way to determine the total loop resistance of the effective ground fault path, than what is? (Wanting to learn)
Click to expand...

One correct way is to use a known reference such as a length of wire long enough to go from one end to the other that in essence lengthens one of your test leads. We've talked about using a long extension cord here before for finding receptacle problems. Plug it into the receptacle in question, extend it to the panel and measure the resistance between the ground bar and ground on the extension cord.

Another possible way is to actually draw current through the ground from the hot at the receptacle and measure the voltage drop. Dunno if that's a good idea though because if the ground path is OK, no problem. But I could see arcing happening at the points in question.

-Hal
 
Status
Not open for further replies.
Top