Three-light tester indications are affected by downstream loads

Many of you may be aware of this, but I just thought I would share this as it may be beneficial to some. For those who rely totally on 3-light receptacle testers, you may want to think about doing something more because these testers are far from accurate and comprehensive. I have been doing a lot of tests on them, and am continually surprised at some of the things that I am learning about them. I just thought I would share my latest blog about these testers.


Good info and blog, Mike.

Good information, any idea what is going on with these testers to give a false reading?

They’re not actually giving false readings, it’s just that they work on a simple principle. They measure the voltage between the three different slots in an outlet. if you have 120 V or close to it between two different slots, then that particular light will be on. In my video, a washing machine is plugged in. Although the washing machine was not turned on, the motors and other components in the washing machine can affect the voltage between two different slots in the outlet where the washing machine is plugged in. These three-light testers are very simple devices, and can only show you if there is a 120 V differential between two slots of the outlet. They cannot analyze circuits like something more complex such as the SureTest circuit analyzer. You really have to be careful how you interpret the readings. I also have a couple of other blog posts that show some other misleading indications that these testers have given me.

Three light outlet testers are not diagnostic devices. The results you obtained required a mis-wired outlet. In the case of a properly wired outlet it doesn’t matter what load occurs downstream.

After thinking about it for a while, I suspect both readings are correct! Three light outlet testers can only identify one problem at at a time. We know that outlet is miss-wired. In the case of a hot-neutral reversed with an open ground, The open ground will take precedence and only the center light will light. This is exactly what occurred when you unplugged the washer. When you plugged the washer in it provided a weak ground and the hot- neutral reversal showed up. The washer is a unique appliance in this case because it is connected to the cold water pipe, which is grounded.


IMO both readings cannot be correct, the miswiring is the condition of the receptacle/circuit with nothing plugged in that is what needs to be repaired.

In order for the tester to display a Hot/Neutral reversed (the red light and the center light) there must be a ground for the tester. Both outside bulbs on the tester require a ground connection. Yet when the washer is plugged in (not operating) the tester lights not only the center bulb (connected to the hot and neutral blades of the outlet) but also the red light which is connected between the neutral and ground. The red light never lights unless it has a hot connection and a ground! Plugging in the washer supplied the ground. You are wrong and have no other explanation.

What good is using a tester when an appliance plugged into a receptacle will change what the actual problem that the tester is showing? Do you report the defect with the appliance plugged in or without the appliance plugged in? The problem cannot be both.

The problem is that a dedicated circuit should be provided for the washer… :wink:

Not just an appliance, and not a downstream load as the OP stated, but an appliance that creates a ground path at the outlet its plugged into.

Reporting the defect is merely a matter of stating the outlet is misswired, we’ve already determined that the tester is not a diagnostic tool and only reports one problem at a time.

Do you mean to insult our intelligence by saying that an outlet can only be wired to have one problem at a time. (i.e. open ground and hot / neutral reversed)

Where were you when the three light tester was discussed in another thread? I am merely responding to the unique situation presented by the OP. You have yet to explain how, if in your opinion the outlet has an open ground, it displays a red (outside) light when the washer is plugged into the same outlet.

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Bob let’s not go off on a tangent, I don’t care what the 3-light tester is displaying that’s not the point. If the outlet is to be reported as a defect and that defect is spelled out in the report (i.e.-open ground or whatever) then what good is testing it with an appliance plugged in that will give a reading that is not the actual problem? If you’re going to say in your report “receptacle in laundry room is miswired” and let the electrician figure it out what that means well then it doesn’t matter if the plugged in appliance actual changes the defect.

My point is if you’re going to report the defect then it should be correct not skewed by an appliance being plugged into the receptacle.

So if the washer was plugged in at the time of testing you would have to report it as hot / neutral reversed because that is what the tester showed. If on the other hand the washer was not plugged in at the time of testing you could report it as an open ground. In either case you would only be reporting half the defect that existed at that outlet.

If you didn’t click on the OP’s link and watch the video you are missing the entire point, no tangent. In your first post you stated that both readings could not be correct. Yet I have shown that they can be. Same outlet, one reading with the washer plugged in and one with the washer unplugged. Both will have to be corrected to have a properly wired outlet.

Yes I did watch the video and Mike did a good job of pointing out this weakness with the simple 3-light tester. But I’m confused, with no appliance plugged in the tester gives one indication open ground, are you saying that it is incorrect and that you need to plug in the appliance to “find” the other defect?

I’m saying the tester can only identify one problem at a time. With no ground the tester can only light the center no ground bulb which is connected across the hot and neutral terminals. It doesn’t care which one is hot and which is neutral.

With the washer plugged in a ground appears and the red light comes on. The red light is connected to the ground terminal and the neutral terminal. So what do you suppose has to occur for this to light. No one has re-wired the outlet, the only difference is the washer is plugged in supplying a feeble ground through the water pipe. Yes, the neutral slot has to be wired to hot for the red light to come on.

Two separate problems with the same outlet revealed one at a time by unintentionally creating a feeble ground with the washer.

Not, I might add, a typical situation, and not one caused by a “downstream load.” Remember the washer is not running so there is no load.

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Thanks for the explanation, I missed the part (the other post in his blog) where Mike set up a test rig and physically removed the EGC from the receptacle to simulate what was depicted in the video. We’re all good. :slightly_smiling_face:

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This is the first time I have logged on in several weeks. I enjoyed reading the discussion about this. My whole point is simply that we cannot take at face value what the three-light tester is telling us. The same effect likely would have occurred had this washing machine been plugged into another outlet on the circuit. We have no control over what things are plugged in downstream or upstream of the outlets that we are testing, and all of these things can affect what we are being told by these 3-light testers. As has been said above, they can tell us that there is a problem, but they often cannot accurately tell us what the problem is.

High input impedance.

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Had 3lighter show good ground… measured H+G and got 89 volts only. How would that work out in case of a ground fault :slight_smile:

Would a SureTest also have the same issue of different readings based on downstream appliances?

I’ll have to do some experiments with my SureTest to see. I’m sure that anything that affects the voltage and impedance of the outlet being tested will affect a SureTest’s readings.