How Three-Light Testers Work

For those who are new to inspecting or don’t understand the limitations of three-light testers, here is an article that shows how they work and their limitations. It shows why you may need more than just a three-light tester to properly inspect electrical receptacles.


If one were to decide to upgrade to something better, what would you recommend in lieu of a 3-light tester Mike?

Any personal recommendations?

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None of the “slots” are at 0 volts unless they are disconnected.

“It has no way of determining which of the two slots is hot, and which is at zero volts.”

We’ve covered this many times. If an outlet is miss-wired, that’s what you report. There is no reason to diagnose the problem. That’s for the person who will repair it to do.

A Wiggy (solenoid tester)..

Thank you sir, I will definitely get one of these and see how many more issues I find.

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I like the SureTest circuit analyzer model 61-164. It’s expensive but works great.

I disagree. The neutral and ground are at zero volts or close enough to zero to consider them at zero.

Unfortunately, if you are inspecting an older house, all of the receptacles are often not grounded. It is not often that the homeowner is going to rewire the house for the buyer; therefore if an inspector understands that his 3-light tester will not detect reversed polarity (for example) if the outlet is ungrounded, he can use other tools such as a voltage “sniffer” to check these receptacles for other issues such as reversed polarity. Issues which are more likely to be corrected before the home is sold.

The fact that neutral is bonded to earth ground doesn’t place it at 0 volts. The house power supply is alternating current. The voltage potential between the neutral wire and ground may be zero (simply because they are both connected together) but so is any wire by itself. The neutrals voltage potential to the other (either in a 240 v supply) leg of the supply is still 120v. Neither leg of an operating (current flowing) AC supply can be at 0 volts (AC current flow changes direction 60 times per second (60 hertz)). The purpose of “grounding” one leg of the power supply is merely to provide an alternate current path (safety).

I recently lost half my lights and outlets intermittently. Checking the voltage at the panel indicated one leg of the panel was dead. This “hot” leg of the panel was at 0 volts (because it was disconnected at the pole). There was no voltage potential between it and the neutral bus. The reason I can say that this hot leg was at 0 volts is because the opposite leg to neutral still indicated 120v.

Capacitive voltage testers have varying sensitivities and are well known for false positives, particularly with ungrounded outlets in older homes. This is usually due to the metal box which changes the capacitive coupling of the “sniffer.”

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This brings up a good point. While I could spend more time “diagnosing” a specific wiring problem, I’m a generalist and my job is to recommend a specialist should that be necessary. Reporting something as simple as “mis-wired” is a great idea. In my honest opinion I don’t see anything wrong with reporting what you’re seeing such as “mis-wired, suspected cause open ground, but further investigation by a qualified electrical contractor necessary”, or something along those lines. Idk.


Brandon, I think that is a good idea. There is really no reason do dive too deep into what the specific cause is. When I report damaged shingles, I don’t determine the cause of the damage. When I come across a smoke detector that doesn’t work, I don’t report if it is the battery or a faulty detector. Why should electrical receptacles be any different?


Right, even if we are say historically an electrician, we’re inspecting the home as a system, not getting technically exhaustive. Like i tell clients, I could spend my entire 3-4 hours diving deep into the mechanicals, but you’re paying me to evaluate the entire home, and a reasonable cost to determine if additional investigation is necessary.


I think some people are missing the point, identifying the specific defect vs calling an outlet mis-wired is not really the point.

The main point to me is that a circuit analyzer is capable of detecting or identifying more deficiencies than a 3-light tester, and therefore is better.

The ability to identify more issues provides more information and more value to clients.

I personally note what the listed deficiencies are according to the tester. Calling something mis-wired is vague and lazy.

If an outlet is ungrounded, has reversed polarity, or a GFCI doesn’t trip when tested, it takes a negligible amount of time to list the defect, especially when it’s just a selection in your narrative or software.

Morning, Patrick.
Hope this post finds you well.

I am not speaking for Mike but I came to realized, 3 bulb testers had limitations and brought undue liability to the table. Purchase a ‘circuit load analyser.’

I purchased my first circuit load analyser, General CA10 AC Circuit Analyzer, many years ago.
After that I upgraded to (EXTECH CT70 and CT80) AC Circuit Circuit Analyzer. Difference. CT70 has no AFCI test. SURETEST work as well. CT 80 has been discontinued. My worked relentlessly.

Go to Inspector Outlet. Best pricing and delivery.

Hope that helps.


I disagree that it is vague and lazy. You are telling your client that this receptacle has an issue and that a qualified electrical contractor needs to asses it and make repairs. Seems prudent and liability-limiting to me. That being said, I currently call out the specific issue but this is now on my list of things to review in the future.


A circuit analyzer can also give vague or incorrect results. No one is saying it is not useful to be more specific. The OP posted a blog post with inaccurate information. If your fundamental knowledge of electricity is wrong a circuit analyzer won’t help you.

Once you start including information that exceeds the SOP in your report you better be right.
Stick to the basics. If someone asks you the time, they don’t want to know how to build a watch. The two basic questions for home inspectors (from the client) are: Is it safe (reliable)? and; Can I afford it (repair / replacement)?

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I’m not arguing for being more or less specific in our reports, but I think it is important to understand the limitations of the tools that we are using.

I did a test of the SureTest Circuit Analyzer versus a 3-light tester and was surprised by the results. The only deficiency that the circuit analyzer detected that was not detected by the 3-light tester was a Bootleg ground.

This comes down to style. But I do not disagree. However, saying it is mis-wired still elevates the problem to an electrical contractor to resolve.

When my tester identifies a defect this is what I say: Front left bedroom electrical receptacle had an open ground observed with my 3-light receptacle tester. Then I give my recommendation.