Thursday, December 8, 2022


Deciphering the 3-light outlet tester


Dear Mike,
I recently purchased a 3-light tester like you talk about in your newsletter, but I wonder what the lights actually mean. Can I use a 30-amp to 15-amp adapter for measuring my pedestal outlet? And how do I know if an outlet is safe to plug into? I’m a newbie so I have lots of questions. —Befuddled in Brooklyn

Dear Befuddled,
Well, I sometimes forget just how confusing this all can be, so thanks for reminding me to write about this topic. I won’t get into ALL the troubleshooting variables here, but let’s cover the basics as they relate to electrical safety.

First of all, yes, you can use a 30-amp to 15-amp adapter for this test. That’s because the RV pedestal TT-30 outlet is virtually identical to the home outlet except that it’s rated for 30 amps of current rather than the 15 or 20 amperes of a home outlet. However, a dedicated surge protector for a 30-amp TT-30 outlet would be a better unit for this test. But the light patterns are generally the same as the 15-amp versions. More on that in a future article.

To proceed you’ll need to get yourself a standard 3-light outlet tester that you can use in any home outlet. Here are a couple from Southwire. You really don’t need the GFCI version unless you want it for additional 20-amp outlet testing since 30-amp pedestals don’t require a GFCI by code. But it won’t hurt anything so go ahead and spend the extra $5 or so on a GFCI version.

You’ll see that there are three lights on the end of the outlet tester: two amber and one red. The pattern of the lights is key to understanding if the outlet is safe, but you really don’t have to know everything that’s going on inside to make a judgment. You just look for a pattern of two amber lights and no red light. When you see that you can assume that the Hot, Neutral and Ground wires are hooked up in the right places.

But if you’re curious, here’s what the other patterns mean:

Open Ground: The outlet has no ground connection, which can easily allow your RV or electrical appliance to develop a hot-skin voltage. DO NOT plug into this outlet.

Open Neutral:  This means that you have no operational appliances, however, they’re actually energized with 120 volts and can be dangerous if you open up an access panel and poke around inside while the appliance is plugged in. Never assume anything is electrically safe to work on unless it’s unplugged.

Open Hot: This means you have no power in the outlet at all, likely from a circuit breaker being tripped or turned off.

Reversed Hot and Ground: Even though your appliances won’t operate, their chassis will be energized with 120 volts, which is extremely dangerous. DO NOT plug into this outlet. EVER!

Reversed Hot and Neutral: Unless something else is miswired, this in not immediately dangerous and won’t cause a hot-skin condition by itself. Use with extreme caution.

Correct Wiring: This means the outlet is safe unless you have a rare miswiring condition I’ve named an RPBG (Reverse Polarity Bootleg Ground), in which case everything will operate normally but there will also be a hot-skin voltage. That’s why I suggest you also use a Non Contact Voltage Tester to confirm there’s no hot-skin voltage after plugging in.

If you want to know more about RPBG wiring and troubleshooting, I wrote an in-depth article where I introduce the concept to the electrical contracting industry. Read it here – but this is a really long and technical article, so don’t go wading in unless you’re ready for Deep Thought. Yes, the answer is 42.

Let’s play safe out there….


Mike Sokol is an electrical and professional sound expert with 40 years in the industry. Visit for more electrical safety tips. His excellent book RV Electrical Safety is available at For more info on Mike’s qualifications as an electrical expert, click here.



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4 years ago

hi Mike,
I am anxiously awaiting your comparison article you mentioned in April. comparing the basic surge protector and the intelligent version. I have no problem protecting my baby with a $500 device if it is needed.
If i missed the article I apologize, do not hesitate to point me to it!
Thanks for all the great content!

4 years ago

Mike, not all 3 light testers use the same pattern. The ideal 61-035 indicates good ckt when both outside lights are on.

Sherry Dawson
4 years ago

Mike, I appreciate all your articles and videos, as I’m a newbie and want to learn all I can about RV electricity. You could do us a huge favor if you could provide printed directions to go along with your videos. To get the information copied into my notes for hookup and maintenance, I have to watch the videos numerous times, stopping and “rewinding” over and over so I can type my notes. Could you possibly provide written instructions below your videos? I, and others, would be truly grateful.

4 years ago

Does a “Correct Wiring” indication from a 3-prong tester, through a dog bone on the 30 amp receptacle, tell me anything about the health of the 50 amp receptacle in the same pedestal? Or, is there an inexpensive gadget to test the 50 amp receptacle?

Solar Steve
4 years ago

Well Mike, you had an article or survey pointing out miswired camp outlets that are 240 rather than 125 volts. I assume that 3 light tester would not reveal this, or more likely would be burned out. Maybe I should take a 30 map plug and make a light box with two bulbs in series between hot and neutral , the brightness of which would reveal any 240 mis-wiring on suspicious outlets?

John Cook
4 years ago

In one of your articles, you indicate that Progressive Industries sells your Generator Neutral Ground Bonding Plug. I was informed when I called that they do not have that item available. Is there another source?

4 years ago

When I plug my RV into my portable 2000 watt Inverter generator I get an open ground. I do not get a hot skin. Apparently this is not a huge concern!?,