Sunday, October 2, 2022


RV Electricity – Possible tester failure on 30-amp outlets

By Mike Sokol

Dear readers,

In this column I’m going to warn you about a potential measurement problem that can occur when testing 30-amp shore power outlets. If you’re evaluating pedestal outlets for proper voltage, polarity and grounds using a digital meter or EMS/Surge-Protector followed up with a Non-Contact Voltage Tester, then you’re still in good shape and have nothing to worry about.

However, if you’re relying on using only a residential 3-light outlet tester plugged into an adapter to test a 30-amp outlet, or using a basic surge-protector with a 2- or 3-light indicator system to check for proper polarity and grounding, then I’ve recently been made aware of probable tester confusion when encountering a 30-amp outlet that’s been accidentally miswired with 2-pole/240-volts. For my original article on how that happens and why it’s so dangerous for your RV’s electrical system, read all about it HERE.

One of my Facebook readers pointed out that if you use a basic 2-light surge protector on a pedestal and it indicates reverse polarity, it’s possible that it’s not just a simple case of reversed hot/neutral polarity on the outlet. And as I’ve written many times, if the only thing wrong with the pedestal outlet is reversed hot/neutral polarity, and all other wiring is correct, then it’s not immediately dangerous and can’t cause a hot-skin condition. However, it does indicate the possibility of more serious wiring issues, so it should be corrected as soon as possible. Read more about how it works HERE.

But what does a 2- or 3-light outlet tester indicate if the 30-amp pedestal you’re testing has been accidentally miswired with 240 volts? Well, in several cases that I’ve looked at the tester indicates REVERSED POLARITY with no indication that the outlet is 240 volts. WHAT!!!! That’s right: Those simple surge protectors don’t have enough smarts to recognize the difference between a 30-amp outlet with Reversed Hot/Neutral Polarity (maybe OK, but a code violation), and a 30-amp outlet that’s been miswired with 2 poles at 240 volts (which will burn up your 120-volt RV’s electrical system in seconds). Yikes!

As far as I can tell, this problem isn’t brand-specific since they all appear to use the same basic 2-light or 3-light polarity/ground indicator circuits. But what I’m going to do for my next RVelectricity™ Newsletter, which publishes the last Sunday of each month, is create a TT-30 outlet that’s been miswired with 240 volts ON PURPOSE. Then I’ll plug in sample 30-amp surge protectors I already have in my lab from Progressive, Surge Guard and Camco, as well as regular Edison outlet testers from Klein, Greenlee and a few others using a 30-amp dogbone adapter. Then I’ll report if any of them can alert you to a 240-volt TT-30 outlet (I don’t think so), or if any of them blow up when encountering 240 volts (I know of at least one 15/20-amp Edison tester that might literally explode, since I did that once accidentally).

I’ll also test all the 30-amp EMS Surge Protectors I currently have on 240-volts to make sure they’ll discover the over-voltage condition and refuse to supply power to your RV. However, I’ve done a little field testing of this important EMS function in the past, so I expect they’ll all work properly without damage. After all, that’s their job!

So watch for my full report on what happens when your Basic or Advanced/EMS surge protectors encounter a miswired 30-amp pedestal outlet that has 240 volts instead of 120 volts. In the meantime, if your basic surge protector or 3-light tester indicates a reversed polarity, perform additional testing with a digital voltmeter BEFORE plugging in your RV’s shore power cord. We don’t want any more toasted RVs.

So as they say in the carpentry business – measure twice, plug-in once (or something like that). Let’s measure carefully and play safe out there….



Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. 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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