Does anyone have experience with setting up protection for circuits that are 120vac nominal, but could be accidentally exposed to 240vac if the wrong plug is used?

In this case the application is for a camper that recently experienced this exact problem when an outlet on a new building was mistakenly powered with 240vac, which fried the fuses on several 120vac camper appliances (fortunately, mostly an easy fix this time). For reference, the camper is rated at 120vac, 30 amps max draw.

I found several over-voltage protection modules that sound like they could work, but have 1 to 2 seconds of delay after powering-on before they would trigger.

Ex: Module 1, Module 2

It seems like that much delay would be too much, but I don't really know. Please let me know if you have any thoughts on this issue, or if there are better solutions.

There are faster options per the Wikipedia surge protection page: crowbar circuits, transient voltage suppression diodes and a few others, but they don't seem to be commonly available as a commercial product. I can solder something up if needed, but off-the-shelf seems safer.

  • 1
    XY problem. What you need (IMHO) is a voltmeter connected to a TT-30P plug, that you use BEFORE you connect your trailer to (code-violating) improperly wired outlets. I found one in a few seconds of searching that has a plug, a receptacle, and bluetooth to display the voltage on a phone, tablet, or computer. You'd plug that in, check the voltage, and ONLY after it showed good plug in your trailer to the receptacle on it. Or you probe the receptacle with a DMM before plugging in. Or use a listed/fused plug adapter to plug in something like a kill-a-watt or other NEMA 5-15P voltmeter
    – Ecnerwal
    May 5, 2022 at 23:05
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    ...and as per usual - you're a few decades out of date on voltages. You are protecting 120VAC from 240 VAC any time in the last 50 years in the USA, at least. Assuming your trailer power panel has a main breaker (fuse?) you could also install a hardwired voltmeter on the line into the trailer before the main breaker or fuse, and turn off the breaker (or pull the fuse) before plugging in the cord, then check the voltmeter before turning the breaker on or installing the fuse.
    – Ecnerwal
    May 5, 2022 at 23:12
  • Don't codes require different shaped outlets for each voltage, so you can't accidently mismatch voltages? May 6, 2022 at 1:03
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    @whatsisname, I believe the issue was that the outlet was mis-wired, so the outlet that was plugged into had the wrong voltage for the requirement for the specific plug shape.
    – Milwrdfan
    May 6, 2022 at 1:36
  • Thank you for the feedback and suggestions (and correction on the voltages haha). It's not my camper, but a relative's who is not particularly tech-savvy, so I was aiming for the fully automated protection route. I agree that a manual voltage check is a cheap and simple solution if they just have to plug something in and look at a screen.
    – Mandias
    May 6, 2022 at 5:35

2 Answers 2


There's a gizmo made precisely for this job

What you want is a Southwire 34930 (plug-in) or 35530 (hardwired). These devices are made for the express purpose of protecting RVs/travel trailers from improper receptacle wiring, and have a built-in overvoltage cutout function as part of their protection features.

  • Thank you for the suggestion. Do you happen to have an opinion on the different joules of surge protection offered? Also, I was under the impression that when choosing power-handling electronics it's a good idea to over-spec to give some "wiggle room", but these models are rated the same as the max current (30A). Does that matter in this case?
    – Mandias
    May 6, 2022 at 5:40
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    @Mandias -- the 30A version is what you want since your trailer is a 30A trailer. I wouldn't worry much about joule ratings (you might want a higher joule rating if you are in say Florida a bunch due to the prevalence of big lightning-induced surges on your grid, but that's about it) May 6, 2022 at 11:46

If you wanted to fully automate "protection", rather than depending on manually checking voltage before plugging in, I guess a 30A or greater relay that's "normally off" and is only turned "on" when some supervisory circuit is happy with the power quality on the input would work. The 120V side would never see 240V input, even for a second, in the situation you are practically trying to prevent (plugging into a miswired outlet.) In a more far-fetched situation (voltage suddenly changes to 240 from 120, which would be unusual in the extreme) how much exposure there would be would depend on how fast it could be switched off. But for plugging into bad power, it would never switch on at all.

The only place I can think to get a supervisory relay and power quality circuit "off the shelf" would likely be prohibitively expensive - a UPS with a 120V 30A in and out connection. But depending on how much it's worth to you, that might be the way to do it. Good ones switch off for voltage too high, voltage too low, and may have capacity to regulate "voltage somewhat too high or low" without going over to batteries. And they do provide some level of battery backup when power input is not good.

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