[NN5I]

Do you need it? What do you expect it to protect you from?

There are three possible conditions from which you may need protection: (1) overvoltage; (2) undervoltage; (3) lightning-induced spikes.

Overvoltage
This will not occur on the power lines. Trust me, I'm an electrical engineer. It can occur in your RV, but only because of an open ground (neutral) that, in effect, places your 120v sockets across the 240v lines. This is more likely to occur in your RV's wiring than in the shore-power supply, but it can occur in the pedestal and sometimes does.

Undervoltage
This may occur anywhere, and is not rare in RV parks that have inadequate wiring. It can also occur on one 120v leg when the other leg gets overvoltage because of an open ground (open neutral).

Lightning-induced spikes
These may occur anywhere during thunderstorms.

Now then: what equipment do you wish to protect from these dangers?
Nearly everything in any RV I ever saw runs on the house batteries (and converter). Nothing that runs at 12v can be hurt by a problem in the shore power. Take that as gospel.

What runs on the shore power, then?

Only a few things: Air conditioners, the converter/charger, microwave, electric space heaters, most TVs, desktop PCs, and any electronics that runs from a wall wart. Let's look at each.

Air conditioners: These can be damaged by extreme overvoltage or undervoltage, but it has to be pretty extreme. Incorrect voltage may cause the compressor motor and fan motors to overheat, but typically they have thermal breakers that shut them off when they get hot. An undervoltage so great that the motor doesn't spin up enough to trip the centrifugal switch can burn up the starting capacitor and maybe even the starting windings -- the compressor is usually a capacitor-start, induction-run motor. Not really much danger there because it doesn't happen instantaneously, but you can avoid that by simple tests when first hooking up. I'll discuss this below. Lightning transients are not going to hurt them at all, being very short, unless the lightning is so close that nothing can stop it. In that case the surge protector won't do any good either.

Microwave oven: This can be hurt by overvoltage, but not by undervoltage, which will simply cause it not to work, or not work very well. Lightning transients, if very strong, might damage a microwave, so see below.

Electric space heaters: can be hurt by large overvoltage that continues for a long time, but not by undervoltage. Immune to lightning surges.

TVs: impossible to hurt by undervoltage, but large overvoltages can hurt them. Lightning can hurt them, too; see below.

Desktop PCs: Typically these won't be hurt by either overvoltage or undervoltage, because the power supply will protect itself by shutting down. Very little danger there. Can be hurt by lightning; see below.

Converter/charger: some are poorly designed and may be susceptible to overvoltage damage.

Anything that runs on a wall wart: Overvoltage may fry the wall wart, but typically won't hurt whatever the wall wart is feeding. Undervoltage won't hurt them. Lightning can fry the wall wart but usually not whatever it's feeding, but with lightning there are no guarantees for anything electronic.

My electric beard trimmer and Water Pik run on 120v too, but I can't imagine any damage to these from overvoltage, undervoltage, or any lightning hit that doesn't burn down the motor home.

What do I do? I certainly won't spend hundreds for a fancy device whose sales pitch, like IBM's in the early days, is based on inducing what is known in the mainframe industry as FUD for Fear, Uncertainty and Doubt.

For the few things that are in danger from lightning, I use small, inexpensive throw-away surge protectors from Lowe's or Home Depot.

Overvoltage can occur only as a result of an open neutral. There are many inexpensive testers that can detect open neutrals. By inexpensive, I mean less than ten bucks. Buy them at Lowe's, Home Depot, Northern Tool, Harbor Freight, or on eBay. You don't need a 240v device; a 120v device will work just fine, even on a 240v, 50a RV. Read on to see why.

Undervoltage can be caused by an open neutral, but only when it's also causing an overvoltage in some other circuit. Undervoltage can also be caused by inadequate wiring in the RV park.

Buy a cheap circuit tester that can detect open neutral by plugging into a 120v outlet. Also buy a little expanded-scale voltmeter that plugs into a 120v outlet.

Before plugging into the RV park's pedestal, test the voltage at a 120v outlet in the pedestal (the household-type outlet in the pedestal). If it's over 130v or under 100v, don't plug your RV into it. Then plug in the open-neutral tester there, and assure that the correct lights light up, so you're pretty sure (not yet certain) there's no open neutral.

Then, with all 120v appliances including the converter/charger turned off, hook up your shore power. Make the same checks inside the RV. If all is well, turn on one robust appliance such as a space heater, toaster, hot plate, or roof A/C, and quickly repeat the tests, especially the voltage tests. If within specs (about 100-125 volts this time), all is well and you can stop worrying.

Why does this work? Your cheap little throw-away surge protectors are protecting the few things that need lightning protection, and you've specifically tested the voltage, both at no-load and at single-load conditions.

Generalized undervoltage isn't rare in RV parks, but you've tested for it with the voltmeter. Open neutrals are a little rarer, but you tested for that too.

Let's discuss that open neutral a bit. The pedestal supplies a 120v/240v circuit. Each of the two legs supplies 120v (between the leg and the neutral), and there is 240v between the two legs. The neutral is maintained midway between. The 240v is always there in the pedestal, even if you have a 30a RV that connects only to one leg. If your shore-power connection is 50a, you're using both legs. (If you use an adapter to plug your 30a RV into a 50a pedestal, you're connecting both legs of your RV to a single 120v leg of the pedestal supply, which is fine because nothing in your RV requires 240v; your appliances are all 120v devices).

You tested at the pedestal and found 120v, close enough, with no load.

Then you tested inside your RV, same thing.

Then you turned on one fairly large load (space heater, toaster, etc) in your RV. If the neutral were open (or partly open), the voltage would drop on that leg and soar on the other leg (which has no load). Your voltage test would detect it either way, no matter which leg you’re reading. If that single voltage test was OK, then you're safe from any problems with the shore power. Pop a cool one (I prefer root beer), and then take a nap.

Even if I had one of those god-awful-expensive devices the RV dealers want to sell you, I’d do these checks anyway, because the god-awful-expensive device might itself have an open neutral or a defective connector; or my shore-power cable could have an open neutral.

You can do all this with ten or twenty bucks worth of equipment that you probably already own (should own!) anyway. Why spend hundreds for stuff whose whole sales pitch is based on inspiring fear of the unknown?