Inexpensive tools are all you need.
By Bill Hendrix, F761S
A few weeks ago we were attending a small rally held at an older RV park when I observed three guys hovering around the utility post for one of the coaches. They were in a friendly but intense discussion, so I walked over to satisfy my curiosity. Turns out they were not getting the proper connection to operate a 240-volt-AC dryer in the coach. This was a very nice converted bus, and all the other electrical appliances worked okay.
Each man held a different tool: one a digital multimeter, another an older analog voltmeter, and the third a test light. They were trying to determine whether the problem was in the 50-amp electrical outlet at the utility post, or in the coach. The multimeter was a recent purchase, and the owner was not very familiar with its functions. The voltmeter was old, and its user really didn’t know whether it was working or not. The guy with the test light was sure the light was working but didn’t know which outlets did what. But they all agreed that the problem just had to be reversed polarity.
Unfortunately, they didn’t know how to test for true 240-volt electricity. Well, isn’t there only one 240-volt electricity? There certainly should be, but some RV parks have been upgraded with whatever labor was at hand and without a thorough electrical inspection to check the work.
Properly connected, the 50-amp/240-volt outlet will have a four-wire service: a neutral, a ground, a line-1 of 120 volts, and a line-2 of 120 volts, and the L1 and L2 will be in opposite phase. In simple terms, you might think of the opposite phase like a teeter-totter “” when one side is going up, the other side is going down. Any 240-volt equipment needs this type of power supply. So if both of the 120-volt pins are wired to the same leg, either L1 or L2, the only difference that would be noticed is that any 240-volt appliance would not work.
This was the dilemma the three guys were facing. Unless you use a meter frequently, you are apt to lose proficiency, and properly checking for correct polarity and true 240 volts does require a little manipulation of the instrument.
For ages, I have carried an inexpensive test light in my toolbox that will display 120 volts and true 240 volts. I can’t even remember where or when it was purchased, so I looked around to see whether testers still are available “” and, yes, they are and still easy on the wallet. I found testers for $1.69 at Big Lots and priced a little higher at Home Depot and Sears.
In Photo 1, note that the tester has two lamps “” one indicates 120 volts and the second indicates 240 volts. Check the left and right outlets, as shown in the photo, and if the 240 light comes on you have true 240 volts with proper polarity. If the 240-volt light does not illuminate, either one of the legs is missing or the two pins are wired to the same leg. If voltage from one leg is absent, half of the 120-volt breaker panel in the coach will be dead. If the two pins are wired to the same leg, the 240-volt appliances cannot operate.
Testing a 15-amp outlet for proper polarity is easily done with a $4 circuit tester (Photo 2), and every coach owner should have one. You can use the same tester for the 30-amp outlet by plugging it into a 30-amp-to-15-amp reducer (Photo 3). I leave my circuit tester plugged into the reducer since we seem to encounter 30 amps more often than 50 amps, and I always test the outlet before plugging in the coach.
With these simple tools, you can check the outlets like a pro and not have to remember how to use a multimeter.