How to use a digital meter

By Mike Sokol 

Everyone,
Here’s Part 2 of my No~Shock~Zone 12-part series on RV electricity fundamentals. My plan is to update and rerun these articles every other week until the series is complete. These are the core principles needed to understand and troubleshoot RV electricity, so take the time to read them a few times until it starts to make sense. After a while it will all become second nature to you, so don’t be frustrated at first.

(Click any image to enlarge.)

Shake and bake

Remember when you were a child and first started to help with baking and there were all sorts of measuring devices and abbreviations to take into consideration? There was a tablespoon (tbsp), teaspoon (tsp), ounce (oz.), with 8 oz. in a cup, and so on. And you better not get your tsp and tbsp mixed up or bad things would happen to your cake. The same types of rules apply when you’re measuring any electrical values. You just need to know how to use a few electrical measuring tools and then you’re ready to test your RV power. As usual, click on any of my graphics to enlarge it to full size.

The meter

Now is the time to familiarize yourself with your Digital Multi-Meter (DMM). Here’s a pretty typical $20 meter that you can purchase at Lowe’s, Home Depot or Amazon. You’ll notice a bunch of strange markings on the selection knob, only a few of which will work to measure AC voltage. Don’t be tempted to just plug the meter leads into a  campsite socket and spin the knob. That will guarantee a burned out meter (at the least).

Note the markings on the control knob are divided into four major groups.

• AC V (AC voltage)
• DC A (DC amperage)
• OHM (electrical resistance)
• DC V (DC voltage)

The only two groups you’ll be interested in are AC V (for measuring the AC voltage in power outlets) and DC V (for measuring the DC voltage in your batteries). For this article we’ll focus on the AC V group since we’re measuring the 120- or 240-volts AC in a campsite pedestal.

Also take a look at where the meter leads (or “probes”) are plugged into the lower right-hand connections on the meter itself. The Black COM (common) input is always connected to your black meter probe, and the red V-Ohm-mA (milliamperes) input is always connected to your red meter probe. Never put either meter probe into the 10A socket, which is designed specifically to check current flow up to 10 amps. Doing so for measuring AC voltage will blow the internal fuse in the meter, and possibly damage the meter itself.

All meters read the difference  between the two lead connections, so if the black probe is touching 0 volts and the red probe is touching 120 volts, the meter will read 120 volts. However, if both the red and black probes are touching wires with 120 volts on them, the meter will indicate 0 volts, which is because 120 minus 120 equals 0 volts. See how it works? Meters indicate the difference (what we call a differential) between any two wires or objects. So the key to using a meter is to connect the meter probes between the two voltages you want to measure.

Now, let’s move back to the meter settings. In the AC V area you’ll see a 200 and a 600 setting. When set to 200 the meter will read up to 200 volts; when set to 600 the meter will read up to 600 volts. Since we could be reading as much as 240 volts, we’ll always set this to 600 and leave it alone during all testing. If you set it to 200 and connect it across a 240-volt outlet, the display will probably stick on 199 volts and start blinking. That doesn’t hurt anything, but it doesn’t tell you the actual voltage. Many meters of this type have a 400- or 750-volt setting, so setting for 400 or 750 volts is fine as well, just as long as it’s set for something more than 250 volts. And if you have an auto-ranging meter, just set it to read AC volts and it will figure out the proper scale for you.

The outlet

Before you graduate to measuring the big 240-volt, 50-amp outlets, you need to start on a common 120-volt, 20-amp outlet like you might find in your living room or throughout your RV. Here’s what one looks like and the connections as standardized by the National Electrical Code. You’ll see a little U-shaped hole, which is the Ground; a taller slot on the right, which is the Neutral; and a shorter slot on the left, which is the Hot connection.

Don’t be confused if the receptacle is mounted with the ground connection to the bottom. The taller slot is always the NEUTRAL, and the shorter slot is always the HOT. Also note that the only difference between a 15-amp and 20-amp outlet is the sideways neutral blade on the 20-amp connection. And while a 15-amp plug can be inserted into a 15- or 20-amp outlet, a 20-amp plug can only be inserted into a 20-amp outlet. But not to worry, since a 15-amp outlet is actually rated to carry 20-amperes of current as long as it’s hooked up with 12-gauge wire and a 20-amp circuit breaker.

This is a GFCI (Ground Fault Circuit Interrupt) receptacle so there are test and reset buttons. More on this later, but pushing the “test” button should cause the “reset” button to pop out and kill the power from the outlet. Pushing the “reset” button in until you feel a click will restore power to the outlet. The job of the GFCI is to kill the power to the plug before it kills you, say from a hot skin condition on your RV. But these GFCI receptacles are only on the 20-amp campsite outlets, not the 30- or 50-amp outlets. In that case you’ll need your own GFCI breaker or outlets in the RV that will help protect you from a shock to ground. We’ll discuss this topic more towards the end of this series.

Also note the difference between the 20-amp and 15-amp versions of the outlets in this photo-shopped picture. A 20-amp outlet will have a sideways T-slot for the neutral connection, while a 15-amp outlet will only have a single vertical slot.

The measurements

Since we’re going to be measuring live voltage, you need to observe the safety rules from Part I of this series:

• Use only one hand to hold the plastic handles of the meter probes.
• Be sure you don’t touch the metal tip portion of either probe.
• Don’t stand or kneel on wet ground when measuring live outlets. For most situations, dry sneakers will insulate you from the earth sufficiently, and if you’re doing this test in your living room then wooden floors or carpet will protect you if something goes wrong. But if you’re going to measure voltage at a waterlogged campsite I suggest standing on a dry rubber shower mat so your feet are insulated from the ground.

Hot to Neutral

With nothing plugged in to the camp outlet, switch on the 20- amp circuit breaker at the power pedestal, set your meter to the 600 or 750 V AC setting and using one hand insert your meter probes into the left and right Neutral and Hot slots. Remember not to rest your opposite hand on the metal box. It really doesn’t matter which side gets the red or black meter probe since you’re reading Alternating Current.

Since the Neutral connection is at 0 Volts and the Hot connection should be around 120 volts, it should read somewhere between 108 and 128 volts on the meter display. If not, then something’s wrong with the power hookup. If you measure 0 volts, then maybe you need to reset the circuit breaker, or if you have an outlet with a GFCI (Ground Fault Circuit Interrupt) remember to push the little reset button on the outlet itself. If it still doesn’t measure 108 to 128 volts, immediately contact the camp manager.

If you measure 220–250 volts, then that power outlet has been jury-rigged inside the box to produce higher voltage. This is illegal and highly dangerous as you’ll surely blow up every piece of electrical gear in your RV if you plug into this outlet. So, if you read 240 volts on the 120-volt outlet do not plug in your RV, and, again, immediately contact the camp manager.

Hot to Ground

If the hot-to-neutral checks out around 120 volts, then it’s time to test the ground, so plug your two meter probes into the HOT (shorter slot) and GROUND (U-shaped hole) connections. Since you’re reading from the Ground connection, which should be 0 volts, and the Hot connection, which should be around 120 volts, your meter should indicate about 120 volts. If you read 0 or something strange such as 60 volts, then the ground wire might be floating (disconnected), which could cause a hot-skin condition that will shock you when touching the body of the RV.

Neutral to Ground

Next, check from Neutral to Ground. That should read very close to 0 volts, but up to 3 volts is acceptable according to the electrical code. If, however, you read 120 volts from Neutral to Ground, then the polarity of the power outlet is reversed. Don’t plug your RV into this outlet. At the very least this demonstrates that nobody tested this outlet after it was installed, so other wiring issues are possible. And a reversed polarity outlet might cause a dangerous hot-skin condition depending on how your RV is wired. But if your RV is properly wired, then a simple H-N polarity reversal will NOT cause a hot-skin voltage.

Final exam

As a final check, a $5 outlet tester from your local home center will confirm that the polarity of the outlet is correct. Plug it into the power outlet on the pedestal and you should see only the two yellow or amber lights light up. If you see any other combination, do not plug in your RV.

If you’re only using 20-amp power for your RV, you’re just about done. At this time I recommend plugging the outlet tester into an outlet inside your RV that you can see from the open door or window. Now, go ahead and switch off the circuit breaker, plug in your 20-amp RV connector, and turn the circuit breaker back on. But before you touch anything on your RV take a peek through the door or window at the outlet tester inside your RV to confirm it’s showing the same Yellow/Yellow pattern. If not, then your extension cord or RV plug has been incorrectly wired. If that’s the case, turn off the circuit breaker and find out what’s wrong before proceeding to power up your RV.

I also like to keep an outlet tester like this plugged into a visible interior RV outlet at all times. That way if something happens to the campground power in the middle of the night that electrifies all the RVs in an area, you’ll get warning from the outlet tester before you get shocked on the door frame while stepping out.

Once you’re familiar with the procedures, all this can be done in a minute or two. It’s a very small inconvenience that will help ensure the safety of you, your family, friends and pets. Stay safe!

Quick Tips

• Always set your meter to read AC volts using the 400-, 600- or 750-volt scale.
  
• Hot (short slot) to Neutral (tall slot) should read approx. 108 to 128 volts.
  
• Hot (short slot) to Ground (U-shape) should read approx. 108 to 128 volts.
  
• Ground (U-shape) to Neutral (tall slot) should read approx. 0 to 3 volts.

Part 3 of this series will cover how to check 30-amp and 50-amp circuits at the campsite pedestal before plugging in, as well as using a non-contact AC tester to check for an RV hot-skin condition, so stay tuned.

Let’s 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 Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

©Mike Sokol 2010/2019 – All Rights Reserved

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