No Shock Zone: Part Two: Multimeters

No Shock Zone: Part Two: Multimeters

 

By Mike Sokol

If you recall the survey we did July, 2010, in RVtravel.com, you know that 21 percent of the RVers who responded had been shocked by their RV.  Review the report here. What follows is part two in a 12-part series about basic electrical safety for RV users and how to protect yourself and your family from shocks and possible electrocution.

This series of articles is provided as a helpful educational assist in your RV travels, and is not intended to have you circumvent an electrician. If you feel you have a dangerous electrical condition in your RV or at a campground, make sure to contact a qualified, licensed electrician.

Shake & Bake
Remember when you were a child and first started to help with baking 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.

The Meter
Now is the time to familiarize yourself with your voltmeter. Here’s a pretty typical $30 meter that you can purchase at Lowes, 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 up 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 are plugged into the lower right-hand connections. The Black COM (common) input is always connected to your black meter lead, and the red V-Ohm-mA (milliamperes) input is always connected to your red meter lead. Never put either meter lead into the 10A socket, which is designed specifically to check current flow. Doing so 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 lead is touching 0 volts and the red lead is touching 120 volts, the meter will read 120 volts. However, if both the red and black leads are touching 120 volts, the meter will indicate 0 volts, which is because 120 minus 120 equals 0. See how it works? That’s the key to using a meter. It must be connected 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 750 setting. When set to 200 the meter will read up to 200 volts, when set to 750 the meter will read up to 750 volts. Since we could be reading as much as 240 volts, we’ll always just set this to 750 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 200 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 600-volt setting, so setting for 400 or 600 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: that is the Ground; a taller slot on the left, which is the Neutral; and a shorter slot on the right, which is the Hot connection. Don’t be confused if the receptacle is mounted upside down with the ground connection to the top. The taller slot is always the NEUTRAL, and the shorter slot is always the HOT.

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. A 20- amp outlet will have another sideways slot for the neutral connection, while a 15-amp outlet will only have a single vertical slot.

The Measurements
Since we’ll 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 leads
•Be sure you don’t touch the metal tip portion of either lead
•Don’t stand or kneel on wet ground. 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 leads 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 lead since it’s Alternating Current.

Since the Neutral connection is at 0 Volts and the Hot connection should be around 120 volts, you should read somewhere between 115 and 125 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 110 to 120 volts, immediately contact the camp manager. If you measure 230-240 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 hot-to-neutral checks out around 120 volts, then it’s time to test the ground, so plug your two meter leads 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 show about 120 volts. If you read 0 or something strange such as 60 volts, then the ground wire might be floating, 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 2 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 in. Again, this can cause a dangerous hot-skin condition depending on how your RV is wired.

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/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 this 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 this same tester plugged into an inside outlet that’s visible at all times. That way if something happens to the wiring in the campgrounds 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 120 volts
•Hot (short slot) to Ground (U-shape) should read approx 120 volts
•Ground (U-shape) to Neutral (tall slot) should read approx 0 volts

* * *
Read all installments of this series.

Mike Sokol is the chief instructor for the HOW-TO Sound Workshops (www.howtosound.com) and the HOW-TO Church Sound Workshops. He is also an electrical and professional sound expert with 40 years in the industry. Visit www.NoShockZone.org for more electrical safety tips for both RVers and musicians. Contact him at mike@noshockzone.org.

Copyright Mike Sokol 2010-2016. All Rights Reserved 

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