Understanding the use of a few electrical measuring tools in order to accurately and safely test stage power...
February 05, 2014, by Mike Sokol
In part 1, we covered what voltage is and a bit on how it’s measured. Here let’s look at how to use a basic digital voltmeter to measure any power outlet or extension cord for proper voltage.
The reason this procedure is so important is that sometimes venues do crazy things with power outlets. For instance. I was teaching a seminar last year in a “gymnatorium” and plugged in my little demo rack along with my RF headset receiver.
As I was getting ready to flip the switch on the Furman rack distro, I noticed the built-in voltmeter was pegged to the right of the 120 volt scale. Luckily, I didn’t go further and did not flip on the switch that powered the full rack.
But unluckily for my RF receiver, it was ahead of the Furman power switch so it was already “on” and burning quite brightly for a few seconds. A quick meter test on the power outlet confirmed it had been rigged for 240 volts, even though it was a standard NEMA-5 “Edison” outlet, which should always be wired for 120 volts.
The janitor told me that was his “special outlet” they had rewired for his 240-volt [floor] buffer. But it should have had a 240-volt plug and outlet, not an Edison outlet modified for 240 volts. My bad for not checking.
My RF receiver was toast. Live and learn….
Shake & Bake
Remember when you were a child and first started to help with cooking, 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 stage power.
At right we see a pretty typical voltmeter that can be purchased 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 wall outlet 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 for measuring voltage 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 wall outlet or stage power distro.
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 250 volts in a standard electrical outlet, we’ll always just set this to 750 and leave it alone during all testing. If you set it to 200 and connect it to a 240 outlet, the display will probably stick on 200 Volts and start blinking.
That doesn’t hurt anything in the meter, 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.
Let’s start on a common 120-volt, 20-amp outlet like you might find in your living room or on any American stage. At right is what one looks like, and the connections as standardized by the National Electrical Code (NEC).
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 chassis condition on your guitar amp.
Generator & Distro Power
If you’re working on a large stage with dedicated power or an outside venue fed by a generator, you’ll often bring along your own electrical distribution system know as the “power distro.”
And many times that power distro system will be fed by large twist-lock connectors generically know as cam locks. These are big brass connectors the size of a banana with rubberized insulating covers that keep you from getting shocked while touching the exterior.
They come in colors corresponding to their connection type, so a green cam lock is ground, white is neutral, and black, red, or blue are hot (at least in America). Sometimes the cam lock covers will all be black with a wrap of white, green, blue or red electrical-tape in the middle to define their usage and that’s legal as well.
Here (below right) is what a portable distro panel looks like with the green, white, black and red cam lock inputs across the bottom.
Cam locks are typically fed by a single, double or triple 100- to 200-amp circuit breaker at the generator or house panel, so you’ll need to provide your own 20-amp breakers downstream to feed your portable backline power outlets in order to prevent the wires from melting in the event of an overload.
You can see the circuit breakers across the top of the panel at the left. Also, you’ll occasionally find a 3-phase house system with green, white, red, black and blue cam locks which will meter as 120/208 volts, but we’ll discuss that topic towards the end of this series.
Also note the difference between the 20-amp and 15-amp versions of the stage outlets as shown a few illustrations back. 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.
Since we’re going to be measuring live voltage, observe the safety rules from part I of this series:
Use only one hand to hold the plastic handles of the meter leads, put your other hand in your back pocket so you don’t lean it on anything conductive
Be sure you don’t touch the metal tip portion of either meter lead
Don’t stand or kneel on wet ground while testing voltages. For most situations, dry sneakers will insulate you from the earth sufficiently, and if you’re doing this test on a dry stage then the wooden floor or carpet will protect you if something goes wrong.
But if you’re going to measure voltage at a waterlogged festival generator I suggest standing on a dry rubber shower mat or dry plywood so your feet are insulated from the ground. It’s cheap insurance.
Hot to Neutral
With nothing plugged in to the wall outlet, switch on the 20-amp circuit breaker at the power panel, set your meter to the 200 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, as that can cause a shock through your heart if something goes wrong. That’s why electricians traditionally stick their unused hand in a back pocket.
It really doesn’t matter which side of the outlet gets the red or black meter lead since it’s alternating current (AC).
Measuring hot to neutral.
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, remember to push the little reset button on the outlet itself. If it still doesn’t measure 110 to 125 volts, immediately contact the stage manager.
If you measure 220-250 volts, then that power outlet has been rigged inside the circuit breaker box to produce higher voltage. This is illegal and highly dangerous as you’ll surely blow up every piece of electrical gear you plug into the outlet. So, if you read 240 volts on the 120-volt outlet do not plug in your amp, and, again, immediately contact the stage manager.
Hot to Ground
If hot-to-neutral checks out around 120 volts (110 to 125 volts), then it’s time to test the ground, so plug one meter lead into the hot (shorter slot) and the other into the ground (U-shaped hole) connections.
Measuring hot to ground.
Since you’re reading from the ground connection, which should be 0 volts (less than 2 volts), and the hot connection, which should be around 120 volts (110 to 125 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-chassis condition that will shock you when touching the strings of your guitar and microphone.
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 around 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-chassis condition depending on how your guitar or PA system is wired.
As a final check, a run-of-the-mill outlet tester from your local home center will confirm that the polarity of the outlet is correct.
Plug it into the power outlet on stage and you should see only the two yellow/amber lights light up. If you see any other combination, do not plug in your guitar amp. 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 and your band.
Measuring Neutral to Ground.
—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 (between 110 and 125 volts AC)
—Hot (short slot) to ground (U-shape) should read approx 120 volts (between 110 and 125 volts AC)
—Ground (U-shape) to neutral (tall slot) should read approx 0 volts (less than 2 volts AC)
Mike Sokol is the chief instructor for the HOW-TO Sound Workshops and the HOW-TO Church Sound Workshops. He is also an electrical and audio expert with 40 years in the industry. Visit the No Shock Zone Website for more electrical safety tips.
This article is provided as a helpful educational assist with sound system setup and musical performance, and is not intended to have you circumvent an electrician or qualified audio technician. The author and the HOW-TO Sound Workshops will not be held liable or responsible for any injury resulting from reader error or misuse of the information contained in these articles. If you feel you have a dangerous electrical condition in your PA system or instruments, contact a qualified, licensed electrician or audio installer.