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Stopping Hums, Buzzes and Shocks on Stage—Meters
Understanding the use of a few electrical measuring tools in order to accurately and safely test stage power...

And "old school" analog volt meter
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Audio Basics, Power, Measurement, Maintenance, Mike Sokol, Electronics

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.

The Outlet
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).

Outlet diagram.

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.

Cam locks.

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.

Power distro.

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.

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