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.