Power Phasing for Safety Issues
by Dave Wendler
If you've played electric guitar long enough, and used a microphone at the
same time, you've been shocked. While there are some amplifier conditions
than can let stray currents escape, most of the time the problem lies in
your power source for the whole band.
In the US (and I'm assuming worldwide), our power comes into the building on
three wires: two lines each carrying 110 volts, and a third leg referenced
to ground - literally tied to the earth. Our two 110v lines are "out of
phase". As each one of the lines is referenced to ground, we can have power
to all of our "consumer" appliances and electronics. If we take our power
from both legs, that in effect doubles the voltage to 220V. That can cause
one heck of a bite on your lip.
So the typical scenario is the stage amps are plugged into a circuit on one
"leg" coming out of the main circuit box, and the PA system is hooked up to
the other - the circuits are out of phase, and you get 220V through your
lips - major ouch. To correct this, you must be sure both the stage amps and
PA system are in phase. You can usually use one 30 amp/110v circuit to power
an entire band. If that is not enough, you will need to access another
circuit breaker in the power box on the same side.
You will also need to pay special attention to power cables, especially on
the stage amps. A lot of "vintage" amplifiers have had the grounding lug cut
off the power cord - an invitation to disaster unless you know what you are
doing. And, to add insult to injury, many clubs and venues have ancient
wiring systems where no attention at all was paid to phasing at the wall
power outlets.
One thing you can do to protect yourself is to carry an inexpensive
VOM (volt-ohmeter). Simply touch a probe to your guitar's strings or bridge,
and the other probe to a microphone. If you get a reading at all, you have
a problem that needs to be addressed. Now a few volts will indicate a power
supply problem with your amp, but anything more than that and you probably
have a piece of equipment that is out of phase with everthing else. All you
can do is start inverting the phase of your power cords until it is located.
And while a GFCI (ground fault circuit interruptor) will help, when you're
talking instrument amplifiers, they all have major sized capacitors in their
power supplies, and those can hold a charge for days. So an instantaneous
zap can occur even while the power is turned off!
So, here's the fix: run the stage amp and PA off of the same leg coming
into the main power box. The same circuit is best; using circuits on the
same side of the power box is just fine also. If you are still getting
shocked, there is a wiring problem with the building. Have an electrician
check it out.
The flip side of this is setting up your stage lights. You want these
coming off the opposing leg in the power box, as the out of phase signal
cancels the 'buzz" common with lighting dimmers.
