Theatre Design & Technology - Summer 2013 - 23

limit electricity enough to protect your
heart is wishful thinking. A path to ground
will limit voltage potential, but the green
grounding wire serves an additional
critical purpose. It facilitates the operation
of the overcurrent device.
Perhaps counter intuitively, the green
grounding wire is meant to cause the current
to rise. How else would the overcurrent device
operate? You need a solid circuit with a relatively predictable resistance between the frame
of the instrument and the electrical supply
source, which for us is usually the dimmer rack
or breaker panel. The lower the resistance of
this circuit, the faster the current will rise and
the faster the breaker will trip.
You might be wondering, why worry about
how fast a breaker will trip if a tiny amount of
electricity-less than a tenth of one amp-can
stop your heart. If the grounding circuit needs
to exceed twenty amps before the 20A breaker
trips, aren't you dead anyway? It needs to be
emphasized that the purpose of the overcurrent
device is to protect the wires and equipment
from fire, not the user from electrical shock injury. Electrical experts are particularly sensitive
about this misconception.
Predicting shock injury severity is a complex matter. Resistance affects the amount of
current running through the body, and while resistance across a straight length of wire is pretty
easy to calculate, resistance through a human
body is not. The NIOSH student manual explains
that some of the variables include where and
across what parts of the body you are shocked,
whether or not you have on protective gear, the
wetness and salinity of your skin, etc.
One critical variable that we can exert
some control over is duration. On a practical
level, the longer the electrical malfunction sustains, the more likely it is that something will
catch fire. If you have received a debilitating
electrical shock, you definitely want the responders to focus on treating you and not on
having to put out the fire or figuring out how get
the circuit shut off. Remember that they can't
touch you until they do; otherwise they become
part of the circuit. It takes only a very small current to cause someone to lose muscle control
and be unable to let go. The longer the current
goes through your body, the more likely that it is
that you will stop breathing or have burns. The

NIOSH student manual says, "At relatively high
currents, death is certain if the shock is long
enough. However, if the shock is short and the
heart has not been damaged, a normal heartbeat may resume if contact with the electrical
current is eliminated" (8).
These time factors are why it does matter that the circuit path be low-impedance and
that we shouldn't try to be creative with how
we ground things. One low-impedance circuit
back to the electrical supply source is what is
necessary and required. NEC section 250.4(A)
(5) ends with the warning, "The earth shall not
be considered as an effective ground-fault current path." In Mike Holt's Illustrated Guide
to Understanding the NEC Requirements for
Grounding versus Bonding (2011), the author
explains, "Because the contact resistance of
an electrode to the earth is so high, very little
fault current returns to the power supply if the
earth is the only fault current path. Result-the
circuit overcurrent device will not open and all
metal parts...will become and remain energized" (25).
Although opening the circuit by way of
"facilitating the operation of the overcurrent
device" will not necessarily prevent injury, it is
the most effective way to limit damage to equipment and people. In order to be responsible
and compliant, supervisors must make sure
that all electrically conductive equipment and
materials have proper equipment grounding
wires and that all the stage cables used to power
that equipment also have reliable grounding
connections. All cable connectors should be
checked periodically to ensure good connections and proper strain relief.
Inspections take time, but the consequences of not doing them can be grave. If a
green grounding wire becomes loose anywhere
in the circuit, the impedance of that circuit will
be higher than if the connection is tight. The
higher the impedance, the more time is lost in
the operation of the overcurrent device.

GettinG Current with CirCuit
CapaCity
We may eventually leave W=VA behind for
good because the nature of electrical loads in
theatre is changing. We have had the luxury
of working with mostly incandescent loads
that draw electricity in a predictable "linear"

s u m m e r 2013

way. When we were taught W=VA, many of us
weren't told, or didn't hear, that it doesn't
work with non-linear loads. Article 100 in
the Code defines a non-linear load as "a load
where the wave shape of the steady-state current does not follow the wave shape of the
applied voltage." Electronic equipment, adjustable speed drive systems, moving lights,
LED fixtures, and dimmable fluorescents are
examples of non-linear loads.
The simplicity of linear loads can be deceiving. No one wants to cause a breaker to trip,
so we learn quickly to estimate the total load on
a circuit as we plug things into it. But how do
you do that when you are hanging a bunch of
lights? We know the wattage of each lamp, but
calculating or remembering how many amps
each of the lamps draws is a lot of work. Wanting to keep things simple, we just calculate the
wattage equivalent of each breaker. One 20A
breaker can handle around 2400 watts. Now,
we just need to make sure the sum of all the
lights is less than 2400W. (Note: in most types
of circuits, the Code restricts loading them over
80% of capacity-a sensible safety precaution.
Most theatre dimmers, however, are rated for
100% loading, and so we routinely and safely
load them up to their full capacity.)
Our friendly formula, W=VA, becomes unusable when we try to apply it to non-linear loads.
For instance, the label on a common LED fixture
says: "Power Consumption: 120 V, 60 Hz: 81 W,
1.15 A (operating)." If you plug these specs into
W=VA, they don't add up. (Try it, you'll see.)
With non-linear loads, there is another variable,
often called the power factor. Unfortunately,
most manufacturers do not give you that power
factor variable in their specifications. In the case
of this particular LED instrument, we can figure
out that the power factor is 0.587, which is the
missing number that makes the equation work
again: 81W=120V x 1.15A x 0.587.
"How," you then ask, "am I supposed to
calculate nonlinear loads if I don't know what
that missing variable is?" The answer is to leave
West Virginia and forget about counting watts.
Manufacturer specifications for this type of
equipment usually tell you what the maximum
current draw will be in amps. Simple math is all
you need to tell you if the load of all the equipment on a circuit is more than breaker's rating.
For example, if you want to know how many
theatre design

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