Doug Fleenor Design DMXOPTO-5 - Manual (Page 4)

3
BREAKING GROUND LOOPS
The term Ground Loop refers to an electrical connection that
results in two paths for current from one point to another, with
one of the paths usually occurring through the building's
electrical ground system. Ground loops result from two pieces of
electrical equipment being connected to electrical power in
different places, and then being connected together. In audio
systems ground loops frequently occur, and can result in audible
hum or buzz. The art of eliminating ground loops in audio
systems is simplified by being able to hear the result; when the
buzz goes away, the job is done.
Serious ground loops in dimming systems are less common but can
be difficult to trouble shoot if they exist. Unlike audio loops
that result in an audible hum, a ground loop in a multiplexed
dimmer control application may not have continuous visible
symptoms. A ground loop in an analog multiplex system (AMX-192
or CD-80) can result in steady flicker, but even a bad ground
loop in a digital multiplex system may not impair performance for
long periods of time (hours or days). From one standpoint this
is an advantage to digital multiplex... it is much more immune to
noise and ground loops. On the other hand, if a problem exists,
it is harder to pinpoint.
When hum exists in an audio system, one useful tool to isolate
ground is an audio transformer. The transformer passes the audio
signal through a magnetic field while breaking the electrical
connection. The DMX Optical Isolator provides a similar function
for a DMX512 control signal. It passes the lighting control
signal (using a beam of light) while breaking the electrical
connection.
If you suspect a ground loop is causing problems in your lighting
system, a DMX Optical Isolator, installed at the console, will
insure that no ground loop can be made through the isolator.
Make sure, however, that there is no connection to the metal
shell of your control wiring connectors. The isolator's shell is
metal and will carry current around the isolator's internal
isolation circuitry if a connection is made to the shell of the
connector (only pins 1, 2, and 3 should have any connections to
them).