Appendix B
93
Multiple devices on the same circuit should be grounded together at a single point.
Furthermore, power supplies and programmable controllers often have DC common tied to
Earth (AC power ground). As a rule, it is preferable to have the APEX615n Iso GND floating
with respect to Earth. This prevents noisy equipment that is grounded to Earth from sending
noise into the APEX615n. When floating the signal ground is not possible, you should make
the Earth ground connection at only one point.
In many cases, optical isolation may be required to completely eliminate electrical contact
between the APEX615n and a noisy environment. Solid state relays provide this type of
isolation.
Transmitted Noise
Transmitted noise is picked up by external connections to the APEX615n, and in severe cases
can attack the APEX615n when there are no external connections. The APEX615n’s sheet
metal enclosure will typically shield the electronics from this, but openings in the enclosure
for connections and front panel controls may leak.
When high current contacts open, they draw an arc, producing a burst of broad spectrum radio
frequency noise that can be picked up on a limit switch or other wiring. High-current and
high-voltage wires have an electrical field around them and may induce noise on signal wiring,
especially when they are tied in the same wiring bundle or conduit.
When this kind of problem occurs, you should consider shielding signal cables or isolating the
signals. A proper shield surrounds the signal wires to intercept electrical fields, but this shield
must be tied to Earth to drain the induced voltages. At the very least, wires should be run in
twisted pairs to limit straight line antenna effects.
Installing the APEX615n in a NEMA enclosure ensures protection from this kind of noise,
unless noise-producing equipment is also mounted inside the enclosure. Connections external
to the enclosure must be shielded.
Even the worst noise problems in environments near 600 amp welders and 25kW transmitters
have been solved using enclosures, conduit, optical isolation, and single-point ground
techniques.
Ground Loops
Ground Loops are the most mysterious noise problems. They seem to occur most often in
systems where a control computer is using RS-232C communication. Symptoms like garbled
transmissions and intermittent operation are typical.
The problem occurs in systems where multiple Earth ground connections exist, particularly
when these connections are far apart.
Ground Loops—
Noise Scenario
Suppose an APEX615n is controlling a motor, and the limit switches use an external power
supply. The APEX615n is controlled by a computer in another room. If the power supply
Common is connected to Earth, the potential exists for ground loop problems. This is
because most computers have their RS-232C signal common tied to Earth. The loop starts at
the APEX615n system limit switch ground, goes to Earth through the drive, and on to Earth
at the computer. From there, the loop returns to the APEX615n system through RS-232C
signal ground. If a voltage potential exists between drive Earth and remote computer Earth,
ground current will flow through the RS-232C ground, creating unpredictable results.
The way to test for and ultimately eliminate a ground loop is to lift or cheat Earth ground
connections in the system until the symptoms disappear.
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