Cutler-Hammer D320 PLC, User Manual

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Chapter 8: Troubleshooting Noise Problems
187
Methods of Handling Large Voltage Spikes Such as Lightning
Surge Absorber
•
A surge absorber reduces the electrical shock to the PLC by taking high-voltage spikes to
ground. Attach a surge absorber in the power line in front of the PLC to prevent damage from
lightning. The surge absorber will clamp the unwanted high voltage and prevent it from
flowing to the PLC power supply. When specifying a surge absorber, the present wiring
system must be carefully reviewed. Some surge absorbers are designed to be placed into the
main power distribution panel while others are designed to be installed in the field close to the
PLC. It is always best to place the surge absorber as close to the PLC as possible.
•
Surge absorbers can consist of either series resistors with capacitors that will couple the spike
to ground, or Zener diodes that safely clamp the high voltage spikes or MOVs (Metal Oxide
Varistors). Some surge absorbers will need replacement after they have suppressed a spike
(similar to a fuse). Others can be reset. In specifying a surge absorber consider how often the
surges are occurring and the maximum amplitude in volts or joules.
Some typical surge absorbers are listed in the following table. For actual installation and
application details, refer to manufacturers manuals
.
Model Name Specifications Manufacturer Remarks
CHSA 470 V Cutler-Hammer 120/240 V power
CHSA01 490 V Cutler-Hammer 120/240 V power
Burying Wire
•
Cabling that is strung from pole to pole in free space is an antenna for lightning. When
possible bury the cable underground. The earth acts like a shield and absorbs most if not all of
the lightning induced noise signals before they are able to reach the cable.
Surge Absorber
Main breaker
Breaker
AC input
Ground
terminal
U
V
AC power
Transformer
Triple ground (Less than 2
Ω
of ground resistance)
Special ground (Less than 2 Ω
of ground resistance)
It is needed to prevent high voltage
noise such as lightning
PLC