APPENDIX B – GROUNDING GUIDELINES
B-6
Hardware
and Installation User Manual
B.4.2 Correcting
Noise Pr
oblems
When
potential noise sources are identified,
two general methods are available
to
correct them. These methods are noise suppression and noise isolation.
B.4.2.1
Noise Suppr
ession
You can use noise suppression (snubbing) to reduce noise at its source.
Applicable
only to devices driven by mechanical contacts, snubbing suppresses
arcing at electrical contacts caused by turnoff of inductive loads (e.g., relays,
motors, motor starters, solenoids, etc.). Load side suppression reduces voltage
transients at the load, preventing them from traveling back to the contacts
where
they would produce a showering arc and couple with adjacent wiring.
You
may achieve an alternative type of suppression by placing an RC or varistor
circuit
(shown in Figure B-2) across the contacts, parallel to the switch.
Both types of snubbing cause the physical devices to come on or go off more
slowly. The RC and varistor elements should have minimal effect on system
timing:
their time constants are substantially less than 1 millisecond.
Discrete
input Load
CR
Discrete input
module protection
RC type noise snubbing
120 V
AC
Discrete
input Load
Discrete input
module protection
MOV snubbing
120 V
AC
MOV
Figure B-2 Examples of Noise Snubbing
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