Correcting Noise Problems; Noise Suppression; Examples Of Noise Snubbing - Siemens SIMATIC TI525 Hardware And Installation Manual

APPENDIX B – GROUNDING GUIDELINES
B.4.2

Correcting Noise Problems

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 Suppression

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
Discrete input
module protection
RC type noise snubbing
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120 VAC
Load
C R
Figure B-2 Examples of Noise Snubbing
B-6
120 VAC
Discrete
input Load
MOV
Discrete input
module protection
MOV snubbing