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Reference DataWiring PracticesElectrical NoiseWhat is Electrical Noise?It is electrical signals, which produceundesirable effects in the electroniccircuits of the control system. Theterm “electrical noise” originatedwith AM radios when the extraneous“noise” heard in the speaker wascaused by lightning or other sourcesof electrical arcing. Electrical noisefrom all sources and its effects oncontrollers are very difficult to define,let alone give exact rules on how toprevent. Noise sensitivity is a functionof more recent electronic controllerdesigns. However, the majority ofnoise problems stem from crude wiringpractices and techniques whichallow “coupling” or the transfer ofelectrical noise into the control circuit.An outstanding resource for informationabout wiring guidelines (sourcefor this summary) is the IEEE StandardNo. 518-1982 and is availablefrom IEEE, Inc., 345 East 47th Street,New York, NY 10017; phone number:800-678-4333.When is Electrical Noisea Problem?Symptoms resulting from an electricallynoisy environment are difficult topredict. One <strong>com</strong>mon symptom is anerratic system, with no evidence of aproblem appearing consistently. Evenworse, the system may exhibit severaldifferent symptoms.Some other <strong>com</strong>monplace symptomsof noise-related problems are fluctuatingdigital indicators, blanked digitalindicators, control instability about setpoint, and outputs turning ON or OFFunexpectedly. Another “red flag” ofelectrical noise raises when high orlow limits trip with no limit faultcondition.Why is Electrical Noise Sensitivitya Problem?How accurately a controller candifferentiate between desired systemsignals and electrical noise is a goodindicator of its sensitivity to noise. Ingeneral, high power controllers suchas mechanical relays or mercury displacementrelays have low noise sensitivity,while low power controllersthat use electronic logic, especiallythose using integrated circuits, aremore sensitive to noise. The developmentof all electronic solid statecontrollers has improved the accuracyof control and expanded immenselytheir capabilities, but they are more<strong>com</strong>plex and operate at very lowpower levels. Electrical noise is morelikely to affect them because of theirlower operating power levels.Where Does Electrical NoiseCome From?Our industrial world is full of equipmentcapable of generating manytypes of electrical noise. A typicalnoise source is any piece of equipmentthat can cause or produce veryrapid or large amplitude changes involtage or current when turned on andoff.Noise Sources:• Switches and relay contactsoperating inductive loads suchas motors, coils, solenoids, andrelays, etc.• Thyristors or other semiconductordevices which are not burst fired(randomly-fired or phase anglefireddevices)• All welding machinery• Heavy current carrying conductors• Fluorescent and neon lights• Thermal voltages betweendissimilar metals that influence thelow voltage thermocouple inputsignal• Chemical voltage produced byelectrolyte action between poorlyconnected leads and interconnectcables• Thermal noise from increasedambient temperatures around thecircuit electronics• Noise could be introduced if thecontrol circuit includes the option ofa mechanical relay output and isused to switch high load currentsover two or three amps. Thispresents a significant source fornoise, including inductive noisefrom the coil and contact arcing,depending on how much power isbrought inside the controller.410