Unwanted electrical signals are called noise. They include all power signal distortions that cannot be classified as harmonics, notching, or transients. Noise can be divided into two categories based on where the noise is present. If the noise is present between reference, phase, or neutral to ground the noise is called common mode. If the noise is present between phase to phase or phase to neutral the noise is called normal mode. (Example)


Noise most often is caused by loose connections in power circuits.  RF equipment for induction heating circuits for metal or using bonding agents that are not shielded properly can cause unwanted noise as well. Power electronic devices, control circuits, solid-state rectifiers, and switchgear may also introduce noise into the power system. One common cause of noise is universal brush type motors that are commonly found in such equipment as vacuum cleaners and blenders.


Noise confuses logic systems, such as microcomputers and programmable controllers, by ‘hiding’ the true signal and providing false positives to the device. Noise also can cause equipment lockup and error in data streams.


Noise may be easily seen on an oscilloscope with a minimum scanning frequency of 200 kHz.


Proper single point grounding, filters, isolation transformers, line conditioners optical coupler or optical isolators for communication lines

Best Source(s) of Information

“Computer Electrical Power Requirements” by Mark Waller

“PC Power Protection” by Mark Waller