This automation industry quiz question comes from the ISA Certified Automation Professional (CAP) certification program. ISA CAP certification provides a non-biased, third-party, objective assessment and confirmation of an automation professional's skills. The CAP exam is focused on direction, definition, design, development/application, deployment, documentation, and support of systems, software, and equipment used in control systems, manufacturing information systems, systems integration, and operational consulting. Click this link for more information about the CAP program.
Common mode noise can be best defined as:
a) noise that appears equally and in phase from each current carrying conductor to ground
b) noise that appears equally and 90 degrees out-of-phase between each current carrying conductor to ground
c) noise that appears between the phase or signal and its return
d) noise that cannot be measured by conventional means
e) none of the above
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Answer B would be classified as normal mode noise since the noise is out of phase with each current carrying conductor to ground.
Answer C is the classic definition of normal mode noise, sometimes referred to as differential noise or metallic noise.
Answer D is not correct because noise can be measured by use of an oscilloscope, current transformer, or other conventional circuitry designed to measure noise (in dB).
The correct answer is A. Common noise must be in phase between conductors, or it converts to normal mode or differential noise.
Reference: Industrial Control Electronics, 2nd Edition