ISA Certified Automation Professional (CAP) program
Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.
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.
The correct answer is A. Common noise must be in phase between conductors, or it converts to normal mode or differential noise.
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).
Reference: Industrial Control Electronics, 2nd Edition (Prentice Hall, 1993.)