ISA Certified Automation Professional (CAP) program

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Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

CAP question

Electromagnetic coupling problems in the electrical installation of a programmable electronic device can be reduced by:

A. separation of signals by voltage level.
B. mounting to eliminate vibration and ground currents.
C. a cone of protection for all PE devices and related wiring.
D. wireless electrical distribution metering.

CAP Answer

The answer is A: "separation of signals by voltage level."

Electrical interference through electromagnetic coupling is caused by the electromagnetic field around one current-carrying conductor inducing a current in another conductor which is in proximity.  This is not an issue for a cable tray or conduit populated solely with 480VAC motor leads, but if an ethernet cable was routed in the same tray or conduit, undesirable currents would likely be induced in the ethernet (low-voltage signal) cable.

As a practice, all cables and wires in should be shielded and properly grounded.  To minimize or eliminate electromagnetic coupling, cabling should be segregated by voltage level in separate conduits or isolated cable tray segments:

  1. High amperage AC wiring (motors, heaters, etc.)
  2. 120VAC outlets and devices
  3. 24 VDC power wiring (power supplies and devices)

Low-voltage signal cabling (Ethernet, Fieldbus, etc.)

Reference: Trevathan, Vernon L., A Guide to the Automation Body of Knowledge, 3rd Ed., 2018