March/April 2012

ISA Certified Control Systems Technician (CCST) program

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

When bundling and running cables in conduit and duct, the listed conductors are normally separated in different duct or conduit runs based on signal type. Which type would normally be shielded?

A. Power wires

B. Signal wires

C. Ground wires

D. Control wires

CCST answer

The correct answer is B, signal wires. In control systems, signal wires are comprised of analog device wiring (such 4-20 mA), reference signals (typically 1-5 VDC or 0-10VDC), and communication signals (low-level VDC or mA). All signal wires carry information over two or more wires in which the voltage level or current levels are varied to represent the information that is being conveyed. The cable shield, connected to an isolated zero potential, is required to prevent nearby inductive loads and EMF fields from altering the low-level voltage or current levels in the signal wiring.

Answer A is not correct. Power wiring requires the power source to be referenced to a common point (ground), but does not require the conductors themselves be shielded. Power wiring is not typically adversely affected by other inductive or EMF forces.

Answer C is not correct because ground wires are connected to earth potential and would not benefit from further shielding of the wire.

Answer D is not correct because control wiring (typically 120VAC or 24VDC) typically conveys on/off (high/low signal level) information, which does not require shielding. However, one conductor of the control wiring is usually referenced to a common or reference potential.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems2nd Edition