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AutoQuiz: How Do You Avoid a Ground Loop on Shielded Cable?

AutoQuiz is edited by Joel Don, ISA's community manager.


Today's automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for information about the CCST program. This question comes from the Level I study guide, Domain 3, Troubleshooting. Level I represents a professional who has a five-year total of education, training, and/or experience.

A ground loop on shielded cable can be avoided by:

a) attaching a safety ground to the instrument casing
b) grounding the shield at only one end of the circuit
c) attaching a safety ground to the control panel
d) grounding the shield at both ends of the cable
e) none of the above

A ground loop refers to an unwanted electrical current in a conductor connecting two points that are supposed to be at the same potential (units are volts), but that are actually at different potentials (not grounded). Ground loops are detrimental to the intended operation of the electrical system.

Potential is the difference in voltage between two points. One of the points should be a ground with voltage equal to 0 (zero).

The best way to avert the ground loop in this question is to ground the shield at only one end of the circuit.

The correct answer is B.

Joel Don
Joel Don
Joel Don is the community manager for ISA and is an independent content marketing, social media and public relations consultant. Prior to his work in marketing and PR, Joel served as an editor for regional newspapers and national magazines throughout the U.S. He earned a master's degree from the Medill School at Northwestern University with a focus on science, engineering and biomedical marketing communications, and a bachelor of science degree from UC San Diego.

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