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Wednesday, September 25, 2019
Question:
Re: CQD answer published Thursday, September 12, 2019 - Fence Grounding
1) In reference to fence grounding at industrial facilities; if there are transmission lines nearby, running parallel to a long fence or pipe line, they can induce current into the fence or other metal structures. This current will seek a ground path in order to return to it's source. I have witnessed arcing between a non-grounded fence where it "almost" touches a grounded fence section. Daniel Cribbs
2)I am not sure about his facility and the reasons, but there are several reasons for the grounding of the fences. My guess from his description is that it is probably for Lightning protection.
A) Lightning arresting – Lightning can travel great distances in a metal fence if it is not grounded well. This could easily cause damage to people, structures or objects thousands of feet away from the strike.
B) Posts that only go down two feet may be in earth that has too high of resistance to effectively ground against lightning. In few soils will two feet be enough for proper grounding. Earth resistance measurements will be required to specify the Grounding rod length. “Rule of thumb” in this case may be due to previous ground resistance measurements, but may not hold true for earth 1000’ away. It is not a safe bet that adding grounding rods every 50’ would provide the proper protection unless tested. Grounding testing should be done on a regular basis to maintain the integrity of the protection, typically once a year for building protection, but maybe less often for fencing as long as there is a regular visual inspection for loose connections, mower damage, etc.
C) Metal fences around Medium and high voltage substations also require Grounding to prevent static and stray voltages from energizing the fence and have much greater Grounding requirements than that for lightning protection.
Thank you, Ratib Baker, CMS, RCT