Requirement for Grounding of the Gas Domes on the Roof of the Digesters

[Shahin Vaez]

I am the electrical engineer responsible for design of new Digesters at a Waste Water Treatment plant and I need your input on the grounding of the Gas Domes on the Roof of the Digesters. The gas dome is a metal structure installed on the roof of the concrete digesters, which houses some gas piping routed from the digesters to other location. The gas domes are included with 120V electrical equipment (lighting, receptacles,...). The
area where the gas dome is located is class 1, Div 1.

My question are:
1. Is grounding of the metal structure installed in a classified area a requirement, preference or not a good practice? Unfortunately I was unable to find a clrea ruling on this issue.
2. Can we ground the structure through the dedicated grounding rods installed in vicinity of the structure? If yes, how many rods (minimum) and what size of grounding conductor to be used? Please note that the distance between the supply source grounding and this site is more than 2000' and 1 ohm resistance can not be acheived.
3. As there are few digesters installed side by side, but with gas domes ( located in the center of the digesters) far apart, can we install separate grounding rods for each Gas Dome?
4. How should be the bonding between this structure, gas piping and the electrical equipment enclosures installed in the dome? My concern is to feed back the energy of the lightning through the structure to the small bonding conductor of the lighting fixtures for example..

 

[don_resqcapt19]

I don't believe that the National Electrical Code addresses the grounding of the parts in question unless it is likely that those parts would become energized as the result of an electrical system fault. The issue of lightning protection is a different subject and is covered by NFPA 780. This document will specify the required ground electrode system and other parts of a lightning protection system. It will also specify how you bond the lightning protection system to the electrical system.
Don

 

[nakulak]

recently we did some work for the navy installing aerial grounding rings, air terminals, etc., along with similar ground base grounding rings for buildings which simply cannot afford to be struck by lightning. I don't know where you find literature on this, but it works extremely well.