Waste water plant

[ryan_618]

I was contacted via email by a gentleman that had a few questions. I told him I would post his questions here, so that a broader range of replies could be given. Please share your thoughts, if you can, on the following questions:



I am the electrical design engineer for upgrade of Digesters at a Waste Water Treatment plant and I desprately 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 structure a good practice or not? Unfortunately the OESC is not very clear.
2. Can we ground the structure through a grounding rods installed in vicinity of the structure?
3. As there are 6 digesters installed side by side but gas domes located in the center of the digesters are far apart, can we install separate grounding rod for each dome?
4. How do we have to make bonding between this structure, gas piping and electrical equipment enclosures installed in the dome?

 

[831]

Apparently he has a path from the domes to the ground i.e. the rod question.

Is there a panelboard in the dome that's providing the 120V for the lighting, recpetacles, etcetera or simple a branch circuit?

Basically, I'm trying to see what distribution is in the dome and what distribution system is feeding the dome. To tie those grounds together is paramount and required (if I'm seeing this right).

I'm picturing a metal "cage" on top of a big conc structure. How does operations access the dome? Standard metal walkway (grating)?

 

[Pierre C Belarge]

I think he is confusing "grounding" with "bonding".

Does he want to ground them for the purpose of lightning protection?
Or
Is he concerned about bonding them for the purpose of facilitating the overcurrent device?

 

[Shahin Vaez]

Waste Water Plant

Waste Water Plant

Apparently he has a path from the domes to the ground i.e. the rod question.

Is there a panelboard in the dome that's providing the 120V for the lighting, recpetacles, etcetera or simple a branch circuit?

Basically, I'm trying to see what distribution is in the dome and what distribution system is feeding the dome. To tie those grounds together is paramount and required (if I'm seeing this right).

I'm picturing a metal "cage" on top of a big conc structure. How does operations access the dome? Standard metal walkway (grating)?

No there is no panelboard in the dome. Digesters are large concrete cylinders (40m radius) installed underground, and dome is located just at the grade level. There are six of them in 2x3 arrangement. The panelboards and MCCs are located at one end of the facility in an electrical room located above ground.

There are currently no ground rods beside the digesters, but my question is whether we need to install the ground rods or other type of grounding provision to tie in the dome to those or not.

 

[Shahin Vaez]

Waste Water Plant

Waste Water Plant

I think he is confusing "grounding" with "bonding".

Does he want to ground them for the purpose of lightning protection?
Or
Is he concerned about bonding them for the purpose of facilitating the overcurrent device?

My questions relate to both item.

1. Is there a requirement for bonding of these domes to the electrical system grounding because they are metal, located in class 1 div 1 area and contain some electrical installation under two different scenario:
- if the dome and electrical installation is totally isolated (meaning that fixtures have fiber galss enclosures and all cables have XLPE jacket with no metal connectors, etc..)
- if the dome is connected to the electrical installation (meaning that the fixture is from metal or there teck cables with metal connectore, etc...)

2. Is it a good practice to provide a separate grounding for the lightning purpose (through installation of new grounding rods in vicinity of each digesters, etc...).

 

[don_resqcapt19]

The NEC would only require that the dome be bonded if it is likely to become energized. See 250.4(A)(4).
The subject of lightning protection is not addressed in the NEC. You need to take a look at NFPA 780, the lightning protection code. There may be other standards that also address lightning protection for this application. If you do install lightning protection, the grounding for the lightning protection and the elctrical system would be required to be bonded together.
Don

 

[tom baker]

There is another NFPA standard that applies to WWTP its 820 Fire Protection for Wastewater, I haven't looked at if for some time, but it may have some requirements.

 

[Pierre C Belarge]

Is lightning a concern for the area the units are installed in?

I think you have just about answered your own question regarding Bonding, and can make an engineering decision based on whether or not "likely to become energized" is a reality.

As has been mentioned, there are other standards for lightning protection. Just as a note, UL has a lightning installation certification that may help you to determine the answer for your needs.

 

[QES]

You should ground all metal structure to your grounding grid.