Equipment grounding of bare steel structure

[MRKN]

Hi,

I need to design an equipment ground for a steel structure (used for operator access to some piping and valves at the top of a ~50' vessel) which will be installed in an existing process plant. Actually we'll be grounding this structure at two separate points.

I know the structural steel will not extend 10' into the ground (this discounts possible credit for the framework as a grounding electrode itself), but am curious on the typical ways such a structure is grounded when installed in an existing location. I would propose connecting (via an approved means) a 2/0 Cu stranded wire to some particularly convenient steel beam and running it via PVC conduit to an existing nearby ground well (with ground rod). This is cheaper than the alternatives, which I see as: to run the EGC 24" underground to an existing ground well, or to simply create two new ground wells with ground rods on diagonally opposite corners of the structure right at the foot of the beams.

I can't find anything wrong with this in the code since the PVC would mechanically shield the ground wire from any damage... thoughts?

 

[petersonra]

Are you grounding it or bonding it? In either case - why?

Is the existing steel grounded? If so, would it serve whatever purpose you have in mind to just run the wire there?

It does appear to me there is a code requirement to do anything at all like you are suggesting.

 

[MRKN]

I believe I am grounding to stabilize voltages and prevent buildup of static electricity (lightning is not an issue here) (I am not bonding so as to provide a low-impedance return path for ground fault current so the OCPD will operate)

However, the only code requirement I can find refers to it as bonding, NEC 2011 250.104(C):
Exposed structural metal that is interconnected to form a metal building frame and is notintentionally grounded or bonded and is likely to becomeenergized shall be bonded to the service equipment enclosure;the grounded conductor at the service; the disconnectingmeans for buildings or structures supplied by a feederor branch circuit; the grounding electrode conductor, if ofsufficient size; or to one or more grounding electrodes used.The bonding jumper(s) shall be sized in accordance withTable 250.66 and installed in accordance with 250.64(A),(B), and (E). The points of attachment of the bonding jumper(s) shall be accessible unless installed in compliance with250.68(A), Exception No. 2.

I suppose then that by just running a wire from a beam to the nearest grounding electrode that I am "intentionally grounding" the structure, and exempt from the above requirements?

I assume the existing steel is grounded, if by that you mean the vessel itself- however that is process equipment and I have just assumed we should not count on that for any type of credit. That we ground at 2 separate points is an internal company requirement.

 

[SparkyHC]

ground grid?

ground grid?

This sounds like a ground grid frequently seen at chemical plants and similar locations...one site
had #4/0 copper about 24" deep in a grid that covered the entire plant (a few acres) and was bonded to all the buildings
and structures...(lots of nasty chemicals)

You might contact an engineer for more review as a 50' high steel structure is most certainly
a lightning risk. What is contained in this structure? Is there anything on site that caused you
to think about grounding / bonding?

 

[GoldDigger]

... as a 50' high steel structure is most certainly
a lightning risk.

Based on the OP's initial description, I see this as a framework erected inside an existing closed building structure, but that may not be the case.

 

[don_resqcapt19]

I believe I am grounding to stabilize voltages and prevent buildup of static electricity (lightning is not an issue here) (I am not bonding so as to provide a low-impedance return path for ground fault current so the OCPD will operate) ...

You don't "stabilze voltages" except at their source so that is not a good reason to bond. A path between then metal parts and or the earth of 1,000,000 or less ohms will prevent the buildup of static, so that is also not a likely reason to bond. The code will not require bonding of the steel unless it is likely to become energized and I really doubt that is the case.

Is there any electrical equipment on the steel? If so what is the wiring method?

 

[MRKN]

The only electrical infrastructure on top of this structure will be the 120V area lighting from a 208/120V step-down transformer nearby- other than that it will be piping, valves, pressure relief devices, etc. It is just so operators/maintenance have access to some new relief system piping being installed on the top of the vessel. It is exposed to atmosphere, but there are much larger structures/reactors/vesseles in the immediate vicinity. Regardless I have contacted a senior engineer to confirm we don't need lightning protection.

We do have a ground grid here and typically they are installed 24" beneath the ground, accessible via the ground wells, surrounding the plant. Of course there are no ground grid drawings for this particular plant however! Our standards do dictate "bonding" all structures as you have described to this ground grid.

Does this help clarify?

 

[don_resqcapt19]

...
We do have a ground grid here and typically they are installed 24" beneath the ground, accessible via the ground wells, surrounding the plant. Of course there are no ground grid drawings for this particular plant however! Our standards do dictate "bonding" all structures as you have described to this ground grid.

Does this help clarify?

Typical standard ...just do what the standard says...this is not covered or required by the NEC and there is no real reason, other than the "standard" to do it.