Ground Wires and MV Ductbanks

[charlie b]

I have received a question from the field regarding a specification requirement they want me to delete. It was part of the boilerplate spec, meaning that I did not write it, and I did not elect to edit it out. The requirement is to run a 4/0 bare copper grounding conductor above (or below, at the option of the designer) any medium voltage ductbank. This conductor is connected to the ground rings of any manholes or equipment pads (e.g., for a pad-mounted transformer). Before I answer the field question, can anyone shed any light on the following questions:

• What does this thing do for a living?
• What would it be called (i.e., it's neither an EGC nor a GEC)?
• Why would the spec say to install it outside the concrete encasement?
• Is this a common requirement, and would it only apply to medium voltage?

 

[publicgood]

I’ve seen requirements to bond shields up to 4 times, per mile. Maybe this is the use.

 

[paulengr]

This sounds like remnants of the requirement to run a wire over lines so that a cable locator works properly. Common in gas line specs. You can detect the shield though so not sure why it would be used in MV.

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[charlie b]

Thanks for the offerings. But I don't think we have it yet. The only things to which the 4/0 is bonded are the ground wires that are installed on the inside walls of any manholes or vaults that are along the MV ductbank's route. Also, the spec section does not speak of distances (i.e., do this every so many feet). So I don't think that could be the reason. As to being a locator aid, the spec section gave the designer the option of calling for the 4/0 to be run either above or below the concrete encasement. So I don't think that could be the reason either.

As I see things, there are two possible reasons for putting a chunk of metal (e.g., rod, pipe, or bare wire) into contact with planet Earth: (1) establishing an equipotential condition, and (2) assisting in the dissipation of a lightning strike. Regarding (1), I don't see why you would need the ground ring internal to one manhole to be at the same potential as the ground ring in a manhole that is 500 feet away. Regarding (2), since this wire is 2 feet and more below the surface, it won't impact how lightning behaves.

The word "ground" is also used in connection with establishing a fault clearing path. But I can't envision a type and location of a fault for which the upstream OCPD would require this 4/0 wire to be present, in order for it to see the high enough current necessary to cause it to actuate.

What am I missing here?

 

[charlie b]

OK. I have thought about it some more, and I found one interesting tidbit. The master versions of the specs are published in such a way as to provide the occasional "note" intended to help the designer make decisions on what to keep and what to delete. These notes are automatically hidden, when it comes time to print the specs. The section in question has a note that mentions that it is preferred to install this 4/0 ground wire under the ductback, in order to afford it some protection against damage. That tells me that the author(s) of that section are really interested in getting the 4/0 installed. It also confirms my belief that the wire has nothing to do with lightning strikes or being a tool for locating the ductbank from the surface.

The MV distribution system at this site, and at all others of which I am aware, is 3-wire. It does not include either a neutral or an equipment ground. That tells me that the only way a fault can be cleared is by passing too much current from one phase to another. There may well be relaying systems that can discern a single phase to ground fault and cause something to trip. But any such system would not take advantage of a "ground wire" that is run parallel to the ductbank.

My conclusion is that the only reason for the 4/0 is to establish an equal potential condition between the ground loops of the manholes and vaults along the run. I am still stuck with not knowing why that condition is important from any safety point of view.

Can anyone help me understand this?

 

[MyCleveland]

Counterpoise

Counterpoise

Charlie
Look over attachment, pg 2, 2nd Col, 2nd paragraph.

Oops...too large of a file.

Just google "Counterpoise Grounding".www.netaworld.org

 

[charlie b]

That Google search turned out a large number of hits. Is there a specific article I should look for? If you can find it again, please copy the web address and post that on your next reply. Thanks.

 

[MyCleveland]

That Google search turned out a large number of hits. Is there a specific article I should look for? If you can find it again, please copy the web address and post that on your next reply. Thanks.

https://www.netaworld.org/sites/default/files/public/neta-journals/NWfall10-Jowett.pdf

 

[Julius Right]

I think a suitable name it could be Ground Continuity Conductor.
See: IEEE 575/2014 6.3.3 Parallel ground continuity conductor

 

[paulengr]

Wait, no ground in the cable? Shields are really able to carry fault current. Sure it isn't an equipment grounding conductor? Or is it high resistance or ungrounded so the shields suffice?

Ground continuity conductors are totally different. For one thing they are insulated. For another they are small, usually #14 For instance. Second they connect only at the farthest point to your equipment grounding system with or without a diode or some other termination device. The ground continuity monitor puts a small test signal on the ground continuity conductor and monitors the equipment grounding conductor at a feeder breaker to verify a return signal. If it loses signal, it shunt tri0s the feeder breaker for the feeder and everything else it is protecting.

This is intended for high resistance grounding systems where loss of ground continuity prevents the ground fault protective relays from operating. This is important in a coal mine where everything around you is covered in and surrounded by combustible dust.

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[gar]

190713-0940 EDT

charlie b:

My guess --- it is to provide reasonably good earthing of conductive parts of the manhole to local earth. Thus, leakage current to these conductive parts would be less likely to produce shocking current from the conductive parts to local earth (others areas of the manhole). Done for human safety.

It takes only a small current, mA range (1 to 5 is detectable), to shock a person.

Whether the 3 hot wires are low impedance earthed somewhere or not there is leakage current via some means, and therefore the hot wires are in someway referenced to earth.

With respect to EGC circuits, if leakage current within a piece of equipment is modest, possibly up to 10 A, then the voltage drop on the EGC at the equipment relative to likely earth potential at the equipment may be only about 1 or 2 V. From 10 A and 0.1 to 0.2 ohms in the EGC.

But what happens when there is a dead short from a hot 120 wire to the EGC? It approaches 60 V.

.

 

[Julius Right]

In my opinion, since the grounding grid horizontal wire has to be minimum 4/0 -from mechanical point of view-this cable could be a sort of a connection between the both cable ends' ground grids. However, it has to be more connections between these grids.

 

[MyCleveland]

Charlie
Don't know if this was resolved for you...found this in a Power Systems text.