Ground Ring

[tom baker]

A ground ring per 250.52(A)(4) [encircle the building, min 2 awg] is commonly called out at telecommuncations sites, etc.
Whats the theory behind a ground ring?

 

[bphgravity]

Other than providing a convenient means of connecting equipment to the electrode system where multiple systems enter a building, and perhaps providing lower ground resistance than a few of the other approved electrodes, they are only as effective as any other electrode.

For some applications, they can act as a counterpoise.

 

[subtech]

Hello Tom
A ground ring is an abbreviated form of what power companies and others call a grounding network or "grounding grid"
Its benefits are many and the reasons to install them are numerous and some aspects are complex. I'll try to hit the high points.

1. A ring provides a common point for all other grounding conductors to tie to. IF the ring and its attached components are of a sufficient size, then during a major electrical fault or even a lightning strike, all attached equipment rises to the same potential. If there is no difference in potential between the equiptment in one panel and the panel next to it, then no current will flow between those two devices and the risk of damage is minimal.

2. Single point grounding is a common method of avoiding "loops" or parallel grounding paths in things electronic in nature. Looped ground paths are a nightmare and can be a personnel hazard. The large grounding ring (again, IF it is of sufficient size and capacity) provides for the common tie point and helps to avoid the dredded "loops".

3 In lightning protection systems, the more copper conductor, ground rods, and real estate you have involved in your system, the more likely you are to survive a major lightning strike.
Here in the Midwest, to effectively dissipate the energy of a lightning hit,
we commonly use thousands of feet of 4/0 copper conductor buried to various depths.(depends on the engineers preference) Also many dozens of ground rods. (usually 3/4 X 10 ft) Several thousand square feet of land area is utilized to construct a "grid" so the those BILLIONS of watts of energy can be safely dissipated into the earth.

That is why I referred to the "ring" as an abbreviated ground network.
Mind you, a ring is much, much better than just a couple of ground rods sunk 6-8 ft. apart, but much better systems can be constructed that will safely handle some god-awful lightning shots.

Well, that is just a drop in the ocean when it comes to grounding. I could go on, but I'll spare you.
Hope this helps
Regards
Mike

 

[petersonra]

2. Single point grounding is a common method of avoiding "loops" or parallel grounding paths in things electronic in nature. Looped ground paths are a nightmare and can be a personnel hazard. The large grounding ring (again, IF it is of sufficient size and capacity) provides for the common tie point and helps to avoid the dredded "loops".

I am curious. Please explain to me how parallel grounding paths can cause a personnel hazard, or for that matter create any kind of nightmare.

Parallel ground paths are inevitable and cannot be eliminated. Think about large factories with metal piping systems and steel structures and rebar in the concrete all over the place. A ground ring just makes this situation "worse".

What is a hazard is when the neutral leg of a power system is grounded at more than one point, causing current flow on the EGC. The ground ring does absolutely nothing to solve that problem.

 

[subtech]

Hello petersonra

Okay, here goes.
In substations where I do most of my work, parallel and multiple ground paths in current transformer ckts. that are connected to protective relays are a scourge. Recently, while I was working on control wiring problem in a substation, a 13 kv feeder that originated in this same sub experienced a hard ground fault. Because the neutral conductors in several ct ckts had been connected to earth ground in multilple locations, a portion of the ground fault return current appeared in the neutrals of these ct circuits.(these neutrals were in effect "in parallel" with the grounding grid) This resulted in a breaker failure relay operation at the 161kv level which dumped part of a large substation. Half of the 13 kv switchgear was also lost due to a differential operation which caused loss of auxilliary operations in a nearby power plant. Two units were taken off line as well as the plant itself going partially dark. In the resulting scramble to find out just what happened, one plant electrician was injured in a fall. All of this from one or two "mis-applied" grounding jumpers.
Another incident involving Pt's and a metering circuit resulted in some damaged digital revenue metering equipment.
Most all literature dealing with protective relays, metering equipment, communications gear warn against multiple and parallel earth connections.
Also, in the electricians or wireman's world, doesn't the NEC mandate that there will only be 1 earth ground connected in a main panel entrance? (I'm not an electrician so my code knowledge is very limited)
I can go on about a substation tech friend of mine who was working on a corner grounded delta secondary and found out the hard way that it was grounded in more than one location(same corner of the delta, different earth ground jumper).
Then there was the flash fried aluminum relay rack that turned out to be in series with a large paralleled run of 4/0 ground grid conductor that turned out to be a favored path of return current for a 69-13 kv 30 MVA power transfomer. The run of 4/0 turned out to be one of only two surving runs left to the X0 bushing after some digging was done in the area.
Then, there was the time...........................
Okay, I'm getting carried away. However, parallel grounding paths placed in the wrong places can cause all kinds of problems and end up getting un-suspecting workers injured. Unfortunately, I've seen it. If you require any more proof of this, I'll be glad to converse with via email. Let me know.
Regards

 

[petersonra]

I noticed in your missive you stated that a neutral wire was grounded in two points. That is generally prohibited because you end up with current flow on the EGC. I mentioned in my previous post that this is a big no-no.

There is nothing inherently wrong with grounding in parallel, as long as the neutral wire is connected at only one point.

 

[subtech]

Oh, by the way, when part of the current that should flow back to the panel in the neutral conductor flows back in the EGC..........uh, doesn't that result in a gfci breaker trip? Thought so.

Regards

 

[jtester]

Quote by subtech
"In substations where I do most of my work, parallel and multiple ground paths in current transformer ckts. that are connected to protective relays are a scourge"

While I agree that instrumentation doesn't always operate well with multiple grounds, utility systems generally operate with thousands of grounds, each causing the "dreaded loop". There is no inherent danger in multiple grounds if properly installed, utilities do it every day.

second quote
"uh, doesn't that result in a gfci breaker trip? Thought so."

Only if there is a gfci breaker or receptacle.

Jim T

 

[winnie]

Hmm. Keep in mind that the _rules_ for power distribution systems are rather different than the NEC _rules_.

Multiple earthing of distribution systems is the norm, and this most certainly does result in 'parallel' current flow through the earth.

I presume that subtech is talking about multiple metallic paths between grounded elements of the system.

Any connection between the grounded conductor system and other conductive materials will result in a parallel path, which can either result in nusiance tripping _or_ desensitization of ground fault detection equipment, depending upon the particular hardware involved.

Back to grounding rings: I've wondered if a grounding ring would act as an 'equipotential shell' to some extent, not only providing the necessary grounding electrode, but also 'shunting' any earth currents around the encircled structure.

-Jon

 

[dave_asdf]

Grounding rings in data centers

Grounding rings in data centers

in a perfect system all current flows on the neutral, none on the EGC, the N-G voltage is minimal and G voltage is the same everywhere. However, in data centers 1. the best equipment isn't always used 2. power supplies can leak a little current to ground. 3. there are many seperately derived systems (UPS and xfmr) and 4. AC and DC power is used through out the floor. All these components can change the G voltage to earth in different areas of a data center. the idea of a grounding ring or grounding grid as some use is to provide a low impedance path for all the equipment in the center to connect. The data lines which connect all these pieces of equipment also have a ground in them, and some a shield. having a low resistance parallel path helps prevent data loss due to G voltage inconsistancies. I guess i thought of the grounding ring as a means to level the ground voltage gradient in different portions of a large area.

 

[bphgravity]

The code promotes the flow of normal operating current on the gorunding system. By permitting the reduction in the size of the service grounded conductor and by encouraging the lowest resistance (impedance) possible for the grounding electrode system, one should expect current to flow were it's really not supposed to be.

Ohm's Law proves this nicely. Raise the impedance of the grounded system while lowering the impedance of the grounding system, less current will flow on the insulated parts and more will be seen on the exposed metal parts.

 

[dereckbc]

I wished I had seen this earlier, maybe could have steered the topic in a different direction. Since I design a lot of protective grounding systems for telephone offices and write the standards, I can tell you the reason(s). Let's just start with the main reason.

In telephone switching and data centers you have multiple entry points along the perimeter of the building. At each entry point we install what we call a cable entrance ground bar (CEGB) which is connected directly to the ring beneath it. The CEGB facilitates connecting discharge devices and terminating cable sheaths that enter the building from the outside plant. This allows the shortest possible to the ground electrode.

The notion that it eliminates loops is incorrect. In fact we design as many loops as possible to minimize impedance between any two given points. Minimize the impedance, you minimize any voltage. However we do use Isolated Single Point Grounding for power supplies and equipment frames for some equipment, but earth has nothing to do with that other than a reference point in time and space.

 

[dave_asdf]

All these components can change the G voltage to earth in different areas of a data center.

The word earth in the above statement was meant to be exactly what you said, a reference point. What i was trying to say is the ground voltage in comparison to "earth" can vary throughout the floor, grounding rings just supply a very low impedence parallel path(s) to keep these voltage differences down therefore preventing them from interfering with data transmission. essentially they don't get rid of grounding loops they just take the loop off transmission lines and put it on the grounding ring (/building steel/floor pedistal grounding/under floor grid/where ever you can connect to something metal).

i think we are talking about the same thing but if i'm way off base please let me know

ps: i don't know how to spell chek so there's probably a million mispelld words. i'll just chalk-up as typos...

 

[tom baker]

All: We need to be carefull here and make sure whos rules we are discussing. A ground ring for a substation or a building supplied by a service under the NEC?
The utility can have a multi point grounded neutral but under the NEC we can't.

 

[dave_asdf]

potatoe po-ta-toe

potatoe po-ta-toe

whoopso, the grounding ring i was talking about is different than the one discussed in 250.52. The one that i was talking about is inside and is kind of a supplemental EGC of sorts, not the grounding electrode discussed in 250.52. guess i should open the book before i open my mouth!