Lightning Protection & Telecommunication Grounding System

According to NFPA 780, all grounding media in or on a structure shall be interconnected to provide a common ground potential. This includes lightning protection, electric service, telephone and antenna system grounds, as well as underground metallic piping system.

Yet, I have gone through one project where the designer used the independent grounding systems. Each grounding electrode system has its own ground resistance from 1 to <10 ohms. Until now, I still have a doubt on it.

But, to interconnect the electrode system to make a common grounding system, we will have only one grounding resistance where there will be different requirements for different application such as electrical service, telecommunication service (< 5ohms), lightning protection service (<10ohms).

In general, common grounding system is mandatory. The question left is how to compromise those requirements in one source of grounding?

A common phrase in the NEC is "or authority having jurisdiction". This means that the final decision is left up to local authorities.(Not always the best informed.)

If your method of grounding is approved or dictated by the inspector, it does not matter, from a liability standpoint, at least, which you use.

You can still annotate your objection in his notes if you wish.

It is a safe bet to send an email to the inspector, and in a friendly way, detail everything that he required you to do.Nothing ever dies if it is on the "Net", so there will always be a digital fingerprint of your correspondence.(CYA)

From an engineering point of view,I favor single point ground system, and "isolated", but not "separate" grounding of sensitive computer equipment;eg: orange receptacles.

I have heard much argument about isolated grounds "all going to the same place, so what difference does it make?"

I will attempt to explain:

Consider a network of rural country roads, all going to the same destination.This is analogous to a typical conduit system in an industrial environment.There are connections at every coupling, fitting, and junction box.Loose connections can induce spurious voltages on conductors within the conduit.

Now consider an interstate highway system,with only emergency traffic allowed, going to the same place.A very direct and simple path with no interference or dependence on other connections.This is analogous to an isolated ground.

It is easy to see the difference.

It is important to remember that the grounding systems described in the NEC are for low frequency systems.

An effective ground at 50 or 60 hz is not necessarily a good ground at 1 Mhz.There are inductive and capacitive factors that come into the equations and are too lengthy to go into here, but if you want empirical proof, connect a balun transformer from a portable radio antenna to an "I" beam in a building.The building structure is a very good antenna.

This is another reason why lightning is so hard to control.The frequency is nearly infinite at the beginning,and consequently, so is the magnetic field, and it oscillates downward in frequency through the whole spectrum as it establishes an ionized path, and oscillates back and forth from earth to cloud, and back.

Then there is the EMP generated from a strike, so unless everything has a Faraday Shield, there will be a potential for damage.The best we can hope for is the attempt to harness the dragon.I doubt we will ever slay it.

HTRN ( A licensed Unlimited Electrical contractor since 1987)

I worked in two industrial situations where we kept lightning grounds separate from equipment / safety / neutral / "building" grounds. In mining, this was required by law. In steel mill power distribution, I'm not sure whether it was required by law or not.

In mining, although I don't think anyone ever gave me an authoritative explanation, I'm sure the reason for the separate grounds was to minimize the chance of, for example, carrying a lightning stroke into the underground portions of a mine and its equipment.

Things that were grounded to the lightning ground (for example, lightning arrestors and the static line on incoming power lines) were always higher and thus (hopefully (I know)) more likely to be struck by lightning than the neutral and such that were lower (in elevation) than the static line and lightning arrestors.

Thus, the intent was (again, I'm sure) to bleed off lightning strokes to a dedicated lightning ground, and minimize the voltage rise in the other ground system. (Lightning and the other ground system "wells" had to be separated by at least 25'.)

I would expect the situation to be similar and more critical in low voltage communication systems--get rid of the lightning somewhere remote from the actual communication equipment--minimize the voltage rise that occurs on the frame, the neutral, and whatever else of the communication equipment.

Rhkramer:

There is the funny thing that this happens not only in mining, that the contractor prefer to make the lightning protection grounding system away from the other grounding. This can be explained by looking at the lightning grounding system design drawings, most of them will be a separated one while we all know that it is best to keep the equipotent within the facility.

Even in some best practice books written for telecommunication grounding, they suggest that there should have a common grounding reference. The grounding reference can be established by connecting electrode grounding systems together.

But if standard says we should have a common grounding system, how we can coordinate the requirement of difference applications?

Pl. search for my paper "Earthing of Sensitive Electronic Equipment" in the net.

Now, to plain answer your question:

1. vide IEC 60364-7-707:

The use of independent, "isolated" earth electrodes for computer or

electronic systems is not recommended/permitted.

2. vide Cl. 9.10.12.2 of IEEE 1100, "the use of any separate, isolated, insulated, dedicated, clean, quite, signal, computer, electronic or other such improper form of earth grounding electrodes for use as a point of connection of the EGC (Equipment Grounding Conductor) is not recommended".

I get into this all of the time with contractors, electricians and engineers. We try to explain that we use an isolated ground, but the negative connotation between what they hear and what they know severly impairs our conversations.

An "isolated" ground using the language of the NEC is specifically allowed. However, the English definition of the word "isolated" does not apply to the NEC definition. An isolated, single point or technical ground can all essentially mean the same thing. It may all depend on the publication and subsections within which are referenced at the time.

I just came from a conversation with yet another electrician and tried to explain that we do not use the commercial/hotel/building ground for our electronic systems. We use the isolated/single-point ground. This carries a myriad of isolation techniques for power and signal that seem to boggle the mind of the traditional commercial and residential electrical technicians.

Our lightning protection scheme is quite unique as well, since we are mostly concerned with lightning strokes to the ocean and earth near our facility. We have no lightning protection for the facility itself, yet the facility grounds are protected with surge arrestors and such through a protection panel for those lightning strokes.

Conservations about grounding do tend to be difficult at times.

Re: I just came from a conversation with yet another electrician and tried to explain that we do not use the commercial/hotel/building ground for our electronic systems. We use the isolated/single-point ground. This carries a myriad of isolation techniques for power and signal that seem to boggle the mind of the traditional commercial and residential electrical technicians.

Are you talking about what I (and some others) call tree type grounding, intended to avoid ground loops?

If so (or even if not), is the final connection to a "ground well" to the same ground well(s) as the building ground system?

We utilize a single-point grounding system. It has been referred to as an isolated grounding system by various entities for over 40 years. All technical systems are isolated from the facility station ground and utilize a dedicated grounding connector to any one of various ground plates throughout the facility. All ground plates are connected either the the master ground in a dedicated space, the ground room. At this point and this point only, in the ground room, a connection is made to the facility ground through structural steel. All structural steel is welded specifically not only for mechanical strength, but for low impedance.

Ground is not delivered via the technical power panel. Ground is only delivered via the isolated/single-point/quiet ground. Sub-panels from the main separately derived service do not carry a ground in order to avoid "accidental" connections between facility ground and/or "accidental" neutral/ground bonds.

Thanks! (It does sound like what is sometimes called a tree-type grounding system.)

rhkramer:

Tree-type grounding system? Could you tell me a little bit more on that?

Hmm, I wanted to find a link, and there are some, but I haven't found a really good one, so far. cuba_pete's description in post #16 is of what I call a tree type system.

I guess what I call a tree-type system may be more commonly known as a star or radial type grounding system. Some of the links I did find for tree type system were long US military PDFs, and I didn't want to read through them to find a good description.

In the end, each piece of equipment (each thing that needs a ground) is grounded with one and only one path to ground. If there are instrument / signal cables carrying a shield or ground, that shield or ground is grounded at only one end, thus not providing a(n additional) ground path to a piece of equipment (i.e., no ground loops).

I should provide more description, but I guess I'm getting old. ;-) If I do find a good link, I'll post it later. Here is something that I did find:

• IEEE Recommended Practice for Powering and Grounding Electronic Equipment (mentions star grounding rather than "tree grounding, iirc")

The instruction for MIL Handbook 419A(B) is MIL-STD 188-124. This is one of the guidelines we utilize in the Navy.

Thanks!

I found that at MIL-STD-188-124 B and skimmed a little bit of it. I was surprised to find the following paragraph, which doesn't sound like tree-type / radial / star grounding, but I may be taking it out of context:

5.1.2.1.1.4 Overall Shields Shields surrounding a cable containing individually shielded lower frequency signal lines or containing only unshielded lines shall be grounded at each end and at junction boxes, patch panels, distribution points and at other intermediate points along the cable run. Overall shields shall be grounded to cases, cabinets or conducting surfaces.

??

rhkramer:

Cases or cabinets, in their turn, will be connected to one common ground to make a start grounding. The purpose of shields connected to cased is to conform to EMC requirement.

Thanks for the reply, but my concern relates more specifically to the idea of grounding the shields at both ends (Shields surrounding a cable containing individually shielded lower frequency signal lines or containing only unshielded lines shall be grounded at each end and), which is not what is done in a tree type grounding system.

Thus I'm confused about whether the document as a whole is describing a tree-type grounding system (I haven't, and almost certainly won't try reading the entire document). Maybe if I did read the entire document I'd find that the document describes several types of grounding systems, only one of which is the tree-type system, and further, that the quotation above comes from a section describing a different type of grounding system.

That is just the shield, not ground wires or current carrying conductors. In HF (30-300kHz or above according to the MIL-STD) both ends of shields are grounded. In LF only one end of the shield is grounded. It depends on which type of interference you are most concerned with.

HF facilities would likely use the equipotential (if it's metal, ground and often) ground plane. LF would likely use the single-point ground.

We utilize shielded twisted pair to carry our signals. The shield is grounded at one end, normally the source of the signal.

And yes, the manual and MIL-STD goes into several methods of grounding. It allows each facility the option to utilize a ground system which suits their needs. In the gov't world, MIL-STD takes precedence over NEC, so some differences may be noted.

Thanks for the followup!

In reply to your other reply (#26), I've only ever used the one-point ground as you call it, and quite a ways above 300 kHz.

Do they approve the TEC(transient earth coupling)which connects lightning earth to other earths during a lightning strike?.

The resistance should then be ≤ the minimum of the resistances specified for each individual requirement, thereby satisfying all.

Here is an excerpt from the National Electrical Code:

"The reason for and the necessity to bond individual grounds or ground electrodes is simple. Soil is an extremely poor conductor and lightning energy that is conducted into it generates rings of voltage potentially around the point where the lightning strikes the Earth. Ground rods in different locations can be thousands of volts apart from each other. If these rods are not solidly bonded, this voltage potential may attempt to equalize in the piece of equipment where there are two grounds, or over the conductors between them. In the most basic installation, the most common examples of this are phone and modem damage or cable TV tuner damage. In the world of microprocessors, this is damage to networked equipment coming in on the data ports. This action is referred to as a difference in potential or a ground loop. "

For further information, google: NEC,Grounding and bonding.

HiTekRedNek:

The NEC says so, but what is the message you want to deliver?

The OP asks for an explanation of the reason for a common grounding point, as opposed to many grounding points.

I think my reply answers that question in a very simple manner.

If it is confusing to you, perhaps because of translation errors, I apologize and will gladly attempt to clear up any confusion if you will identify the confusing parts.

Oh, it is quite clear.

Just remind the OP that he should not bond the lightning protection grounding and telecommunication grounding above ground.

I found your question somewhat confusing?? I`m not a technical person, or even one with a higher education than most. But, I am a ham radio operator with some hands on exsperience,and this is the way I do it. All my antennas are grounded through a double throw switch. When the storm hits, I unscew the coax, and throw the switch from transmitter to ground.This goes outside of my radio shack to a stake driven 8 ft. into the ground. Since earth is a poor conductor, I use Holeplug Bentonite in the bottom of each hole about 12 inches deep. Then you add water, and stir. This stuff looks like fine chat for your driveway, but with water, it dissolves to a snot like consistency.It never dries out. I have a piece of PVC pipe in the hole also. I fill in the hole, and in August I can dump some water down the pipe to freshen it. This is the best ground know to man. I do the same for my ham radio transmitters, which are all old tube radios and receivers. Each has its own ground. I have never lost a radio, or antenna yet. My ham gear works really good with a really good grounds. Just food for thought. You might also check out ARRL forums on grounding. KC0VEA

NFPA 780: Standard for the Installation of Lightning Protection Systems clearly states that, "All grounding media in or on a structure shall be interconnected to provide a common ground potential. This interconnection shall include lightening protection, electrical service, telephone, and antenna system grounds, as well as underground metallic piping systems."

Do you have any new idea since this was posted already?

Hi, Shafikhan; you can grounding both lightning and Telecom systems, but it´s necessary to use HF coils to block out the high frequency signals coming from lightning system, Common grounding systems of course is more secure & economical than separated grounding systems.

I personally adhere to the NEC,( a single point common ground),but no matter which grounding method you use, it is important to have the grounding system tested at a prescribed interval.A lightning strike can oxidize or glaze the surface of a ground rod, making it ineffective due to high resistance.

I do believe there is value in an isolated equipment ground for computer systems due to the sensitivity of the electronics.This ground is still going back to the service mains ground, so it is not truly "isolated", as in separate ground rod, but is more aptly described as a "dedicated" ground.This type of ground is free of the spurious grounds of hundreds or even thousands of conduit connections that are in parallel\series with each other and ambient conditions, such as moisture,vibration, chemical corrosion etc. that can make mechanical connections at junction boxes and conduit connections unreliable.

If using MOV's for surge protection, put a fuse in series with the MOV.If it activates and goes shorted, the fuse will blow.A neon lamp(in series with the proper resistor) across the fuse will give a visual indicator of the MOV status.If it is on, the MOV has been activated and should be replaced.There are fuse holders with a built in indicator lamp made for this purpose.

Hope this helps.

Just to stir the pot a little, let's not forget "Driven Shields" which are sometimes used in instrumentation signals.