Grrrounding!

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E.C.

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I am installing a video/data link for our helicopter and a relay to our main office. They will be on top of a 22 story building downtown. There is a lightning protection system to bond the 10' aluminum masts. Inside the penthouse is a 120v ups circuit that fed old eq and the ups panel is located several floors down. There is a MCC 277/480v in the penthouse. The eq rack and it's components have to be bonded but this place has no 'building ground'. I know the grounding should be done at the service entrance and proceed from there. That's 22 floors down. Should I bond to building steel? Or the ground bus in the MCC? I'm worried about making a loop between the MCC and the UPS panel, especially with the difference in potential. I know alot of this rack stuff the green with lands on the eq case. The Ups is hardwired neutral and ground so it is not a sepratley derived system. Thanks in advance for any help you guys can give.
 

dereckbc

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Location
Plano, TX
I can help you out, but I am not clear what kind of ground you are looking for.

The best ground in any high-rise building no matter which floor you are on is Building Steel. Now with that said here is how Telco's and Wireless Carriers do pent-house grounding.

  • Run a bonding jumper from the nearest building steel located on the same floor as the equipment being used to either the case of/or MCC ground buss.
  • Bond antenna mask, coax shield, and antenna protectors to LPS which should also be bonded to the building steel.
  • Run a bonding jumper from building steel to the UPS case of ground buss.
  • Supplemental grounds to equipment racks are sourced to building steel.
 

brian john

Senior Member
Location
Leesburg, VA
Dereck:

Assuming an all concrete building do you feel if exposed rebar is available this would be adequate for you grounding?

I have seen the cellular guys in an all steel building want a 750 kcmil run all the way down. Never understood this use of copper.
 

dereckbc

Moderator
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Location
Plano, TX
brian john said:
Dereck:

Assuming an all concrete building do you feel if exposed rebar is available this would be adequate for you grounding?

I have seen the cellular guys in an all steel building want a 750 kcmil run all the way down. Never understood this use of copper.
Rebar yes if it is bonded to the service, as it serves the same purpose as BS.

I am also guilty of running a 750 MCM riser cable myself in my ignorant youthful days. Now that I have 30 or more years under my belt, never again. In fact this practice can be very dangerous if the Telco or Carriers uses what we call Isolated Single Point Grounding, or Isolated Ground Zone (IGZ) using a DC battery plant.

To briefly explain in an IGZ we install what is called either a Master Ground Bar (MGB) or a Ground Window Bar in close proximity to the DC power plant. The DC plant, and all the equipment racks powered from the DC plant are installed isolated from the building steel and structural components. In other words the DC plant and all the equipment is floating magically in the air isolated from everything. Then from the MGB we bond the battery return buss in the DC plant and all the equipment frames to form the Single Point Isolated Ground Zone. What this gains us is it prevents any outside currents (noise, faults, lightning, etc?) from flowing in the IGZ. It is extremely effective and still used today in digital switching equipment.

However it can be very dangerous in a high-rise building if implemented incorrectly. One of those incorrect ways is to run a insulated riser cable from the basement up to the higher floors. Here is the deal Imagine you are located on the 20th floor of a 40 story building, and lightning strikes the building. Your switching equipment is referenced to earth potential in the basement. But when lightning strikes the building forms a voltage divider from top to bottom. At the top of the building the voltage develops say 50KV, and as it divides working its way down to you on the 20th floor the voltage is 25KV. I think you see the problem, the floor you are standing on has risen to 25 KV, and your equipment is at 0 earth potential. Someone is going home in a dust pan.

The solution is simple, bond the MGB to the building steel with a short bonding jumper. In fact we mount the MGB right next to a steel column to facilitate the connection. In fact that is all we use any more without a dedicated ground riser.

A typical installation now days in a high rise is we install isolation/step down transformer, bond Xo to BS, install the various ground busses like MGB near steel columns and bond to BS. If UPS are involved same thing bond to BS and call it done.
 

dereckbc

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Location
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E.C. said:
Thanks guys, so the mcc is ok to bond? should i use it and building steel?
Yes bond the MCC, it is permitted. Not sure I understand the second part of your question. Are you needing a supplemental ground for equipment racks in addition to the EGC supplied with circuit conductors from the MCC? If so it is permitted but not required. This is really a design issue, and not something you should go willy nelly about. One of three things can happen, one is desire deffect, the other two can create problems if you do not know what you are doing.
 

sandsnow

Senior Member
dereckbc said:
However it can be very dangerous in a high-rise building if implemented incorrectly. One of those incorrect ways is to run a insulated riser cable from the basement up to the higher floors. Here is the deal Imagine you are located on the 20th floor of a 40 story building, and lightning strikes the building. Your switching equipment is referenced to earth potential in the basement. But when lightning strikes the building forms a voltage divider from top to bottom. At the top of the building the voltage develops say 50KV, and as it divides working its way down to you on the 20th floor the voltage is 25KV. I think you see the problem, the floor you are standing on has risen to 25 KV, and your equipment is at 0 earth potential. Someone is going home in a dust pan.

We still have designers who want this riser in High Rise Buildings. All I seen all different scenarios form the riser bonded on every floor. every third floor, just at the ground level and not even bonded at all. By bonded I mean connected to BS.

Unless I'm mistaken, minimum Code req's are just bonding the riser to some part of the GES, but at really no specific location.

Is there any problem with just following the Code in this case? As long as everything is at the same potential, I'm thinking we're OK.

Thanks
 

dereckbc

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Staff member
Location
Plano, TX
E.C. said:
also, should I bond the masts to the ground buss and the lightning ring?
The MCC ground buss should have its own bonding jumper to the building steel. The masts gets it own bonding jumper to either the LPS or BS whichever is the closest to keep the jumper short and straight as possible. If the LPS is properly installed it should already be bonded to BS in multiple places.

The LPS and BS make for one large grid of interconnecting loops, or a equipotential ground plane. This means the high frequency impedance measured between any two points is extremely low with large current handling capacity, so that potential differences will be extremely low under normal and fault conditions.
 

E.C.

Member
Thanks. I wanted a second opinion as this is the capitol building for the state of Florida. I would hate to burn it down, or blow FDLE's data center. They really should of got a PE but they wanted to value engineer it so I'm getting stuck with it. I have alot of experience with cell sites but nothing up in the air. :cool:
 

megloff11x

Senior Member
It's ironic that a state capitol building might have a code compliance issue, but God help some poor schlepp running a small business with lesser issues in his building.

Do as I say, not as I do.

Matt
 

E.C.

Member
Compliance??!!! There are three systems already running in the penthouse. Two of which are critical police radio repeaters. They have a ground buss and all the trimmings the run to...of all things....a screw on the first jbox in the room. Some old electrician just 'put it to the box' like the old days. I don't work for the state of Florida so I can say they have some jackleg electricians working for them.
 

dereckbc

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Location
Plano, TX
sandsnow said:
We still have designers who want this riser in High Rise Buildings. All I seen all different scenarios form the riser bonded on every floor. every third floor, just at the ground level and not even bonded at all. By bonded I mean connected to BS.

Unless I'm mistaken, minimum Code req's are just bonding the riser to some part of the GES, but at really no specific location.

Is there any problem with just following the Code in this case? As long as everything is at the same potential, I'm thinking we're OK.

Thanks
Sandsnow I still see the riser being used myself. As long as it is bonded at every floor or every third floor is fine. Where it can become a problem is the specialized application as I lined out with Isolated Ground Zones used by telephone companies, but these applications are rare.

In any event a riser cable impedance is extremely high if not bonded to BS periodically and can be considered an open circuit to high frequencies like lightning or RF. Whereas building steel with the thousands of loops and interconnections has almost 0 impedance between any two points with tremendous current carrying capacity.

Now this excludes what I see in a lot of high rise buildings where they run MV buss up a shaft, then tap transformers at every floor for service.
 
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dereckbc

Moderator
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Location
Plano, TX
EC, I am still having a problem wrapping my brain around your scenerio. I cannot draw a mental picture of where everything is at, how the equipment is being powered, etc... Therefor I am hesitant to make firm reccomendations. Can you spell it out for me a little better?

Where is the radio equipment?
How is it being powered?
Where is the MCC
Where is the UPS.
What kind of coax/antenna?

I need to be able to draw a 1-line in my head or better yet see it.
 

E.C.

Member
The eq is in the penthouse on top of the building. It is being (will be) powered by building ups 3 floors down. The MCC is in the penthouse and is 277/480 and will not power the eq but as I believe it has a direct ground to the service entrance. The antenna is a 6.3 ghz motorized parabolic TXRX in a geodeisic dome. The relay to the office is line of site microwave system.
 

dereckbc

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Location
Plano, TX
So the MCC is not part of the system? If true don't worry about it, it has nothing to do with the system or have any value from ground prospective.

The problem as I see it is the UPS being 3-floors below. I would add an isolation transormer in the penthouse fed from the UPS via a feeder to derive a new N-G bond point to BS close as possible to the equipment.
 
Not trying to be a nudge here, but I am confused by all of the different terms being used for these conductors???

Bonding jumper, grounding conductor, equipment ground, Building system

This "riser" being bonded on each floor...is it a GEC or an equipment ground conductor of some sort...some kind of bonding conductor. How is it being used as an equipotential ground plane, Building system , Lightning protection system (am I correct that you may be saying the LPS is part of the Equipotential ground plane?)


What is the purpose of bonding the MCC to the building steel. The steel is not permitted as an equipment grounding conductor [250.136(A) - the second sentence], and grounding per se is performed at a separately derived system or at the service. How is the MCC equipment considered as either where it is located up in the penthouse?
[to me grounding is a conductor connected to earth, as in 250.4(A)(1) & Bonding generally is as in 250.(A)(3)...I do understand about the principle of bonding jumpers, such as bonding of different systems together]

I am somewhat confused by this thread.
maybe if you have the time you can outline what is trying to be acheived, and the way it can be acheived.
 

dereckbc

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Location
Plano, TX
Pierre, I might be able to clear things up a bit, the terms being used are from the telecom industry from ATIS, Bellcore, etc?

Vertical Riser (aka Veritical Riser): The main vertical GROUNDING CONDUCTOR used to obtain a ground reference on each floor in a building.

Equipotential Ground Plane: A mass, or bonded masses, of conducting material that offer a negligible impedance at high frequencies to current flow between any two points. Think along the lines of a copper sheet, sheet metal. In this case of a building; it is all the structural steel, pipes, raceways, LPS everything intentionally bonded together at as many places as possible to form a low impedance path between any two points.

Bonding the MCC ground buss is not being used as a fault path, although it will aid in that function, it is to lower the overall high frequency impedance between any two points and make the EGC and raceways part of the Equipotential Ground Plane.

In short what is being talked about is beyond the scope of the NEC and pertains to special needs and practices of the telecom industry. Although these techniques are not required by the NEC, it is permitted and considered supplementary.

Many times when I refer to the Telco sector you will see me us terms like:

OPGPB: Office Principle Ground Point Bus: A ground bus located near but external to the AC service entrance switchgear. It serves as a central connection point for conductors such as Vertical Riser, Bonding the AC service switchgear, bonds to steel, bonds to grounding electrodes, other bonds to equipment frames requiring a ground reference.

Hope that helps a little. If you will send me a PM with your e-mail address, I can send you an industry engineering practice I wrote that will help exlain a lot.
 

E.C.

Member
:-? Ok, sorry, my bad. I used alot of different terms during this thread. I know that a ground is at the service entance and every other connection past that is a bond. I understand the 750 mcm riser as that has been spec'd on many of my jobs. ( Yes in PVC outside and no conduit inside :fire code: with semi loop straps and plastic bushings so as to have no complete metal ring around the cable :that's a resistor: and megged with a digital meter that costs mor than my truck.) I am not used to having no 'telco' or 'building' ground. I'm afraid that in 96 I was still in high school when the telcom internet boom hit. Everything after that had a separate 'ground'. Thanks again for you help. I just want to do it right.
 
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