Data Rack Grounding

[infinity]

What is the purpose of grounding data and communications racks to a grounding riser that originates at a grounding bus adjacent to the building service equipment? Recently we looked at the drawings for a grounding riser that runs through the building to a ground bus in each IDF closet. After looking at the drawings a discussion came about and a few questions were raised.


1) What is the purpose of this riser? Maybe the better question is why couldn't the building steel be used to perform the same function.

2) What is this riser? Is it a GEC?

3) Are conduits and sleeves that it passes through required to be bonded to it?

4) Is this riser required by code or is this a design issue?

 

[Gmack]

In my zip, great and elaborate means are effected to "bond" the rack.

Most noteably in HCF's.

Take a peek at:

http://www.panduit.com/enabling_technologies/102200.asp

P.S. It appears to be a NEC issue. See 800 and 830. "Gounding methods"

 

[RayS]

well, here's my stab at it.

1. It is used to provide a single point ground so you don't have stray ground currents on signal cables/shields. Steel would also work, but may become "contaminated" by other voltages being applied.

2. GEC? no. Dont want to use it as such either. As far as power AC stuff, I think it should all have it's own, seperate GEC

3. I'd say ya gotta bond it all to maintain low impedance at hi. freq's. Hopefully won't be needed for fault current.

4. Not req'd for NEC, maybe some telco reg?

 

[dereckbc]

Edited to clarify answer 1.

Well as usual on this subject, I see a lot of misinformation as has already been seen in the replies. To be short I will answer your questions with brief comments.

Q1. What is the purpose of this riser? Maybe the better question is why the building steel couldn’t be used to perform the same function.

A1. Vertical riser is used to obtain a ground reference between the office ground bus on each floor and the principle ground point (GES) in the building. Building steel is required to be bonded to the riser for every floor in which it passes.

As to the second part of the question, building steel can be used in lieu of a riser. In fact building steel will have much lower impedance than any single cable of any size. The reason the riser is often used is because in most applications building steel integrity (bonding) cannot be verified after the initial construction of the building. So as a contractor coming in say 10-years after the building is constructed a riser is used. Nothing in the NEC to prevent its use.

Q2. What is this riser? Is it a GEC?

A2. Yes, in essence it is as it is the point where all SDS systems obtain the ground reference.. As stated in the first question it extends the earth potential up through a high rise building. The riser is bonded to what is called Office Ground Bus on each floor or by some other name. This bus is used to reference all AC equipment (transformers, etc) DC plants, equipment racks, and signal grounds on that floor.

Q3. Are conduits and sleeves that it passes through required to be bonded to it?

A3. Yes, if the sleeves are ferrous metal, no otherwise.

Q4. Is this riser required by code or is this a design issue?

A4 The answer is both yes and no. The riser makes for a very easy means to obtain a ground point for all systems. Building steel can be used, and in fact is as building steel is required to be bonded to the floor ground bus. However building steel is not always easy to be access.

 

[infinity]

Thanks Dereck for you detailed response. One other question, you stated:

A4 The answer is both yes and no. The riser makes for a very easy means to obtain a ground point for all systems. Building steel can be used, and in fact is as building steel is required to be bonded to the floor ground bus. However building steel is not always easy to be access.

Do you have a code reference that requires the ground bus on each floor to be bonded to the building steel? Over the years I have seen this done with and without the individual bonding on each floor and was wondering why some designers would include this in their design and some wouldn't.

 

[dereckbc]

Trevor, good question, and I have a good answer, but it takes some explaination.

Loosely the requirement comes indirectly from 250.30, as all SDS is required to be bonded to the nearest electrode which in high-rises is BS. However that is not the real reason IMHO.

A method of grounding data and telecom equipment racks is what we call “ISOALTED SINGLE POINT GROUNDING” or it comes under a lot of acronyms like IBN (isolated bonding network), IGZ (isolated ground zone), etc. Point is here this equipment is isolated from any incidental electrical contact with the building structure (like building steel, conduit, supports, etc) and single point grounded to the floor bus bar called the Office Ground Bar (OGB, lots of names for this bar)

So now let’s assume we do not bond the OGB to the building steel on the floor in which it is used for a moment. Lets also assume the equipment is installed on the 20th floor of a 40-story building. This means our equipment is referenced to earth potential all the way down in the basement via the riser cable. Life is good so you think.

Then a thunderstorm comes along and lightning hits the building. As the discharge current passes along the length of the building, a voltage divider is developed. So at the top of the building a potential of lets say 40 KV develops between the top and basement, which means us down on the 20th floor are around 20KV to earth. Our isolated equipment is setting at the basement potential of 0 volts, while the floor and all surrounding objects are at 20 KV from the lightning strike. So if you are the poor SOB working on the equipment at that time, you sure are going to make a heck of a mess for the janitor to clean up

So how do you prevent the problem?

Simple bond the OGB or riser to each floor it passes through thereby shorting the problem out. Now when lightning strikes, everything rises and falls at the same potential on the same horizontally plane.

There are other reasons for bonding to building steel like fault clearing paths (shortest route), but none better than lightning.

Hope that helps.

FWIW, those designers who do not bond to each floor are ignorant IMHO, and do not know what they are doing.

Dereck

 

[cosmos]

TIA/EIA-942 infers that correct grounding will improve the reliability of data centers. This standard also gives some installation guidance, such as component details and avoiding daisy-chaining.