Multiple Grounding Rings

[horsejoe]

I am looking for standards on the installation of multiple facility grounding rings. I have heard of facilities that have installed multiple grounding rings, one for system ground, one for safety ground, and one as a low noise ground (for connecting shields of sensitive testing equipment cables). The purpose for the multiple ground rings was to isolate sensitive testing instruments from harmonic and other types of electrical noise caused by other equipment. Are there standards for the design of such ground systems; burying depth, clearance between rings, installation geometry (concentric circles), etc?

 

[busbar]

There are two (non-exclusive) US-based standards that deal with grounding. There are sample sections at:

http://standards.ieee.org/colorbooks/sampler/Greenbook.pdf

IEEE Std 142-1991

http://standards.ieee.org/colorbooks/sampler/Emeraldbook.pdf

IEEE Std 1100-1999

Another is:

http://www.nfpa.org/Codes/NFPA_Codes_and_Standards/List_of_NFPA_documents/NFPA_70.asp

I believe that isolated "power"/"instrument"/"quiet" grounding systems for one facility are often significantly restricted. Certainly this is a subject with a long history of considerable debate.
There are many regional variations of code enforcement, and discussion with local inspection authorities may be time well spent.

 

[dpc]

The NFPA 70 that Busbar references is also known as the National Electrical Code. If your facility is subject to the requirements of the National Electrical Code, all grounding systems must be bonded together, for safety. There are no exceptions to this that I am aware of. This includes lightning protection, computer grounds, intrumentation grounds, whatever.

The NEC does provide for separate "isolated" grounding systems but these must ultimately be bonded to the system ground at some point. It is also permissible to install supplemental ground rods and grounding systems provided these are bonded to the main grounding system.

As Busbar indicates there is a long history of contention and dispute regarding system grounding, particularly with regards to instrumentation and computer systems. At one time it was common for control system suppliers to require a separate "quiet" ground for their process control equipment. Since this is a clear violation of the NEC, this requirement has been modified to include bonding to the main system.

There are special provisions in the NEC for patient-care areas of hospitals.

Hope this helps.

dpc

 

[busbar]

Bonehead electrician’s old low-tech war story—we did a little experiment about 15 years ago that proved to me in at least one case there was a serous problem with blindly following equipment-installation instructions.

A CNC manufacturer insisted that their new machine had to be installed per their printed instructions, and they wanted a single “flowerbed” ground rod and a plastic [“isolating”/plumber’s] coupling installed in the galvanized rigid conduit run for the associated 20A 480V circuit. After some head scratching we temporarily rigged up a powerstat and transformer to put a metered AC potential between the [installed by code] machine tool and the counterfeit ground rod. We found—with ~277V potential difference—that a little less than 15 amps flowed. So, essentially the serving breaker would never open, but the machine might very well kill the first person who touched it.

After a lot of wrangling the buyer determined that the purchase order for the ~$20K machine was incomplete and that the CNC mill did not meet its manufacturer’s own specifications. [Of course, the vendor continued to insist there were hundreds of these out there that worked safely installed their way.]

The (government research) site’s “authority having jurisdiction” decided that NEC compliance stood, and that life and property protection prevailed over misoperation of the mill. What a pissing contest that was.

 

[dpc]

Busbar's anecdote sounds all too familiar - I had similar experiences with control system suppliers many years ago. The insulating bushing in the conduit was supposed to eliminate concerns about interconnecting the two "separate" grounding systems.

I pointed out to them that a lightning bolt that just traveled 15,000 feet **through the air** probably wouldn't notice this little piece of plastic.

I think these "quiet" ground concepts came about because field engineers found that *in some cases* when they disconnected the control system from the plant ground, certain problems were eliminated, even if they didn't really know why.

Sort of the way I troubleshoot problems with Windows, and about as scientific.

 

[horsejoe]

Good information guys, thanks! I downloaded the IEEE standards and will study them diligently. The facility that used multiple grounding rings is a famious jet engine manufacturer in England. They conduct vibration analysis on their engines and the vibration signals themselves look like noise. It would be interesting to find out what they use as grounding standards and the theories that support the grounding design that they have adopted.

DPC is right about the lightning strike. I read about a facility that used a 40' well casing as its main ground rod. Good ground? Wrong! The first 30' of the casing was in dry ground. Only the last 10' was below the water table. The impedance of the 30 dry feet to the 10+MHz component of a lightning strike caused an instantanious voltage drop at the connection to the well casing of over 8500 kV. All of the delicate electronic equipment was damaged.