Isolated Signal Reference Grid

[maxbillington]

I have a client who is insisting on an isolated signal reference grid. I have referenced the 2008 NEC Article 645.15 that says "Where signal reference structures are installed, they shall be bonded to the equipment grounding conductor provided for the information technology equipment."

I am insisting that their signal reference grid be bonded to the main service ground bus for the facility. They are insisting on complete isolation.

Does anyone know of a good succinct reference document on this subject or white paper to which I can refer my client?

Max Billington, P.E.

 

[ceb58]

Look at 250.112

 

[SparkyHC]

I have a client who is insisting on an isolated signal reference grid. I have referenced the 2008 NEC Article 645.15 that says "Where signal reference structures are installed, they shall be bonded to the equipment grounding conductor provided for the information technology equipment."

I am insisting that their signal reference grid be bonded to the main service ground bus for the facility. They are insisting on complete isolation.

Does anyone know of a good succinct reference document on this subject or white paper to which I can refer my client?

Max Billington, P.E.

Max, Take a look at the IEEE 1100-2005, recommended practice for "Powering and Grounding Electronic Equipment.".....

"Article 8.5.4.8 - Follow the NEC and other related applicable codes and standards for safe grounding. There is no conflict between safe grounding for people and effective higher frequency grounding for electrical systems and their associated electronic equipment."

This sounds similar to the attempts made in the past at isolating IT equipment from the building grounding system by "only" connecting to ground rods driven in the IT room....and orange receptacles....both of questionable benefit.

This sounds like an uphill battle.....good luck.

Harry, PE

 

[tom baker]

An isolated signal reference grid can cause equipment problems from lightning, due to a difference in potential between the signal reference grid and the building grounding system. The EPRI has published a study on this, Mike Holt has a graphic on it, that explains the dangers.

 

[dereckbc]

Max I can help you out with this as I have done quite a bit of it in the Telecom sector and have contributed to a lot of the standards written on the application including IEEE-1100.

What your client is insisting on is rubbish and frankly have no idea of what they are asking for. If you were to build it as they suggest if there were a fault severe equipment damage would occur and possible fatal injury. Not to mention would have the exact opposite results of what they would expect.

Isolated Ground Planes come under many names. The most documented is Isolated Bonding Network. One of the best documents covering the subject and design guidelines is ANSI T1.333.2001. If you PM me with your email address I will mail it too you.

With that said I can tell you if you do this, as soon as they terminate anything to the SRG, they will corrupt it unless all their equipment is in stalled completely isolated with single point grounding of all power supplies be it AC or DC. Once you read the document you will understand what I am talking about.

For the most part just about all equipment made for data and telephone offices has now switched to Intergrated or MESH Grounding, which roughly means everything is bonded to everything and all parts of the building. Only some stubborn relics like Lucent still use ISG on their 5ESS switch gear.

 

[dereckbc]

The EPRI has published a study on this, Mike Holt has a graphic on it, that explains the dangers.

I can explain it real quick with just a drawing. 20-story building on left uses an isolated ground bar and uses a 2/0 insulated cable passing all the way to the basement without being bonded to building steel through the floors it passes en route to the AC service ground, or isolated ground rod whichever floats your boat because they both have the same disastrous result.

Lightning strikes building, a voltage divider is formed along the vertical length of building. Voltage at the 10th floor where isolated BG is installed rises to half the voltage of the top of the building. GB is referenced to earth 150 feet below at 0 volts. You know have 5000 volts potential difference between the isolated ground plane and building steel. All equipment is destroyed, and anyone standing near it is dead.

Solution is simple shown in the right hand drawing. Bond the bar to the building steel on the same floor where it is used. Or you can run a dedicated cable to the basement if you like collecting large checks, just be sure to bond the cable to all the floors it passes by en route to the basement.

I know this sounds silly, but it has happened, I have seen it and the results of millions of dollars of Telephone switching equipment destroyed because of improper use of Isolated Ground Planes.