Yes, we must have 5 ohms impedance!

[tom baker]

I have a copy of the US Army grounding manual, its about 300 pages. it discusses two ground systems, one for the crypto type equipment and one for every thing else.

 

[sparky_magoo]

Close to twenty years ago, I was wiring facilities for black world clients. TEMPEST referred to computers, cableing, & communications equiptment. The systems had to be shielded to prevent RF from leaking. The secure facilities were built to a DIA code called DIAM 50-3. A newer practice came into play where the room would be shielded, rather than the equiptment. Originally, the rooms were surrounded with grounded wire screen, hence the term screen room. When I started wiring the facilities, they were surrounded with steel plates. There were double sets of steel doors with copper "knife-edges", around the perimeter. Only one door could be opened at a time to prevent RF from escapaping.

Part of my job was to certify the facility's integrity. We used an NSA spec called 65-6. It required 100 db isolation from DC to 2.2 ghz. I would set up a sweep generator & amplifier inside the secure facility & measure for leaks outside the facility with a spectrum analyzer.

After the evil empire collapsed, funding for black programs started to dry up & I had to find a real job. Thats why I work for an electrical contractor now.

 

[kingpb]

In an effort to try and steer people the right way, and avoid being chastised for inadvertently mis-stating a minute detail, I will say little to nothing on this subject, other then, the 25 ohm value is not magic, voodoo or anything else.

There is ample discussion in IEEE 142, Grounding of Industrial and Commercial Power Systems. Of note, a statement found therein is as follows: "This should not be interpreted to mean that 25 ohm is a satisfactory level for a grounding system."

Further, reference is made to IEEE 80: IEEE Guide for Safety in Substation Grounding.

Between these two references, your questions should all be addressed, and to avoid unnecessary dialog, any further discussion in regards to grounding should include these documents.

 

[iwire]

Between these two references, your questions should all be addressed, and to avoid unnecessary dialog, any further discussion in regards to grounding should include these documents.

Why?

Are we building a substation or are we trying to ground a premise wiring system operating at 120 volts to ground.

The effectiveness of a ground rod is greatly influenced by the voltage.