Re: Performance Grounding?
Chuck, I will try to answer some of your questions, and might be able to shine some light on the subject.
Try IEEE ?Emerald & Green Books? as a reliable source and available to the public. There are other sources published by Belcore and other phone companies, but they are proprietary and hard to come by.
I can?t answer why health care facilities require 1 to 5 ohm?s, but I can answer why telephone companies and data centers do. Reason is simple; it is required by the equipment manufactures for warranty. However from the performance perspective, it is ludicrous.
It is easy to obtain a 1 to 5 ohm resistance in large facilities by use of a combination of ground rings, rods, water pipe, structural steel, and ufer ground electrodes to form an electrode system. Such connections to the electrode system are only used for electrical safety (power-safety range) and not suitable for use at high frequencies (performance range). Measured resistance values of 1 to 5 ohms are only meaningful at dc and possible low frequencies below 100 Hz. In any case, these are not HF impedances. A 10-foot length of 750 MCM cable exhibits about 230 ohms @ 10 Mhz. Add that to 5 ohms dc and what do you get? About 230 ohms 230 uh? So what difference does 5 or 25 ohms make at HF. Take the same problem with a 25 ohm ground and 230 ohm reactance @ 10 Mhz and what do you get? About 231? HA!
The equipment inside the facilities requires both safety grounds and performance grounds. They are two separate but interconnected systems. The safety ground consist of the ground electrode system, GEC, MBJ, EGC?s, DC reference, DC equipment ground, cable entrance ground bars, etc. The performance ground systems consist of the ground electrodes, logic, and signal grounds.
The performance ground system has several configurations depending on the company philosophy. Some use single point isolated ground planes using single dedicated ground conductors, while others use an integrated ground plane approach using a grid connected to every electrode possible. Most use a hybrid of both isolated and integrated.
Radio sites are an exception to some point. They need a low impedance cable entrance point to bleed off a direct lightning strike before the coax's and tower lighting circuit enters the facility. Again they use the same principles as phone company's using single point isolated grounding so no external curents can flow through the equipment or circuits.
The one industry I know of that really needs the low resistance are electric utilities in generator plants and sub-stations. They pretty much use a grid to minimize step-rise potential differences during a ground fault.
Hope that helps.
Dereck
