Re: Grounding rods and Ufer
Bennie, your statement of single point grounding for sensitive equipment is only a half-truth. In modern communications facilities they use both single-point and multi-point grounding techniques. They call them isolated and integrated ground planes. Basically a ring ground is used and tapped at various points around the building where services and cables enter/leave the building.
Single-Point Isolated Ground Planes start by forming a ground window with a master ground bar (MGB) installed inside the ground window. The MGB is connected to the ground ring, AC service, and any other ground electrode available. From the MGB the DC power plant return bus and frames are bonded, and all frame and signal grounds used in the isolated ground planes. In addition any AC power supplying the isolated ground plane has to be routed to the ground window and have the raceway and ECG?s terminated to the MGB. From that point everything is completely isolated from incidental contact from concrete, building steel, and structure. Equipment cabinets and everything else is isolated and fed from the MGB. This method makes for a true single point ground. No fault current of any kind can flow other than internal power faults. Due to the complex nature this method is only used on telephone switching equipment and where required by equipment manufactures. The down side is it is very expensive to install, maintain, and can be very dangerous to personnel if lightning energizes the surrounding building steel. It is extremely easy to compromise, and locate/troubleshoot faults after installation is completed.
On the flip side of the coin is the integrated ground plane or multi-point ground. It is used for radio, transmission, UPS, DC plant, fiber-optic, and data processing equipment. It involves constructing a ground grid of copper wire on 2 to 4 foot centers installed below the raised floor or overhead on cable racks and is bonded to the equipment frames and signal grounds. The grid is also bonded to every metallic object it passes by including building steel, cable racks, raised floor structure, water pipes, and multiple connections to the ground ring, service panel ground bars, and any possible ground electrodes. Equipment frames, conduits, raceway etc are installed with incidental contact (not isolated) to building structures to further lower the impedance. Its purpose is not to be a fault-clearing path (however it will supplement the EGC?s), but rather form a high frequency low impedance path back to earth, and between signal points and equipment frames. True it does allow small amounts of common mode current to flow, but the low impedance formed by the multiple loops of the grid keep the impedance so low the voltage developed is negligible. It is an extremely safe and proven method.
[ April 06, 2003, 12:50 PM: Message edited by: dereckbc ]