ADP manufacturer's grounding instructions and recommendations are important and should be followed. Occasionally, however, the grounding techniques specified for various units are inconsistent, especially if they have been supplied by different manufacturers. Sometimes they are inconsistent with the interpretations of wiring codes by contractors, electricians, and inspectors. When there is doubt, there must be no compromise with safety. The system must be safe and must be capable of operating reliably without compromise.
The central Grounding Point of an ADP system should be readily identifiable. It should be the point where the interconnected parts of the computer's grounding system are connected to other ground conductors which extend beyond and outside the ADP room. If there are two such points, each being interconnected to each other and to separate external grounds, the noise voltage difference between those separate grounds will cause noise current to flow through the ADP ground system via the ground loop which is formed.
The single point of entry wiring strategy for ADP rooms, in above diagram, is a practical approach to establishing a virtual single point ground for the ADP system, the communication system, the power source, and the life-safety system. If all external conductors penetrating the ADP room were to enter at a single point rather than at multiple points around the room, their respective ground conductors could all be interconnected at the point of penetration.
The expected benefit is that noisy ground currents flowing between the power, communication, and other ground conductors can flow through the short ground interconnections at the entry point rather than going through the grounding conductors, within and among ADP system units. The remaining question is, "How short is short?" The answer depends upon signal frequency. The measuring yardstick is often related to the wavelength of electrical signals. (Wavelength is approximately the speed of light divided by the frequency of a signal.)
Electrical interface location: Entry of power, communications, and life-safety in one place rather than opposite sides of the room permit use of short interconnections to equalize ground voltage differences without passing equalizing currents through the ADP room.
