With that background in a place, it’s time now to discuss how to install a system that meets the requirements of technical grounding systems with a minimum of compromises.
Star ac and ground
The basic connections for technical, systems and equipment grounding are shown in Figure 22. The bottom half of Figure 22 illustrates the connections that exist in all residential, commercial and industrial electrical systems with regard to system and equipment grounding. All of the basic grounding connections are made to the neutral or equipment ground bus. We see the equipment ground, the system ground and the utility ground electrode are bonded together. The top half of Figure 22 illustrates the additional conductors that are put in place to serve the purposes of technical grounding of the electronic equipment. Note that an additional wire is run between the system equipment central ground point and the master technical ground point.
The master technical ground point is the center of the star ground system. From that point we distribute to the local equipment centers. This would include each control room in a studio or broadcast facility or a theater’s power amplifier room, the state left or right main equipment center and the rear house-mix position. The key to integrating the power system with the technical ground system is to note that wherever there is a sufficiently large amount of electronic equipment to warrant a local or area ground point, there is typically a requirement to have a power-distribution center—in other words, an ac distribution panel (breaker panel). Consequently, the main power-distribution center (breaker panels) will also have a local isolated ground bus.
By implementing the system this way, we run the technical grounding conductors (and buses) with the phase and neutral conductors. This is a critical point9pecified in most electrical codes. (Remember, electrical codes require that the ground conductors be run with the phase and neutral conductors they service.)
Figure 23 illustrates the result of this approach. The system consists of a subdistribution panel and area or local branch panels. These area panels then feed the electronic equipment. A larger system might have several levels of subdistribution panels. In a smaller system, the subdistribution panel may not exist, and the area technical panel may in fact also be the main panel for the building. It is only in larger facilities or systems where a completely separate set of panels is provided.
Conductors
Table 2 suggests conductor sizes to be used in implementing the technical ground systems (Giddings, 1990). In all cases, the ground conductor must be suitably rated for the overcurrent device of its associated power conductors. Table 3 gives the minimum conductor sizes required from a safety standpoint.
Table 2: Technical ground conductor sizes in AWG (mm squared) (suggested only)
Table 3: Minimum equipment ground conductor sizes (based on National Electrical Code table 250-95, “Minimum Size Equipment Ground Conductors for Grounding Raceway and Equipment)
It is important that the terminations of the wires in a technical grounding system be done in such a way to ensure a low resistance connection over a long period of time. Several factors can affect the long-term reliability of these connections.
Galvanic corrosion occurs when dissimilar metals are in contact and exposed to humidity in an atmosphere of ionized salt content. This can be a problem in areas near the ocean or near heavy industry. The further apart the two metals are on the electromotive-series chart, the greater the reaction. The effect of the reaction is to reduce the contact area and, consequently, connection integrity. Most corrosion problems occur with aluminum, which should only be put in contact with aluminum, solder-dipped copper, tin-plated brass and copper or tin-plated aluminum with no undercoat. Also to be avoided is silver- or gold-plated copper in contact with solder-dipped copper.
Creep is another factor that can affect contact integrity. It is defined as a dimensional change with time of a material underload. Certain materials will continue to deform as long as a pressure is applied, such as the loosening of aluminum wire. Special crimps and materials must be used to control this effect. However, aluminum wire is never recommended for grounding purposes.
Oxidation occurs when materials combine with oxygen. Oxides are nonconductive and make contacts of poor conductive quality. Material such as gold, palladium and rhodium do not oxidize easily and make good contact materials. However, they are quite expensive.
Copper oxidizes quite easily. When copper is used for electrical contacts, the high voltages used in most power contacts are able to clean the area because of arcing across the oxide.
Although it might seem that copper contacts are perfectly reliable because they never fall in 120V circuits, they are not reliable in very low-voltage circuits, such as grounding circuits. There may only be one or two volts of ground noise, and these voltages are unable to clean the contact.
Of course, another way of cleaning contacts on ac terminations is to pull out and reinsert the plug. This causes a wiping action that rubs the oxides off the surfaces. Obviously, over a long period of time, an oxide can build up: it then becomes necessary to remove and insert the plug. It is for this reason that very high-quality ac receptacles such as hospital grade should be used. These typically have higher contact pressures that prevent the oxide from building up on the contacting surfaces.
Separation and routing
The physical separation of cables has a significant effect on how they interact. The effects of the separation of parallel wires are governed by the 3dB per doubling of distance rule, which applies to electric and magnetic fields. For example, when cables are spaced from one unit of distance to two, four, eight or 16 times that unit, there is 3dB less coupling per step. Once the small initial separation has been achieved, much greater separations are needed for further improvement.
When routing cables, electromagnetic compatibility can be obtained free of charge by simply taking care of how they are routed. It is recommended that technical ground conductors should not be routed near any other high-powered ground conductors in the building—for example, main feeders servicing other non-technical equipment, such as motors and lights. These lines might contain high transient fields because of the switched ac lines. Further, routings should be kept far from motors and transformers that generate significant electric and magnetic fields. Routing and separation are forms of electromagnetic control that are almost free of charge.
