Copper busbar

Electrical Connections. Electric current is brought from the transformers by air-cooled copper busbars and close to the electrode by water-cooled bus tubes and flexible cables, connecting to water-cooled copper contact plates at the electrode. The plates are held against the electrode by hydraulic pressure. The connectors are as short and as balanced as possible to allow cancelling of magnetic fields associated with individual conductors. 

Busbars. Fitting the tank for d-c power is usually accompHshed usiag round copper busbars, both for supporting the anodes and the work or cathodes. Size of the copper bus is determined by the amount of current flow expected 1000 amperes requires about 6.5 cm of cross-sectional area. The bus is iasulated from the tank, and any other sources of grounding, and coimected to the d-c power supply. Shorter distances from the tank as well as fewer electrical connections keeps the voltage drop to a minimum. 

In the traditional parallel-plate cells, the Walker system is the most commonly used electrical arrangement (Fig. 13). In this system, the current flows from a copper busbar on one side of the cell to the anodes, and the cathodes are connected to another busbar on the opposing side of the cell. The second busbar feeds current to the anodes of the second cell, and so on. In the Walker system, only one side of each electrode is connected to the electric circuit. The intercell busbars do not require so much thickness as the end busbars as the current flows through the path of least resistance. 

Cubicle steelwork is invariably bolted to the floor, a floor frame or to a wall structure, and therefore bolted bonding straps are used in a similar manner as described above. Most cubicles are fitted with an internal copper busbar which is bonded internally to the steel. The busbar is used to receive the bonding connections from internal components, partitions, screening panels, cable glands, cable armouring, cable screens and gland plates. 

Install a vertical ground bus in each rack (as illustrated in Fig. 10.270). Use about l-y2-in.-wide, y4-in.-thick copper busbar. Size the busbar to reach from the bottom of the rack to about 1 ft short of the top. The exact size of the busbar is not critical, but it must be sufficiently wide and rigid to permit the drilling... 

The main incoming mate coniacts are generally made ot copper or brass and are cither hotted or damped on die vertical bus. Since tlic bus is generally of aluminium. Ihe coniacts may form a bimelaltic join wilh Ihe busbars and cause corrosion and pilling of ibe melal. This may result in a failure of the joinl in due course. To mininii/e rnelal oxidation and bimetallic corrosion, the conlacls must be silver plated. 

A) 1 For busbars and busbar connections of aluminium or copper 2 For busbars and busbar connections of aluminium or copper silver plated or equivalent 3 Terminals for external insulated cables 50 65 70 90 105 110... 

Figure 28.14 Ratio of a.c. current ratings for different configurations of busbars of the same cross-sectional area (Courtesy The Copper Development Association, U.K.)...

Electrical conductivity The electrical conductivity of refractories is important when they are used in electric furnaces. Except for graphite and metals, all other refractories are poor conductors of electricity. Graphite is a highly refractory material, and is used for electrodes and furnace linings in all high-temperature electric furnaces. Metals are not important as refractories in electric furnaces, but copper wires or busbars, for example, are utilized to carry current to the graphite electrodes. 

The current is carried to the electrolyzer by copper or alluminium busbars from which it is distributed by cables or by a system of elastic copper lamellae to the terminals of the electrodes. The current passes through many points of contact which determine the magnitude of so called contact resistance. The more imperfect are the contacts at these points the greater the losses in electrical... 

The earthing (grounding) bnsbar is separate from the neutral busbar, and is nsed to earth all conductors that need to be earthed as well as the metallic frame and casing of the switchboard or motor control centre. The earthing bnsbar is made of high-grade copper and is usually located at the front or rear of the enclosnre at gronnd level. 

The local processing equipment will be similar to that described in sub-section 13.2.1, and therefore the earthing practices will be similar, for example the use of copper interconnected busbars and bonding conductors. However, if the plant is mounted on concrete foundations then extra earthing rods will usually be needed at each foundation site. All reinforcing steelwork in concrete should be earthed to busbars or through their own rods. 

The water cooled copper power feeds are then screwed into place and copper shims of the appropriate thickness are used to obtain the correct rotational position for the assembly to the busbar system, which could utiUze the copper pipes used for the cooling water. Temperatures up to about 2650°C can be used, but erosion will limit the life of the element. At 2625°C, the total resistance is about 10 milliohms, which rises fairly quickly to about 11 milliohms towards the end of the element s life. Monitoring the transformer voltage and kilowatt readings can be used to calculate the total resistance from from the equation R = F /kW. 

Cast copper does not have outstanding mechanical properties. Work hardening by rolling or drawing improves these properties. In sizes typical of chlor-alkali busbars, hard-drawn copper bars or strips have tensile strengths of about 2.5 tons cm . Cold working destroys about 2-3% of the maximum electrical conductivity. Annealing restores this at the expense of some of the enhanced physical properties [18]. 

A. Capacity of Busbars. Current densities through busbars are high. With copper conductor, l,500kAm is not rare. Even with the low resistivity of copper, this produces enough I R loss to raise the temperature of the metal significantly. The crqracity of the... 

TTie rate of heat generation in a conductor is proportional to the square of the current. Both radiation and convection dissipate the heat. Compact arrangements of busbars restrict the opportunities for radiation, and convection usually is more important. Polished copper may be more attractive visually, but keeping the surface free of oxide is counterproductive. The emissivity of a polished copper surface is about 0.05 the emissivity of an oxidized surface is 10 times higher. Aluminum tends to have lower emissivities ( 0.02), but again, heavy oxidation produces a 10-fold increase. Outside surfaces of conductors sometimes are painted to increase the emissivity even more. 

Istas [20] described an aluminum busbar system in a mercury-cell plant. The design-basis current density for the buswork was about 900kAm operation was close to 800kAm. The buswork joints were clamped. Joints between aluminum and copper or steel were through 19 ftm of nickel plate. Exposed aluminum surfaces were covered... 

We can extend the comparison between aluminum and copper buswork to the stability of joint resistances. Aluminum flows more easily under stress than does copper, and so the problem of loss of contact pressure as buswork temperature cycles is more severe when using an aluminum bus. Aluminum oxidizes more easily and more rapidly than copper and forms an oxide with a higher resistance. Furthermore, copper oxide has a negative temperature coefficient of resistivity, so that its contribution to joint resistance actually declines as temperature increases. Aluminum oxide, like the conductors themselves, has a positive temperature coefficient For these reasons, welded butt joints are often a better solution for aluminum busbars than clamped or bolted overlap joints. 

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