Mounting busbars

Figure 13.30 Possible arrangements for busbar mounting systems...

To obtain a strong busbar mounting system, suitable to withstand the electrodynamic forces arising out of a system fault, modern practice is to make use of thermosetting plastics, such as DMC (Dough Moulding Compounds)... 

The mechanical endurance of the current-carrying parts of all the equipment, bus system, deviees and components, used in a particular circuit as well as the load-bearing members and supports on which they are mounted. The electrical parts of a device (breakers and switches, etc.) are the responsibility of the component manufacturers. The manufacturer of the switchgear assembly is responsible for the busbar systems, metallic links and wires. 

Manufacturers may adopt different practices to mount the main and auxiliary busbars, depending upon the size, rating and fault level of the system. Some of the recommended and more common of these are illustrated in Figure I3.30(a)-(d) and discussed briefly below,... 

Busbars are mounted one below the other, horizontally but in a vertical disposition. The cooling is better and requires less derating. The short-circuit withstand capacity is high due to high sectional modulus but occupies more vertical space. This configuration is also adopted by some manufacturers. 

This is similar to (a) above but each busbar now is mounted horizontally. Due to obstruction in heat dissipation, this arrangement requires a higher derating. It is also prone to collecting dust and provides a habitable surface for lizards and rodents etc. Therefore this is not a recommended configuration. 

Only high tensile (HT) fasteners must be used for busbar Jointing and their interconnections or links not only to take care of the fault level but to also maintain the recommended contact pressure over a long period of operation as noted in Table 29.1. An ordinary fastener may not be able to withstand or sustain this torque for long. Similarly, the busbar supports, which are mounted on only two or three fasteners, should also be fitted with these fasteners. 

Forces on each set of busbar insulators and mounting fasteners, when 400 mm apart... 

Often a failure on a fault may be due not to the inadequate size of busbars, fasteners or insulators but to poor alignment of the insulators or to too large a gap between the busbar and the insulator slots. It may be a consequence of an inappropriate mounting or unequal width of the busbars or insulator slots. In such cases, load sharing w ill be uneven and the weakest section may fail. This can be illustrated as follows ... 

A loose fit of busbars itiside the slots may cause e.xcessive vibrations on a fault and may lead to loosening of the fasteners and shearing of the wedges and/or the edges and the fingers of the insulators. Even the insulator mounting section. v - x may become vulnerable to failure. 

Figure 29.2(b) A typical mounting and fixing arrangement of a conical insulator and a flat busbar... 

The purpose of a flexible joint is thus to make an electrical connection and to absorb the busbar s expansion and vibrations of the generator or the transformer and to prevent transmission of these vibrations to the bus system and mounting structure. 

Precautions in mounting insulators and conductors Making a joint Bending of busbars... 

Despite the advanced technology of the AQUATECH System, its installation is not complex. In fact, it is simpler and less costly than conventional electrolysis processing. AQUATECH Systems need only two electrodes for an entire 100-150 cell unit stack, avoiding complicated busbar arrangements. Furthermore, scale-up and installation of the unit are facilitated by the modular skid mounted cell stack design. 

Busbars are mounted on insulated bushes that are strong enough to withstand the peak short-circuit currents and forces. The busbars may be air insulated or enclosed in an insulating sleeve. 

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. 

Switchboard An integrated, factory-coordinated combination of circuit protective devices, control devices, meters, relays, busbars, and wireways enclosed in a single, preplanned unit designed to be a self-contained center. Protective devices may be individually compartment-mounted (switchgear) or group-mounted in barriered compartments. The entire structure is designed and built to operate as a coordinated unit. 

Post-World War II installations will usually have a consumer unit, which is a small distribution board with a double pole isolating switch and a phase busbar to feed the fuses or MCBs controlling the final circuits. There will be terminal blocks for the neutrals and protective conductors. The earth terminal block may be utilised as the consumer s earthing terminal. Some of the older models may have wood frames and/or may be backless. As BS 7671 requires connections to be made in non-flammable enclosures, the wooden framed type do not comply and the backless ones are acceptable only if mounted on non-flammable material. If rewirable fuses to BS 3036 are used, check that the correct size of fuse wire has been used. For cartridge fuses, check that blown fuses have not been repaired with a bit of fuse wire spanning the contacts or the cartridge replaced by a nail, hairpin or the like. There should be only one final circuit connected to each fuse carrier or MCB. 

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