Busbars enclosures

Figure 28.33(a) Busbar arrangement and enclosure size bus duct of Example 28.12... 

The busbars and the inside of the enclosure may be painted with matt finish black paint to make this section suitable for... 

As discussed later, the enclosure of an IPB may carry induced currents up to 95% of the current through the main conductors. Accordingly, the enclosure is designed to carry longitudinal parasitic currents up to 90-95% of the rated current of the main busbars. The cross-sectional area of the enclosure is therefore maintained almost equal to and even more than the main conductors to account for the dissipation of heat of the main conductors through the enclosure only, unless an additional forced cooling system is also adopted. The outdoors part of the enclosure exposed to atmospheric conditions is also subjected to solar radiation. Provision must be made to dissipate this additional heat, from the enclosure. 

Figure 31.5(b) Cross-sectional view of an iPB with insulated (discontinuous) enclosure illustrating busbar-supporting arrangement (Courtesy Best Crompton)... 

In metal-enclosed assemblies, the equipment is contained within a grounded metal enclosure. In metal-clad types, separate compartments are provided for the circuit interrupter, the components connected to one side of the interrupter (e.g. the cable box) and those connected to the other side of the interrupter (e.g. the busbar chamber and the control equipment). 

Table 6.36 Caloric values of enclosure and busbar materials...

To give an impression of the significance of these numbers in terms of quantities of melted or vaporized metals, Table 6.36 summarizes specific values for metals used for enclosure and busbar materials. Obviously, the energy release of a half-period arc can melt some 10 grams or vaporize some grams of steel. 

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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