Neutral busbars

Most commercial multistory installations contain busways that serve as the primary source of electrical power to various floors. Busways that incorporate sandwiched busbars are susceptible to nonlinear loading, especially if the neutral bus carries large levels of triplen harmonic currents (third, ninth, etc.). Under the worst possible conditions, the neutral bus may be forced to carry a current equal to 173% of the phase currents. In cases where substantial neutral currents are expected, the busways must be suitably derated. Table 4.10 indicates the amount of nonlinear loads that may be allowed to flow in the phase busbars for different neutral currents. The data are shown for busways with neutral busbars that are 100 and 200% in size. 

Neutral earthing arrangements, e.g. solid, with a neutral earthing resistance (NER), with a common busbar, switches or circuit breakers for isolation. 

The sleeve is used where the atmosphere may be damp or corrosive. Neutral busbars are usually rated at half the phase busbar current ratings. If the neutral is likely to carry harmonic currents then it is good practice to use a fully rated neutral busbar i.e. the same as the individual phase busbars. 

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. 

Whether earthing connections and neutral busbars need to be isolated during maintenance, e.g. as may be required when two transformers feed a common switchboard. 

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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