In case of ac supply 3-phase 3-wire system is employed for balanced loads such as power load, 3-phase 4-wire system is employed for unbalanced loads such as light and power loads combined and single phase two wire system is used for lighting and small power appliances.
(a) Three-Phase 4-Wire Distribution. Three-phase, 4-wire distribution layout is shown in fig. 7.6. Transformer substations are built at or near the load centre of the area where the load is to be supplied. The substation contains high voltage feeder cable from the generating station. The low voltage bus-bars are distinguished by colour marking; red, yellow, and blue, for the phases, and black for the neutral. The local 4-wire distributors are connected in parallel to the bus-bars through the fuses or links, and the separate cores are also distinguished by similar color marking. Domestic and other low voltage consumer’s premises are connected to the distributors by 2-wire service cables, which are too jointed to the distributors. In order to balance the three phases, the consecutive services are connected to the different phases and the neutral in turn: red-neutral, yellow-neutral, and blue-neutral. Larger non-domestic consumers are supplied with a 4-wire service, to give 230 V single phase for lighting, as well as 400 V-phase for power.
(b) Single Phase Two-Wire Distribution. For purely domestic loads single phase two wire distributors are used. These are connected to single phase and neutral at the substation bus-bars. Balancing of 3-phase supply is affected by connecting the different distributors to different phases.
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