Stepped and Tapered Mains

Stepped and Tapered Mains

AU : Dec.-12, May-15

The current along the length of the distributor will vary when a series of loads are tapped off from the mains. If the area of cross section is same throughout the length of the distributor from the curent carrying point of view then the requirement of copper is large.

The cross sectional area for the distributor is based on fixed voltage drop rather than on fixed current density. The series of loads is shown in the Fig. 7.14.1.

Consider a distributor as shown in the Fig. 7.14.2 with two loads having currents I₁ and I₂ taken at points Q and R respectively. Let the length and cross-sectional area for section PQ be l₁, l_a while for section QR it is l₂ and a₂.

Total resistance for section PQ (go and return) = 2 ρ l₁ / a₁

From equations 1 and 2, it can be seen that the current density is not same in the sections.

In practice it is not possible to manufacture conductors which are having varying cross sections. If the conductors of different cross sections are used then it involves more joints. Technically it is better to have minimum number of joints.

The point that is to be considered in this method is the alteration of current distribution in the sections. Sometimes it is not feasible and practicable to use stepped conductors.

Review Question

1. Explain stepped or tapered distributor.