Overview

It is the function of cable connected secondary switchgear to accept electrical power from a primary switchboard (see Chapter 9). The secondary switchgear then distributes the power to points in the network where the voltage is either transformed to a lower value or where it is consumed without transformation, as would be the case when supplying high-voltage machines.

There are basically two types of application. The first is that used to provide power to housing developments and small industrial estates and the second is within the local network of relatively large consumers. In the first type of application, a typical network used is the ring-main as shown in Figure 10.1.

Figure 10.1: A typical ring-main network for secondary distribution

Within the typical distribution network shown, there are two primary incoming supplies, feeding the ring-main via two incoming circuit breakers and a normally open bus-section unit in the primary switchboard. The network is shown to be operating with one switch at the remote end open in the ring circuit. This means that each side of the ring will run as a radial feeder. This arrangement helps with fault location, reduces the number of consumers who would lose their supply in the event of a fault, giving the system greater flexibility and increased availability. In the event of a permanent fault occurring in one of the primary incomer circuits, the primary incoming circuit breaker would be opened/isolated and the bus-section circuit breaker closed to re-establish supplies to all consumers fed by...