As a Construction Electrician second class, you may have to supervise the installation, maintenance, and repair of overhead primary and secondary power distribution systems. This chapter will provide the necessary information to enable you to calculate electrical loads and perform fundamental tasks in the selection, by size and type, of distribution equipment. When you perform the above-mentioned tasks, remember, your primary goal should be the safety of your troops.
A power distribution system includes all parts of an electrical system between the power source and the customer's service entrance. The power source may be either a local generating plant or a high-voltage transmission line feeding a substation that reduces the high voltage to a voltage suitable for local distribution. At most advance bases, the source of power will be generators connected directly to the load.
The configurations of four distribution systems are defined in the following paragraphs. These four distribution systems - radial, loop (ring), network, and primary selective - are briefly described. For additional information, review the Electric Power Distribution Systems Operations, NAVFAC MO-201.
A representative schematic of a radial distribution system is shown in figure 4-1. You should note that the independent feeders branch out to several distribution centers without intermediate connections between feeders.
The most frequently used system is the radial distribution system because it is the simplest and least expensive system to build. Operation and expansion are simple. It is not as reliable as most systems unless quality components are used. The fault or loss of a cable, primary supply, or transformer will result in an outage on all loads served by the feeder. Furthermore, electrical service is interrupted when any piece of
Figure 4-1. - Radial distribution system.
service equipment must be de-energized to perform routine maintenance and service.
Service on this type of feeder can be improved by installing automatic circuit breakers that will reclose the service at predetermined intervals. If the fault continues after a predetermined number of closures, the breaker will lock out until the fault is cleared and service is restored by hand reset.
The loop, or ring, system of distribution starts at the substation and is connected to or encircles an area serving one or more distribution transformers or load centers. The conductor of the system returns to the same substation.
The loop system (fig. 4-2) is more expensive to build than the radial type, but it is more reliable. It may be justified in an area where continuity of service is of considerable importance, for example, a medical center.
In the loop system, circuit breakers sectionalize the loop on both sides of each distribution transformer connected to the loop. The two primary feeder breakers and the sectionalizing breakers associated with the loop feeder are ordinarily controlled by pilot wire relaying or directional overcurrent relays. Pilot wire relaying is used when there are too many secondary substations to obtain selective timing with directional overcurrent relays.Continue Reading