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 (Figure 1-5), distribution is at the utilization voltage. A single primary service and distribution
transformer supply all the feeders. There is no duplication of equipment. System investment is
the lowest of all circuit arrangements. Operation and expansion are simple. Reliability is high if
quality components are used, however, loss of a cable, primary supply, or transformer will cut off
service. Further, electrical service is interrupted when any piece of service equipment must be
deenergized to perform routine maintenance and servicing.
1.5.2.2 Expanded Radial Distribution System. The advantages of the radial system may
be applied to larger loads by using a radial primary distribution system to supply a number of unit
substations located near the load centers with radial secondary systems (Figure 1-6). The
advantages and disadvantages are similar to those described for the simple radial system.
1.5.2.3 Primary Selective Distribution System. Protection against loss of a primary supply
can be gained through use of a primary selective system (Figure 1-7). Each unit substation is
connected to two separate primary feeders through switching equipment to provide a normal and
an alternate source. When the normal source feeder is out of service for maintenance or a fault,
the distribution transformer is switched, either manually or automatically, to the alternate source.
An interruption will occur until the load is transferred to the alternate source. Cost is somewhat
higher than for a radial system because primary cable and switchgear are duplicated.
1.5.2.4 Loop Primary-Radial Distribution System. The loop primary system (Figure 1-8)
offers nearly the same advantages and disadvantages as the primary selective system. The failure
of the normal source of a primary cable fault can be isolated and service restored by
sectionalizing. Finding a cable fault in the loop, however, may be difficult and dangerous. The
quickest way to find a fault is to sectionalize the loop and reclose, possibly involving several
reclosings at the fault. A section may also be energized at both ends, thus, effecting another
potential danger. The cost of the primary loop system may be somewhat less than that of the
primary selective system. The savings may not be justified, however, in view of the
disadvantages.
1.5.2.5 Secondary Selective-Radial Distribution System. When a pair of unit substations
are connected through a normally open secondary tie circuit breaker, the result is a secondary
selective-radial distribution system (Figure 1-9). If the primary feeder or a transformer fails, the
main secondary circuit breaker on the affected transformer is opened and the tie circuit breaker is
closed. Operation may be manual or automatic. Normally, the stations operate as radial systems.
Maintenance of primary feeders, transformer, and main secondary circuit breakers is possible
with only momentary power interruption, or no interruption, if the stations may be operated in
parallel during switching. With the loss of one primary circuit or transformer, the total substation
load may be supplied by one transformer. In this situation, however, if load shedding is to be
avoided, both transformers and each feeder must be oversized to carry the total load. A
distributed secondary selective system has pairs of unit substations in different locations
connected by tie cables and normally open tie circuit breakers. The secondary selective system
may be combined with the primary selective system to provide a high degree of reliability.
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