Reliability of service generally is interpreted to mean the continuity
of service or the lack of interruption to service. For a distribution system,
or any of its parts, absolute reliability or continuity of service 100
percent of the time for 100 percent of its consumers is an impossibility,
although this goal can be approached. The costs to achieve such goals,
even partially, are usually not warranted.
Degree of Service Reliability
As a practical matter, all consumers may not require a uniformly
high degree of service reliability. For some consumers, an extremely
high degree of service is essential; these may include hospitals, military
establishments, some larger theaters, department stores, apartment
buildings, hotels, etc., where the safety of the public is concerned; often
auxiliary sources of supply are provided to supplement the utility
company supply.
For some other types of loads, a high degree of reliability is desirable
but not so essential from the public safety viewpoint; smaller
apartments and theaters are examples of these, as well as some
manufacturing or service processes where interruption may result in
substantial monetary losses. To the average residential or commercial
consumer, however, a short interruption (and in some cases even an occasional
long one) is more of an inconvenience than a hazard or cause
for monetary loss.
As a rule, provisions for higher degrees of service reliability involve
higher expenditures, for both additional facilities and increased
maintenance. The expenditure to provide reliability should bear some
proportion to the degree of reliability needed. Various system designs,
outlined in Chapter 2, provide for varying degrees of service reliability,
from a simple, unsectionalized radial feeder to a low-voltage secondary
network supplied from a multiplicity of primary feeders isolated from
each other. Each type of service should produce revenues to justify the
additional expenditures for achieving the service reliability desired or
required; exception may be made for such public services as hospitals
and military establishments.
Overhead versus Underground
In this regard, comparisons between overhead and underground
systems should be borne in mind. Overhead systems are generally much
less costly but are more vulnerable to the hazards of nature (wind, ice,
lightning, flood, etc.) and to the actions of people (vehicles hitting poles,
kites, etc.); they are, however, easier to maintain, especially as faults can
be more easily located and repaired. Underground systems, generally
more expensive and less vulnerable to the vagaries of nature and people,
nevertheless require longer times for the location and repair of faults
that may occur.
Reliability Indices
Service reliability indices are maintained to measure and obtain
trends in the performance of a distribution system and its components.
Some of these include number of interruptions per consumer served;
number of consumers affected per consumer served; number of consumer
hours of interruption per consumer served; average duration of
interruption (hours) per consumer affected; average number of consumers
affected per consumer served; and average duration (hours) per consumer
served. Further indices are maintained as to causes and duration
of interruptions in the several parts of the system, e.g., on the basis of
miles of conductor installed, on the miles of circuit, by voltage classification,
or by geographic divisions. Compilation and analyses of these data
lend themselves to computer application.
Trends
The trends, over a period of time, not only measure the effectiveness
of system designs (and operating procedures), but also point out
areas of need for further improvement of service continuity.
