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 temperature limit for each type of insulation for short-circuit durations not in excess of 10
seconds has been established, and many times this criterion is used to determine minimum
conductor size. Insulated Power Cable Engineers Association (IPCEA) standards define the
maximum conductor temperature limits allowable under worst-case fault conditions.
2.4 TYPES OF CABLE INSTALLATIONS. There are a variety of ways to install power
distribution cables. Each method ensures distribution of power with a unique degree of
reliability, safety, economy, and quality for any specific set of conditions. These conditions
include the electrical characteristics of the power system, the distance and terrain of distribution,
and the expected mechanical and environmental conditions.
2.4.1 Open-Wire. Open-wire construction consists of uninsulated conductors on insulators
which are mounted on poles or structures. The conductor may be bare or it may have a thin
covering for protection from corrosion or abrasion. The attractive features of this method are its
low initial cost and the fact that damage can be detected and repaired quickly. On the other hand,
the uninsulated conductors are a safety hazard and are also highly susceptible to mechanical
damage and electrical outages resulting from short circuits caused by birds or animals. Proper
vertical clearances over roadways, walkways, and structures are critical. Exposed open-wire
circuits are also more susceptible to the effects of lightning than other circuits, however, these
effects may be minimized by the use of overhead ground wires and lightning arresters. In
addition, there is an increased hazard where crane or boom truck use may be involved. In some
areas contamination on insulators and conductor corrosion can result in high maintenance costs.
2.4.2 Aerial Cable. Aerial cable consists of fully insulated conductors suspended above the
ground. This type of installation is used increasingly, generally for replacing open wiring, where
it provides greater safety and reliability and requires less space. Properly protected cables are not
a safety hazard and are not easily damaged by casual contact. They do, however, have the same
disadvantage as open-wire construction, requiring proper vertical clearances over roadways,
walkways, and structures.
2.4.2.1 Supports. Aerial cables may be either self-supporting or messenger-supported.
They may be attached to pole lines or structures. Self-supporting aerial cables have high tensile
strength for this application. Cables may be messenger-supported either by spirally wrapping a
steel band around the cables and the messenger or by pulling the cable through rings suspended
from the messenger.
2.4.2.2 Distance. Self-supporting cable is suitable for only relatively short distances, with
spans in the range of 100-150 feet. Messenger-supported cable can span relatively large
distances, of over 1000 feet, depending on the weight of the cable and the tensile strength of the
messenger. For this reason, aerial cable that must span relatively large distances usually consists
of aluminum conductors to reduce the weight of the cable assembly. The supporting messenger
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