Custom Search
|
|
|
||
 (b) Longitudinally corrugated metal sheaths (corrugations or bellows formed
perpendicular to the cable axis) have been used for many years in direct-burial communications
cables, but only recently has this method of cable core protection been applied to control and
power cables. The sheath material may be of copper, aluminum, a corrosion resistant steel or
copper alloy, or a bimetallic composition of materials selected to best meet the intended service.
(c) Lead or lead alloys are used for industrial power cable sheaths for maximum cable
protection in underground manhole and tunnel or underground duct distribution systems subject
to flooding. While not as resistant to crushing loads as interlocked armor, its very high degree of
corrosion and moisture resistance makes lead attractive in the above applications. Protection
from installation damage can be provided by an outer jacket of extruded material.
(d) Extruded aluminum, copper, die-drawn aluminum, or copper sheaths are used in certain
applications for weight reduction and moisture penetration protection. While more crush
resistant than lead, aluminum sheaths are subject to electrolytic attack when installed
underground. Under these conditions, aluminum sheathed cable should be protected with an
outer extruded jacket.
(e) A high degree of mechanical protection and longitudinal strength can be obtained by
using spirally wrapped or braided round steel armor wire. This type of outer covering is
frequently used in submarine cable and vertical riser cable for support.
2.2.5 Single-conductor and Multiconductor Constructions. Single-conductor cables are
usually easier to handle and can be furnished in longer lengths than multiconductor cables.
Multiconductor constructions give smaller overall dimensions than an equivalent
single-conductor cable, providing a space advantage.
2.3 CABLE RATINGS AND SELECTION CRITERIA. Cables come in various sizes. The
size of a cable depends on the ampacity or voltage rating of the cable. Cables may contain
various conductor sizes, and the electrical characteristics of the cable depends on the conductor
size used.
2.3.1 Electrical and Environmental Specifications. The selection of power cables involves
the consideration of various electrical and environmental conditions. These conditions include
the quantity and characteristics of the power being distributed and the degree of exposure to
adverse mechanical and thermal stresses. The selection of conductor size is based on the
following criteria:
2.3.1.1 Voltage rating.
2.3.1.2 Load current criteria (as related to loadings, thermal effects of the load current,
2-5
|
 
|
|
|
||
