Introduction to installing power cables
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.
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 .
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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.
2.1 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.
2.2 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 provides high strength to withstand climatic
rigors or mechanical shock. It may also serve as the grounding conductor
of the power circuit.
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3. Above-Ground Conduits
Rigid
steel conduit systems afford the highest degree of mechanical
protection available in above-ground conduit systems. Unfortunately,
this is also a relatively high-cost system. For this reason their use is
being superseded, where possible, by other types of conduit and wiring
systems.
Where applicable, rigid aluminum, intermediate-grade
steel conduit, thin-wall EMT, intermediate-grade metal conduit, plastic,
fiber and asbestos-cement ducts are being used.
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4. Underground Ducts
Underground
ducts are used where it is necessary to provide a high degree of safety
and mechanical protection, or where above-ground conductors would
be unattractive.
4.1 Construction
Underground ducts use rigid steel, plastic, fiber, and asbestos-cement conduits encased in concrete, or precast multi-hole concrete with close fitting joints.
Clay
tile is also used to some extent. Where the added mechanical protection
of concrete is not required, heavy wall versions of fiber and
asbestos-cement and rigid steel and plastic conduits are direct buried.
4.2. Cables
Cables used in underground conduits must be suitable for use in wet areas, and protected against abrasion during installation.
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5. Direct Burial
Cables
may be buried directly in the ground where permitted by codes and only
in areas that are rarely disturbed. The cables used must be suitable for
this purpose, that is, resistant to moisture, crushing, soil contaminants, and insect and rodent damage.
While direct-buried cable cannot be readily added to or maintained, the
current carrying capacity is usually greater than that of cables in
ducts. Buried cable must have selected backfill.
It must be used
only where the chance of disturbance is unlikely. The cable must be
suitably protected, however, if used where the chance of disturbance is
more likely to occur.
Relatively recent advances in the design and operating characteristics of cable fault location equipment and subsequent repair methods and material have diminished the maintenance problem.
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6. Underwater (Submarine) Cable
Submarine cable is used only when no other cable system can be used. It supplies circuits that must cross expanses of water or swampy terrain.
6.1 Construction
Submarine cable generally consists of a lead sheathed cable and is usually armored. Insulation material
should be XLP or EPR, except when paper insulation is justified because
of its high resistance to, and freedom from, internal discharge or
corona.
Multiconductor construction should be used, unless limited
by physical factors. The lead sheathing usually consists of a
copper-bearing lead material, however, other alloys may be required when
special conditions warrant nonstandard sheathing. The most common type
of armoring material used for submarine cables is the spirally wrapped
round galvanized steel wire.
In this type of cable, asphalt
impregnated jute is usually applied over the lead sheath and the
wire armor is applied over the jute to reduce mechanical damage and
electrolytic corrosion. An additional covering of the asphalt
impregnated jute may be applied over the wire armor.
Nonmetallic
sheathed cables are sometimes suitable for certain submarine
applications. The cable must be manufactured specifically for submarine
service and, generally, has an increased insulation thickness. The cable
may require wire armor and should have electrical shielding for all
voltage ratings above 600 V.
6.1 Installation
Submarine
cable should lie on the floor of the body of water and should have
ample slack so that slight shifting caused by current or turbulence will
not place excessive strain on the cable. Where the cable crossing is
subject to flow or tidal currents, anchors are often used to prevent
excessive drifting or shifting of the cable. In addition to
laying cables directly on the bottom, burying cable in a trench using
the jetwater method should be considered.
Cables must be buried in
waters where marine traffic is present. The depth of burial should be
enough to prevent damage caused by dragging anchors, which may be in
excess of 15 feet for large ships on sandy bottoms.
Warning signs
located on shore at the ends of the submarine cable should be provided
to prohibit anchoring in the immediate vicinity of the cable.
Grounding of Cable Systems
For safety and reliable operation, the shields and metallic sheaths of power cables must be grounded. Without such grounding, shields would operate at a potential considerably above ground. Thus, they would be hazardous to touch, and would incur rapid degradation of the jacket or other material intervening between shield and ground.This is caused by the capacitive charging current of the cable insulation which is approximately 1 milliampere (mA) per foot of conductor length. This current normally flows at a power frequency between the conductor and the earth electrode of the cable, normally the shield. In addition, the shield or metallic sheath provides the fault return path in the event of insulation failure, permitting rapid operation of the protection devices.
Grounding Conductor
The grounding conductor, and its attachment to the shield or metallic sheath, normally at a termination or splice, should have an ampacity no lower than that of the shield.In the case of a lead sheath, the grounding conductor must be able to carry the available fault current over its duration without overheating. Attachment to shield or sheath is frequently by means of solder, which has a low melting point; thus an adequate area of attachment is required.
Grounding Methods
The cable shield lengths may be grounded at both ends or at only one end.
If
grounded at only one end, any possible fault current must traverse the
length from the fault to the grounded end, imposing high current on the
usually very thin shield conductor. Such a current could damage or
destroy the shield, and require replacement of the entire cable rather
than only the faulted section. With both ends grounded, the fault
current would divide and flow to both ends, reducing the duty on the
shield, with consequently less chance of damage.There are modifications of both systems. In one, single-ended grounding may be attained by insulating the shields at each splice or sectionalizing point, and grounding only the source end of each section. This limits possible shield damage to only the faulted section.
Multiple grounding, rather than just double-ended grounding, is simply the grounding of the cable shield or sheath at all access points, such as manholes or pull boxes. This also limits possible shield damage to only the faulted section.
source: electrical engineering portal.
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