Quantcast Grounding of Cable Systems

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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.
2.4.6.2 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.
2.4.7 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.
2.4.7.1 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.
2.4.7.2 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
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