For safety and reliable operation, the shields and metallic
sheaths of power cables must be grounded. Without grounding, shields would
operate at a potential considerably above ground.
Thus, they would be hazardous to touch and would cause 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
that is on the order of 1 mA/ft of conductor length.
This current normally flows, at power frequency, between the
conductor and the earth electrode of the cable, normally the shield. In
addition, the shield or metallic sheath provides a fault return path in the
event of insulation failure, permitting rapid operation of the protection
devices.
The grounding conductor and its attachment to the shield or
metallic sheath, normally at a termination or splice, should have an ampacity
no less than that of the shield. In the case of a lead sheath, the ampacity of
the grounding conductor should be adequate to carry the available fault current
without overheating, until it is interrupted.
Attachment to the shield or sheath is frequently done with
solder, which has a low melting point; thus an adequate area of attachment is
required.
There is much disagreement as to whether the cable shield
should 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 light shield conductor.
Such a current could readily 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 to 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 grounding at both ends,
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.
No comments:
Post a Comment