POWER CABLE SYSTEMS GROUNDING TUTORIALS and BASICS


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.

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