Question:
Bonding of the cable is done to:
Bonding of the cable is done to:
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To minimize these circulating sheath currents while maintaining safety grounding, modern installations use special Cross-Bonding schemes, which cancel out the induced sheath voltages over three consecutive cable segments.
Updated On: Jun 25, 2026
- Decrease the effective R and L
- Increase the effective R and L
- Decrease the effective R but increase L
- Increase the effective R but reduce L
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The Correct Option is D
Solution and Explanation
Concept:
High-voltage underground cables are equipped with metallic sheaths (usually made of lead or aluminum) to protect the insulation from moisture and mechanical damage. When alternating currents (AC) flow through the inner conductor, the time-varying magnetic flux induces voltages within these metallic sheaths via mutual electromagnetic coupling. If the sheaths are bonded together at multiple points along the run, a closed loop path is formed, allowing circulating sheath currents to flow.
Step 1: Evaluate the effect on effective line inductance (\(L\)).
The circulating currents induced in the metallic sheath flow in the opposite direction to the primary currents in the core conductor. This behavior mimics a short-circuited transformer secondary winding. The magnetic flux produced by these sheath currents opposes the main conductor magnetic flux, reducing the total net magnetic flux linkages around the core conductor. Since inductance is defined as flux linkages per unit current ($L = \frac{\lambda}{I}$), this reduction in net flux reduces the effective inductance of the cable system.
Step 2: Evaluate the effect on effective line resistance (\(R\)).
The circulating sheath currents encounter ohmic resistance within the sheath material, causing extra $I^2R$ power losses. These losses add to the power losses occurring in the main core conductor. From the power system's perspective, this increase in overall power loss manifests as an increase in the effective AC resistance of the cable.
Step 3: Combine the structural results.
Bonding underground cables increases the effective AC resistance due to sheath losses, while reducing the effective inductance via opposing magnetic flux cancellation. Therefore, the correct statement is Option (D).
Step 1: Evaluate the effect on effective line inductance (\(L\)).
The circulating currents induced in the metallic sheath flow in the opposite direction to the primary currents in the core conductor. This behavior mimics a short-circuited transformer secondary winding. The magnetic flux produced by these sheath currents opposes the main conductor magnetic flux, reducing the total net magnetic flux linkages around the core conductor. Since inductance is defined as flux linkages per unit current ($L = \frac{\lambda}{I}$), this reduction in net flux reduces the effective inductance of the cable system.
Step 2: Evaluate the effect on effective line resistance (\(R\)).
The circulating sheath currents encounter ohmic resistance within the sheath material, causing extra $I^2R$ power losses. These losses add to the power losses occurring in the main core conductor. From the power system's perspective, this increase in overall power loss manifests as an increase in the effective AC resistance of the cable.
Step 3: Combine the structural results.
Bonding underground cables increases the effective AC resistance due to sheath losses, while reducing the effective inductance via opposing magnetic flux cancellation. Therefore, the correct statement is Option (D).
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