Single-Core Cable Sheath Bonding: Design and RFQ Guide

Why voltage appears on the sheath

AC conductor current produces a changing magnetic field. Magnetic coupling induces longitudinal voltage in nearby metallic sheaths and screens. The magnitude depends on load current, cable geometry, spacing, formation, section length and the electrical connections between sheaths and earth.

If the sheath circuit is closed through bonding at both ends, current can circulate. That current produces loss and heat, which may reduce usable ampacity. If the circuit is open at one end, circulating current is restricted, but a standing voltage appears at the insulated end. Neither consequence should be evaluated alone.

Three common bonding arrangements

Solid bonding

The metallic sheaths are bonded to earth at both ends and may also be bonded at intermediate points. This provides a defined earth connection and generally limits standing sheath voltage. The closed loop can carry circulating current in normal operation. The rating calculation must include applicable sheath losses for the installed geometry.

Single-point bonding

The sheath circuit is bonded to earth at one point and insulated from earth at the other end or at section boundaries. This prevents a normal closed loop, but induced voltage rises along the section. The design must set permissible section length, standing voltage and insulation level, provide an earth-continuity conductor where required, and coordinate sheath voltage limiters for transient protection.

Cross-bonding

Long routes can be divided into minor sections. Sheath connections are transposed so induced voltages from three balanced sections largely cancel over a major section. The result depends on phase arrangement, section balance and correct link-box connections. Unequal lengths, route geometry, parallel circuits and transposition errors must be included in the study rather than assumed away.

Information needed before the bonding study

• System voltage, frequency, normal and emergency load current.
• Maximum fault current, fault duration and earthing arrangement.
• Cable conductor, insulation, metallic screen or sheath and oversheath construction.
• Conductor and sheath electrical data, dimensions and temperature assumptions.
• Route length, joint positions, cable formation, spacing, phase sequence and transpositions.
• Buried, duct, tunnel, tray or air installation conditions and nearby circuits.
• Termination earthing, substation earth-grid data and earth-continuity conductor arrangement.
• Permitted standing and touch voltages under the owner’s rules.
• Switching, lightning and fault transient cases used for insulation coordination.

A cable quotation without these inputs can price the cable construction, but it cannot establish a complete, project-specific sheath-bonding design.

Specify the bonding equipment

Link boxes: state the schematic, number of ways, removable links, test access, enclosure material, ingress and corrosion requirements, internal clearances, labels, locks and mounting location.

Bonding leads: define conductor size, insulation level, length, routing, short-circuit duty, terminal interfaces and mechanical protection. Excessive lead length and poor routing can affect transient performance.

Sheath voltage limiters (SVLs): select them from a project insulation-coordination study. Cover continuous voltage, residual voltage, energy and current capability, temporary-overvoltage duty, environmental protection and required test evidence.

Earth-continuity conductor: where required, define conductor size, insulation, route, connection method and fault-current duty. It is not an unspecified accessory to add after the route is fixed.

Current rating and sheath losses

Do not transfer an ampacity value between bonding arrangements without recalculation. IEC 60287 includes methods for cable current rating and losses; its parts address sheath and circulating-current effects under stated assumptions. Use actual cable dimensions, material resistivities, installation geometry and bonding configuration, then document assumptions and sensitivity cases.

Sheath-current measurement can support commissioning or condition assessment, but an unexpected value does not identify the cause by itself. Load imbalance, geometry, parallel paths, incorrect links, damaged insulation or instrumentation error can alter the result.

Installation and commissioning checks

1. Issue an approved bonding schematic and link schedule for every location.
2. Trace cable drums, phases, joints, minor sections, link boxes, bonding leads and SVLs.
3. Inspect oversheath integrity under the approved test procedure.
4. Verify link positions and phase-to-sheath identification before energisation.
5. Check earth connections and earth-continuity conductor continuity.
6. Perform specified insulation, continuity and component tests with calibrated instruments.
7. Record load, sheath voltage and sheath current where required, then compare with calculated expectations.
8. Seal link boxes, preserve photographs and issue an as-built bonding schedule.

Common procurement failures

Buying link boxes without a design. A box count does not define the arrangement, component duty or terminal schedule.

Assuming cross-bonding automatically cancels everything. Cancellation depends on section balance, geometry and correct installation.

Ignoring maintenance access. Flooded or inaccessible link boxes, unidentified links and missing as-built records can defeat a sound design.

Treating an SVL as a generic surge arrester. Its voltage and energy duties must be coordinated with sheath insulation and project transients.

Separating bonding from ampacity. Sheath losses are part of the thermal design, so sizing and bonding studies must use consistent inputs.

Primary references

• CIGRE TB 797 summary — Sheath bonding systems of AC transmission cables
• CIGRE TB 801 summary — Safe work under induced voltages or currents
• IEC 60287-1-3:2023 — Current sharing and circulating-current losses
• CIGRE TB 283 — Special bonding of high-voltage power cables