Single-Core Cable Sheath Bonding: Design and RFQ Guide

Why does voltage appear on the sheath?

AC conductor current produces a changing magnetic field. This field couples into nearby metallic sheaths and screens and induces a longitudinal voltage in them. The size of that voltage depends on load current, cable geometry, spacing, cable formation, section length and the electrical connections between the sheaths and earth.

If the sheath circuit is closed by bonding at both ends, current can circulate. That current produces loss and heat, which may reduce the usable current rating. 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 on its own.

The sheath is therefore part of the electrical system, not only a construction layer. For background on voltage classes and accessories, see the medium-voltage cable buying guide.

How do the three bonding methods compare?

Solid bonding. The metallic sheaths are bonded to earth at both ends and may also be bonded at intermediate points. This gives a defined earth connection and generally limits standing sheath voltage. The closed loop can carry circulating current in normal operation, so the rating calculation must include the 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 the permissible section length, standing voltage and insulation level, provide an earth-continuity conductor where required, and coordinate sheath voltage limiters (SVLs) for transient protection.

Cross-bonding. Long routes can be divided into minor sections. Sheath connections are transposed so that 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.

The table summarises the arrangements described above. Intermediate bonding is shown as a variant of solid bonding. The method, section lengths and component ratings must come from the project bonding study.

What information does the bonding study need?

This guide explains specification boundaries. It is not a sheath-bonding calculation, and final bonding, earthing, SVL, insulation and touch-voltage requirements must be set by the project cable-system engineer using the governing standards, the owner's rules and verified route data. The study needs the following inputs:

• System voltage, frequency, and 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.

What should the bonding equipment specification cover?

Bonding components are engineered parts, and each one needs its own specification.

• 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.

Standing-voltage limits are set by the owner's rules and vary by utility and country. IEEE 575 includes an annex on standing voltage, which is a useful starting point for that discussion, but a single limit should not be copied from another project.

How do bonding choices affect rating and losses?

Do not transfer an ampacity value between bonding arrangements without recalculation. The IEC cable current-rating standards include methods for losses and circulating-current effects, under stated assumptions. Use actual cable dimensions, material resistivities, installation geometry and bonding configuration, then document the assumptions and sensitivity cases. For the general rating approach, see the cable ampacity and derating guide.

Armour material also affects losses. Single-core AC cables commonly use non-magnetic aluminium wire armour (AWA), because steel armour around a single AC conductor causes magnetic losses and heating. BS 5467 covers single-core AWA cables and multicore SWA cables at 0.6/1 kV, and BS 6724 covers LSZH-sheathed armoured equivalents.

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 all alter the result.

Where do bonding projects go wrong, and what must be checked at commissioning?

Common procurement failures include the following:

• 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.

Commissioning should follow these 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 the specified insulation, continuity and component tests with calibrated instruments.
7. Record load, sheath voltage and sheath current where required, then compare them with calculated expectations.
8. Seal link boxes, preserve photographs and issue an as-built bonding schedule.

Frequently asked questions