The metallic sheath in an HV cable is not simply a shield wrapped around XLPE insulation. It forms an electrical boundary around the insulation, participates in induced-voltage and loss calculations, carries a defined share of earth-fault current, and connects the cable to joints, terminations, link boxes, earths, and sheath voltage limiters. In lead- or aluminium-sheathed constructions, it can also provide a radial moisture barrier.
Those duties interact. Increasing metallic area may improve fault-current capability and reduce resistance, yet lower resistance can increase circulating-current loss under both-end bonding. Isolating a sheath can reduce that loss, yet creates standing voltage and transient-protection requirements. A technically complete design must therefore check the same sheath through normal load, fault, transient, and long-term route conditions.
Five cable layers are often collapsed into the word “shield.”
A shielded high-voltage cable contains several layers with different functions. The conductor screen smooths the electrical interface around the stranded conductor. The XLPE insulation carries the main dielectric stress. The insulation screen creates a controlled outer boundary for that insulation. The metallic screen or sheath sits outside the insulation screen and provides an electrically continuous metallic path. The polymeric oversheath protects and, where required, electrically insulates that metallic layer from earth.
These layers are not interchangeable. A semiconductive insulation screen controls the field interface but is not a substitute for a metallic earth-fault path. A copper-wire screen may provide electrical continuity and a configurable cross-section, but the wire layer alone is not a continuous radial water barrier. A welded aluminium or extruded lead sheath can provide a metallic moisture barrier, but material selection still needs mechanical, thermal, corrosion, manufacturing, and accessory-interface checks. Armour, where used, has a primary mechanical function and cannot automatically replace the designed sheath circuit.
| Layer or component | Primary system role | Design question that remains |
|---|---|---|
| Conductor screen. | Creates a smooth electrical interface over the conductor. | How does the extrusion system control geometry, cleanliness, and compatibility? |
| Insulation screen. | Defines the outer equipotential boundary of the main insulation. | How will joints and terminations restore that boundary? |
| Metallic screen or sheath. | Provides metallic continuity, bonding connection, and specified current duty. | Which material, resistance, area, bonding arrangement, and fault duty apply? |
| Metallic moisture barrier, when present. | Limits radial moisture entry and may also perform the electrical sheath function. | How will welds, joints, corrosion protection, bending, and fatigue be controlled? |
| Polymeric oversheath. | Protects the metal and can insulate it from earth for special bonding. | Which electrical integrity, mechanical, environmental, and after-installation checks apply? |
One sheath design must pass four different system states.
A datasheet that names only “aluminium sheath” or “copper-wire screen” leaves the important work unfinished. Material and nominal dimensions describe the physical layer; they do not establish its performance in the connected cable system.
State 1: normal load creates induced voltage and heat.
AC conductor current produces magnetic flux that links the metallic sheath. The resulting induced voltage depends on current, frequency, cable geometry, phase arrangement, section length, and nearby conductors. The bonding arrangement decides what happens next.
When sheaths are bonded at both ends, the closed path can carry circulating current. That current produces additional loss and can reduce cable current-carrying capacity. IEC 60287-1-1 provides the calculation framework for conductor and sheath losses in a single circuit. IEC 60287-1-2 addresses eddy-current losses for defined double-circuit flat formations and explicitly distinguishes one-point or cross-bonded sheaths from both-end bonding with significant circulating current.
State 2: an earth fault divides current among several paths.
The metallic sheath may carry only part of the total earth-fault current. Current can divide among cable sheaths, earth conductors, station grids, parallel metallic paths, and earth according to system impedances and bonding. The maximum prospective fault current at a bus is therefore not automatically the current assigned to one cable sheath.
IEC 60909-3 addresses partial short-circuit currents through earth and includes procedures for current distribution and reduction factors of metallic sheaths or shields earthed at both ends. After current division is established, the sheath, bonding leads, joints, link-box connections, and earth conductors need thermal and mechanical checks for the applicable clearing sequence. IEC 60949 provides a method for thermally permissible short-circuit current that accounts for non-adiabatic heating; it does not remove the need to define material, initial and final conditions, duration, and the actual current path.
State 3: switching and lightning events test insulation coordination.
Special bonding deliberately allows parts of the sheath system to operate above earth potential. Power-frequency standing voltage is only one condition. Switching events, lightning impulses, earth faults, and interactions with connected equipment can impose transient voltage across sheath sectionalizing insulation, the oversheath, link-box insulation, and sheath voltage limiters.
An SVL is not selected from system voltage alone. Its continuous voltage, protective level, temporary overvoltage behavior, energy duty, connection length, earthing arrangement, and relationship to the protected insulation must come from the insulation-coordination study. A lower protective level is not automatically safer if normal or temporary voltage overstresses the limiter. A higher rating is not automatically safer if it fails to protect the insulation.
State 4: route conditions challenge the barrier for decades.
A metallic sheath may share electrical and environmental duties. Lead and aluminium sheath systems can provide radial moisture protection when their continuity and accessory seals remain intact. Copper wires can define a fault-current path and bonding connection, often with a separate foil or laminate where a radial barrier is required.
The route then introduces corrosion exposure, cyclic movement, bending, vibration, joint-bay water, pulling forces, thermal expansion, and installation damage. The outer covering becomes important because damage can expose the metal, compromise corrosion protection, or defeat an insulated-bonding scheme. IEC 60229 covers extruded oversheaths with a special protective function and includes electrical tests after installation. A main-insulation withstand result does not demonstrate that the oversheath remains intact.
Bonding changes the answer; it is not an accessory selected later.
Solid bonding, single-point bonding, and cross-bonding are circuit arrangements rather than quality grades. Solid bonding provides a continuous sheath-current path and generally limits standing voltage, but circulating loss can become material. Single-point bonding interrupts continuous circulation in a section, while standing voltage, earth continuity, sheath insulation, and transient protection need control. Cross-bonding connects successive minor sections so induced voltages can largely cancel when lengths, cable geometry, phase relationships, and connections remain suitably balanced.
A route transition can disturb that balance. Changes from trefoil to flat formation, unequal minor-section lengths, phase-order changes, joint relocation, parallel circuits, or different cable constructions alter the induced-voltage model. The recent XWA article on MV cable screen bonding arrangements explains the basic circuit options. At HV transmission level, the same names require a project-specific insulation-coordination, loss, fault, touch-voltage, and maintenance model.
CIGRE TB 797 treats AC transmission cable systems at and above 66 kV as requiring a uniquely designed sheath-bonding system. Its scope joins the cable and accessories with bonding leads, link boxes, SVLs, earthing, installation tests, and maintenance. That system boundary is the reason a cable cross-section alone cannot settle the bonding design.
Material choice follows the failure mode, not a hierarchy.
Copper wires allow the metallic cross-section to be adjusted around the insulation screen and remain compatible with flexible cable construction. A separate water-blocking or metallic-laminate solution may be needed when the route requires a radial barrier. Welded aluminium can combine metallic continuity with a continuous moisture barrier and lower mass than lead, while weld quality, forming, corrosion protection, bending behavior, and accessory sealing remain critical. Extruded lead provides a continuous barrier and useful chemical resistance in suitable designs, but adds mass and requires attention to mechanical support, cyclic strain, environmental controls, and compatible jointing methods.
No material name proves fault capability. Electrical resistivity, effective metallic area, construction tolerances, joints, connection hardware, current division, fault duration, and permissible temperature all matter. No material name proves route durability either. The selected metal, oversheath, water-blocking system, seals, installation method, and soil or tunnel environment must work together.
A single fault-current number is not a sheath specification.
A statement such as “fault current for one second” remains incomplete unless the document states whether the current is the total system value, zero-sequence current, or the portion allocated to the metallic sheath. It also needs the fault type, grounding arrangement, clearing sequence, contribution from both ends, parallel paths, bonding state, initial load condition, and treatment of repeated duty.
The design sequence should begin with the network fault cases and current division. It should then identify the most severe sheath and bonding-component duty and check thermal capability and connections. The same cases govern the review of touch voltage, earth-potential rise, oversheath stress, and SVL energy. The official IEC current-distribution procedures provide a neutral reference for the current-split problem without supplying project-specific inputs.
The final drawing must join cable construction to the route circuit.
The 64/110(123) kV cable range and 76/132(145) kV cable range show voltage-specific construction context. A project record must go further by joining each cable section and accessory to the bonding calculation.
The controlled record should identify the metallic material, nominal area, longitudinal resistance basis, radial barrier, and oversheath duty. It should also fix cable formation, spacing, section lengths, phase order, bonding connections, link-box locations, earth connections, SVL data, and bonding-lead construction. Operating currents, fault cases, clearing times, standing-voltage limits, transient cases, touch-voltage basis, installation tests, and maintenance access complete the record.
The decisive check is consistency. The cable drawing, manufacturing lengths, joint schedule, link-box diagram, loss calculation, fault-current split, insulation-coordination study, and as-built route must describe the same electrical system. If one record changes, the affected calculations and interfaces require review before the change becomes a field instruction.