Short answer: MV cable screen bonding controls three connected outcomes: induced screen voltage, current circulating in the metallic screen, and the path available for earth-fault current. Both-end bonding keeps screen voltage near earth potential but can add circulating-current loss. Single-point bonding breaks the steady-state loop but allows voltage to rise toward the open end. Cross-bonding divides the route into coordinated sections so induced voltages largely cancel.
No method is universally best. The selection depends on circuit length, লোড কারেন্ট, phase formation, ব্যবধান, screen resistance, earth-fault duty, earthing arrangement, allowable touch voltage, cable rating, joint locations, and maintenance strategy.
First separate the metallic screen from the semiconductive screen
An extruded MV cable normally has conductor and insulation screens made from semiconductive compounds. These layers shape the electric field at the insulation boundaries. They are not the components connected through link boxes.
The bonding system acts on the metallic layer. That layer may use copper wires, তামার টেপ, a concentric neutral, or a continuous metallic sheath. Project documents often use পর্দা, shield, এবং খাপ differently. The connection diagram must therefore identify the actual metallic component, its cross-section, প্রতিরোধ, fault duty, and accessory interfaces.
The existing XWA guide to এমভি তারের পর্দার ধরন explains the construction differences. Bonding begins after the metallic design and earth-fault duty are known.
Why voltage appears on a grounded cable screen
Alternating conductor current creates a changing magnetic field. That field links the metallic screen and induces a longitudinal voltage. Its magnitude and phase depend on conductor current, ফ্রিকোয়েন্সি, তারের ব্যবধান, trefoil or flat formation, screen position, রুট দৈর্ঘ্য, parallel circuits, and nearby metallic paths.
A ground connection at one location fixes that point near local earth potential. It does not remove induction along the full route. If the screen also connects to earth at another location, the two connections close a conducting loop. The induced voltage can then drive circulating current through the screen, bonding leads, earth connections, and connected metallic parts.
These losses matter because they generate heat. আইইসি 60287-1-1 includes sheath and armor losses in steady-state current-rating calculations. আইইসি 60287-1-2 makes the trade-off explicit for single-core circuits: significant circulating current can occur with both-end bonding, while single-point or cross-bonded arrangements avoid that closed-loop component when correctly designed.
Both-end bonding favors voltage control but creates a loop
Both-end bonding, also called solid or multiple-point bonding in some documents, connects the metallic screens to earth at both route ends. Intermediate earth points may also appear in a multiple-point arrangement. The grounded connections limit steady-state screen-to-earth voltage along the cable.
The same connections form a closed loop. Induced voltage drives current around that loop during normal load operation. The resulting screen loss increases total cable-system heat and can reduce permissible conductor current. The effect is not a fixed percentage. It changes with cable spacing, formation, screen resistance, load balance, circuit arrangement, and earth-path impedance.
| Both-end bonding characteristic | Engineering consequence | Required check |
|---|---|---|
| Earth connection at both ends. | Screen voltage remains controlled relative to local earth. | Earth potential difference, bonding-lead duty, and touch-voltage basis. |
| Closed metallic loop. | Load current can produce screen circulating current and heat. | আইইসি 60287 loss calculation and resulting ampacity. |
| Continuous fault-current path. | The screen system participates in earth-fault return according to the network design. | Screen area, connection resistance, earth-fault current, and clearing time. |
| Simple connection concept. | Fewer special joints and transpositions may be required. | As-built continuity and avoidance of unintended parallel paths. |
Both-end bonding can remain practical for shorter MV circuits or routes where calculated losses stay acceptable. Length alone does not decide the method. A compact trefoil circuit and a widely spaced flat circuit of equal length can produce different induced voltages and losses.
Single-point bonding removes the steady-state circulation path
Single-point bonding intentionally earths the metallic screen at one point. The other end remains insulated from earth under normal operation. Without a second intentional connection, the screen has no closed path for normal circulating current.
The trade-off is standing voltage. Screen-to-earth voltage increases with distance from the bonded point and reaches its highest normal value near the insulated end. The design must check that voltage against sheath insulation, joint and termination interfaces, accessible metalwork, touch-voltage criteria, and the selected sheath voltage limiter where one is required.
A sheath voltage limiter is not a normal earth bond. It limits transient overvoltage by conducting above its protective level. Its continuous voltage, energy duty, temporary overvoltage behavior, lead length, insulation coordination, and failure mode belong to the project study.
The fault path still needs a deliberate design
Breaking the normal screen loop does not remove the need for an earth-fault return path. A parallel earth continuity conductor may connect the earth grids at both ends. Its location and transposition affect induced voltage and current. The design must also consider fault-current sharing among screens, the continuity conductor, earth electrodes, structural metal, and other parallel paths.
Single-point bonding can use end-point bonding or midpoint bonding. Midpoint bonding divides the route so induced voltage rises toward both ends, which can reduce the maximum standing voltage for the same total length. It also changes joint, link-box, and earth-continuity arrangements.
Cross-bonding cancels voltage across three coordinated sections
Cross-bonding divides a three-phase route into three minor sections. At sectionalizing joints, the metallic screens cross-connect so each screen occupies a different induced-voltage position in each section. One set of three minor sections forms a major section.
When section lengths, cable formation, phase order, and loading are sufficiently balanced, the three induced screen-voltage phasors largely cancel over the major section. This limits end voltage while avoiding most circulating current associated with solid bonding.
Cancellation is not automatic. Unequal section lengths, different spacing, a formation change, parallel-circuit asymmetry, phase transposition errors, or an incorrect link-box connection leaves residual voltage. Cross-bonding therefore needs a route model and a connection schedule, not only a note on the cable datasheet.
| Cross-bonding input | কেন এটা গুরুত্বপূর্ণ | Evidence needed |
|---|---|---|
| Minor-section lengths. | Unequal induced-voltage contributions reduce cancellation. | Approved route and joint-position schedule. |
| Phase and sheath sequence. | A wrong cross-connection defeats the intended phasor sum. | Link-box diagram, labels, and continuity record. |
| Cable formation and spacing. | Trefoil, flat, and transitions change mutual inductive coupling. | Route cross-sections and installation survey. |
| SVL arrangement. | Transient protection depends on insulation coordination and lead geometry. | Study, component data, and installed connection check. |
| Earth continuity. | Fault-current return must remain available across sectionalized screens. | Earthing diagram and fault-duty calculation. |
Cross-bonding often serves longer, higher-capacity single-core circuits, but it is not automatically justified for every MV feeder. Its thermal benefit must exceed the extra joints, link boxes, design work, installation control, পরীক্ষা, and maintenance burden.
The choice changes the cable current rating
A current-rating study should use the selected bonding configuration rather than adding it after conductor size is fixed. Screen losses become part of the thermal balance. Both-end bonding can lower ampacity through circulating-current loss. Single-point and cross-bonded systems can reduce that loss, but eddy-current and other metallic losses may remain.
The rating model also needs route conditions. Soil thermal resistivity, duct arrangement, পরিবেষ্টিত তাপমাত্রা, কবরের গভীরতা, তারের ব্যবধান, load factor, neighboring circuits, and drying assumptions may dominate the final result. A bonding change cannot compensate for an incorrect thermal route model.
দ MV power cable construction and the voltage-specific 15কেভি তারের page provide product context. The project rating still needs the actual screen, রুট, formation, and bonding data.
Bonding design must cover normal, fault, and transient conditions
Normal operation determines standing voltage and circulating losses. Earth faults determine screen, bonding-lead, earth-continuity, and connection duty. Switching events and lightning can impose transient voltage across sectionalized sheaths and link-box components.
Local earth potential rise also matters. Two cable ends may connect to earth grids that do not remain at the same potential during a fault. A bonding design that considers induced voltage alone can miss stress transferred from one substation earth system to another.
The CIGRE B1.50 review treats the bonding system as a lifecycle system that includes screens or sheaths, armouring, insulating jackets, bonding leads, link boxes, SVLs, পরীক্ষা, এবং রক্ষণাবেক্ষণ. Its technical overview also notes that lower-voltage circuits use the same principles, even when utilities apply more standardized arrangements.
A connection schedule should exist before manufacture
The cable factory can define screen material, ক্রস-সেকশন, প্রতিরোধ, construction dimensions, oversheath, and accessory interface data. The system designer must connect those properties to the network, রুট, earthing, and protection study.
A complete schedule normally records:
- Electrical system: ভোল্টেজ, ফ্রিকোয়েন্সি, maximum load current, phase balance, earth-fault current, পরিষ্কার করার সময়, গ্রাউন্ডিং পদ্ধতি, and transient basis.
- Cable data: কন্ডাকটরের আকার, screen or sheath material, metallic area, প্রতিরোধ, cable diameter, formation, ব্যবধান, and parallel circuits.
- Route data: section lengths, joint positions, trefoil or flat zones, transpositions, cable crossings, and earth-grid locations.
- Bonding scheme: both-end, single-point, midpoint, or cross-bonded diagram with link-box connections and labels.
- Protection: SVL location and rating basis, bonding-lead length, নিরোধক স্তর, earth continuity, and touch-voltage limits.
- Verification: factory screen data, oversheath tests, ধারাবাহিকতা চেক, link-box inspection, connection resistance, labels, and as-built drawings.
Commissioning should prove the installed diagram
A correct design can fail through one crossed lead, one unintended earth connection, or one open continuity conductor. Commissioning should therefore verify the actual topology before energization.
- Trace every screen section. Confirm phase identity and each link-box terminal against the approved drawing.
- Check intentional and unintended earth paths. Temporary construction bonds, গ্রন্থি, বর্ম, supports, or test leads can bypass the design.
- Verify continuity and connection quality. Apply the project procedure and acceptance criteria to screens, bonding leads, and earth-continuity conductors.
- Test the oversheath and sectional insulation. Follow the applicable cable-system and accessory procedure without exceeding component limits.
- Inspect SVLs and leads. Confirm location, orientation, connection length, অন্তরণ, environmental sealing, and identification.
- Record the final configuration. Keep link positions, readings, photos, deviations, and approvals with the commissioning file.
Later maintenance should preserve the same topology. A link box left in a test position, a failed SVL, ক্ষয়, জল প্রবেশ, or an unrecorded earth bond can change losses and screen voltage without changing the power conductor.
Common questions about MV screen bonding
Which method gives the highest ampacity?
No method guarantees the highest project rating in isolation. Single-point and cross-bonded arrangements can reduce circulating-current loss, but route thermal conditions, eddy losses, section balance, পর্দা নকশা, and operational limits still control the result.
Can the cable screen remain ungrounded at both ends?
That is not a normal default for an energized MV circuit. The metallic screen needs a defined reference, fault-current path, voltage control, and accessory coordination. Special arrangements require a system study and approved diagram.
Does cross-bonding require phase conductors to change position?
Cross-bonding primarily transposes the sheath connections at sectionalizing points. Phase-conductor transposition is a separate route decision. The model must reflect the actual conductor formation and any physical transposition.
Is a sheath voltage limiter the same as an earth link?
না. An earth link provides a metallic connection. An SVL remains non-conducting at normal voltage and limits specified transient stress. Its selection requires insulation coordination and energy-duty checks.
Why can a short MV feeder still need a bonding calculation?
High current, wide flat spacing, parallel circuits, low-resistance screens, earth-grid potential differences, or strict touch-voltage criteria can matter even on a short route. Length is only one input.
ইঞ্জিনিয়ারিং উপসংহার
MV cable screen bonding is an electrical and thermal system decision. Both-end bonding controls screen voltage but can create heat-producing circulating current. Single-point bonding removes the normal loop but creates a standing-voltage and fault-path problem. Cross-bonding balances those effects across coordinated sections but demands accurate joints, link boxes, phasing, SVLs, পরীক্ষা, এবং রক্ষণাবেক্ষণ.
The defensible method is to calculate normal voltage and losses, verify fault and transient duties, then freeze the connection and commissioning schedule before manufacture. A bonding label without those inputs does not complete an MV cable specification.
Send the Cable System and Bonding Basis
For technical alignment, provide system voltage, ফ্রিকোয়েন্সি, load and earth-fault duties, গ্রাউন্ডিং পদ্ধতি, conductor and metallic-screen data, cable formation, ব্যবধান, route and section lengths, joint positions, parallel circuits, earth-grid locations, proposed bonding diagram, SVL requirements, পরীক্ষা, পরিমাণ, and destination. এক্সডব্লিউএ পাওয়ার & Cable can align cable construction and interface documents with the approved system basis.