Route length alone cannot determine the number of cross-bonding link boxes. Start with an engineered sheath-bonding layout, count its cross-bonding and direct-earth locations, then use the accessory schedule to establish the physical enclosure quantity.
Route length alone cannot determine a cross-bonding link box count. You need the cable construction, joint locations and an engineered sheath-bonding layout first. The count then follows from the required cross-bonding and earthing connections, plus how those connections are packaged into enclosures. Use the checklist below to prepare a design request and review the resulting schedule, not to select connection positions yourself.
Confirm what the count includes
A cable sheath or metallic screen is a conductive layer associated with the cable insulation system. In a single-core AC circuit, load current can induce voltage along that layer. Its bonding arrangement affects sheath voltage, circulating current and cable losses.
Cross-bonding connects the metallic sheaths between phases in a planned sequence. Over a suitably balanced group of sections, the induced voltages partly cancel. Cancellation depends on the electrical characteristics of the sections, not simply their combined length.
Before asking for a quantity, agree on these definitions:
- Cross-bonding location: a position where sheath connections transfer between phases according to the approved diagram.
- Direct-earth location: a position where the design connects the sheaths to the earthing system.
- Link box: the physical enclosure containing specified links and, where required, sheath voltage limiters.
- Sheath voltage limiter: a protective device selected to limit transient voltage across specified parts of the sheath insulation system.
A connection location does not necessarily equal one purchased enclosure. Termination arrangements, separate phase enclosures and accessory construction can change the physical box quantity.
Also distinguish cable sheath cross-bonding from conductor phase transposition. They are different design decisions and should appear separately on the drawings.
Collect the design inputs
Give the cable engineer a consistent set of route and electrical information. Mark missing values rather than replacing them with assumptions.
Cable and circuit
- Confirm whether the circuit uses three single-core cables or another construction. Do not assume a three-core cable needs the same bonding arrangement.
- Provide cable construction drawings, including metallic sheath or screen details and oversheath insulation information.
- State the number of circuits and identify parallel cables or nearby circuits that may affect the assessment.
- Supply the operating voltage, frequency, intended load current and applicable loading cases.
- Provide earth-fault current information and fault-clearing times from the protection study.
Route and installation
- Confirm route length and units against the route drawing.
- Identify cable formation, spacing and changes in geometry along the route.
- Mark joint bays, terminations, crossings and locations where access is restricted.
- Include installation conditions needed for the cable rating assessment, such as burial arrangement and relevant thermal information.
- Identify available earthing connections and provide the earthing study information.
The induced sheath voltage depends on current and cable geometry as well as length. A route with changing spacing cannot automatically be treated as several electrically identical sections.
Check the section layout
A minor section is an electrically defined length of sheath between the relevant sectionalizing connections or terminations. A conventional cross-bonded major section groups three minor sections.
The three minor sections should have suitably balanced induced voltages. Similar lengths help where the installation geometry is similar, but equal physical lengths can still produce electrically unbalanced sections when formation or spacing changes.
Ask for a layout that identifies:
- The start and end of every minor section.
- The three minor sections belonging to each major section.
- Which joints interrupt the metallic sheath and which maintain its continuity.
- The bonding arrangement at both circuit terminations.
- Any incomplete group, unusual route segment or transition to another bonding method.
Cable drum lengths influence joint positions, but they do not establish the bonding scheme by themselves. A straight-through joint need not create a new minor section. Equally, a convenient joint bay may not provide an acceptable section length without further assessment.
Do not divide the total route into equal pieces merely to produce a convenient box count. Request the sheath-voltage and loss calculations that support the proposed grouping.
Count locations only after the scheme is fixed
For a simple arrangement containing M complete major sections, with two internal cross-bonding locations per major section and direct earthing at each major-section boundary and both circuit ends, the functional location count is:
| Connection function | Conditional location count |
|---|---|
| Internal cross-bonding locations | 2 × M |
| Direct-earth locations, including both ends | M + 1 |
| Combined connection locations | 3 × M + 1 |
This arithmetic applies only to the arrangement just described. It is not a general enclosure-ordering formula.
For example, two complete major sections would give four cross-bonding locations and three direct-earth locations under those assumptions. The shared boundary is counted once. The accessory schedule must still establish how many physical enclosures serve those locations.
Before accepting the quantity, check that the schedule accounts for:
- End connections incorporated into termination equipment rather than separate boxes.
- Separate or combined enclosures for the phase connections.
- Additional circuits and any independently specified bonding arrangements.
- Route portions using a different bonding method.
- Spare equipment, listed separately from installed equipment.
An unexplained difference between the location count and the enclosure count is a question for the designer, not a reason to adjust the drawing to match the order.
Review protection and access requirements
The link box is part of an electrical system, not just a container for removable links. Ask the engineer to specify the required insulation, fault-current capability, bonding leads, earthing connections and any sheath voltage limiters.
The review checklist should include:
- Calculated sheath voltages for the relevant normal and fault conditions.
- The basis for sheath voltage limiter selection at each applicable position.
- Bonding-lead construction, routing and permitted lengths.
- Expected sheath losses and their effect on cable current rating.
- Earthing arrangements and the assessment of accessible metalwork.
- Box placement, identification and controlled maintenance access.
Do not open links or disconnect sheath earths on an energized cable. Metallic sheaths can develop hazardous induced or fault voltages. Inspection and testing require an approved procedure, competent electrical personnel and the specified isolation and earthing controls.
Request evidence that the installed scheme matches the design
Commissioning requirements should come from the approved cable and accessory documentation. Do not substitute generic test voltages or informal acceptance limits.
Request records covering:
- Sheath continuity and the connection sequence against the approved bonding diagram.
- Oversheath insulation testing under the specified procedure.
- Earthing connections and the applicable acceptance criteria.
- Correct accessory and sheath voltage limiter identification.
- As-built joint positions, bonding-lead routes and enclosure labels.
- Any specified operating measurements, assessed against the design calculations under a controlled procedure.
If alarms are repeated on a plant information screen, distinguish a factory status display from an operator HMI. An information display is not evidence that a cable is isolated or safe to access.
What to send with a quote request
Send a coordinated package rather than a route length and a requested box quantity:
- Project information: circuit identification, cable construction, electrical design inputs and route drawing.
- Section schedule: joint positions, minor-section lengths, major-section groups and termination details.
- Approved bonding design: connection diagrams, earthing requirements and the supporting calculation reference.
- Accessory schedule: connection function, enclosure quantity, required ratings, bonding leads and sheath voltage limiters by location.
- Delivery scope: which accessories, installation work, testing records and as-built documents the quotation must include.
If the bonding design is not yet available, request design support before asking for a final equipment quantity. A defensible count can be traced from the approved connection diagram to the location schedule and then to the enclosure list.
Questions
Can route length determine the cross-bonding link box count?
No. Route length does not establish joint positions, electrical section balance or enclosure arrangements. The count needs an approved sheath-bonding layout based on cable construction, loading, route geometry and fault conditions. Use length as a design input, not as a direct box-count formula.
Does every major section require two link boxes?
In a conventional three-minor-section major section, there are two internal cross-bonding locations. Direct-earth connections at major-section boundaries and circuit ends are separate functions. The physical number of enclosures depends on the approved accessory arrangement, so two locations do not always mean two purchased boxes.
Can cable drum length set the minor-section length?
No. Drum length helps establish practical cable joint positions, but the electrical design determines the sheath sections. Some joints maintain sheath continuity rather than creating a new minor section. The designer must also assess induced voltage, geometry, losses and any incomplete section groups.
What should be available before ordering link boxes?
Request the approved bonding diagram, joint and section schedule, sheath-voltage calculations, earthing requirements and accessory schedule. The schedule should distinguish connection locations from physical enclosures and identify the required links, bonding leads and sheath voltage limiters at each applicable position.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.