Cross-Bonding of HV Cable Sheaths: Major Sections and Transposition Logic
Engineering guide: Cross-bonding connects sheath sections so the induced voltages from the three phases approximately cancel over a complete major section. The result depends on section balance and correct field connections.
On this page: Minor and major sections · Cancellation is not automatic · What the designer coordinates · Commissioning focus · Maintenance record
Minor and major sections
A typical major section contains three minor sections separated by sectionalizing joints. Sheath connections are transposed at link boxes so each sheath occupies the equivalent induced-voltage position over the complete set.
Cancellation is not automatic
Perfect cancellation assumes balanced phase currents and appropriately balanced section geometry and length. Route constraints, unequal sections, nearby circuits and transposition details produce residual voltage and current that must be calculated.
What the designer coordinates
The cable route, joint schedule, link-box locations, bonding leads, earthing points, sheath voltage limiters and insulation levels must agree. A change in joint location can change the electrical result even when cable length is similar.
Commissioning focus
Verification should confirm sheath continuity, insulation condition, connection sequence and correspondence with the approved diagram. CIGRE emphasizes after-installation testing because a misplaced link can defeat the intended transposition.
Maintenance record
Keep a durable major-section identifier, link-box schedule, photographs and test baseline. When residual currents change, compare against load, phase balance and any route or earthing modifications.
Related engineering guides
Continue with the main topic guide. The design concepts here require project-specific studies, applicable standards and qualified personnel.
Next in this series: Single-Point Cable Sheath Bonding: Purpose, Limits and Design Inputs · Both-End Bonding of Cable Sheaths: Circulating Current and Rating Impact · Sheath Voltage Limiters in HV Cable Systems: What the Design Must Prove
Sources and image credit
Technical reference: primary engineering source. Photo by Anton Dmitriev under the Unsplash License. Original educational content by ST College.