Both-End Bonding of Cable Sheaths: Circulating Current and Rating Impact
Engineering guide: Bonding a cable sheath at both ends provides a continuous path to earth. That helps control standing voltage, but the induced electromotive force can drive sheath current and create continuous loss.
On this page: How circulating current appears · Why ampacity changes · When the arrangement is considered · Measurements need interpretation · Design review output
How circulating current appears
The conductor magnetic field induces voltage in the metallic sheath. With both ends bonded, the closed loop permits current to flow. The result depends on phase geometry, sheath impedance, conductor current and bonding network.
Why ampacity changes
Sheath loss becomes heat near the cable and must be included in the thermal rating. A design that ignores it may overstate available conductor current. The effect can differ by phase when installation geometry is not symmetrical.
When the arrangement is considered
Solid bonding may be practical for short sections or where standing voltage control and simple earthing are priorities. Selection is not made from length alone; fault duty, losses, touch conditions and project standards also matter.
Measurements need interpretation
A clamp-current reading can identify imbalance or an unexpected path, but it is not a complete diagnosis. Compare phases, load, bonding diagram and historical baseline. Work on link boxes and bonding leads requires the approved safety procedure.
Design review output
Show the closed sheath-current paths on the drawing and report calculated sheath loss, cable-rating impact, earth connections, fault-current duty and inspection points.
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 · Cross-Bonding of HV Cable Sheaths: Major Sections and Transposition Logic · Sheath Voltage Limiters in HV Cable Systems: What the Design Must Prove
Sources and image credit
Technical reference: primary engineering source. Photo by American Public Power Association under the Unsplash License. Original educational content by ST College.