cross-bonding

Cross-bonding is a technique used in high-voltage power cable installations where the return path (typically the cable sheath or armor) is deliberately connected and disconnected at intervals along the cable run, with the connection points rotated between different phases. In a three-phase cable system, this means the sheath might be bonded to phase A at one location, phase B at the next location, and phase C at the third location, then the cycle repeats. The purpose is to distribute induced currents more evenly across the cable and its surroundings, rather than allowing them to concentrate in a single return path.

The electrical benefit is twofold. First, cross-bonding reduces the voltage gradients that develop around the cable, lowering dielectric stress on insulation. Second, by cycling the return current among the three phases, no single sheath or armor sees the full unbalanced current load, which would otherwise create significant losses and heating. This allows the cable to carry more current before reaching its thermal limit, raising the ampacity rating of the installation by 5 to 15 percent depending on soil conditions and burial depth.

Practical implementation requires careful planning at the design stage. The cable route must be divided into three or more equal sections, with bonding points spaced so that the electrical characteristics remain stable. At each bonding point, the sheath is connected to the phase conductor through a link box or cross-bonding chamber; at intermediate points, the sheath is left floating (insulated). Incorrect spacing or missing bonding points can create eddy currents in the sheath and actually worsen losses, so this is not a field retrofit but a planned permanent feature.

Cross-bonding is most common in underground extra-high-voltage (EHV) and ultra-high-voltage (UHV) transmission cables, where cable length is substantial and sheath current losses become economically significant. It is rarely used on shorter distribution cable runs or on cables in conduits where other cooling methods are available. The technique becomes impractical if the cable route is shorter than about 300 to 400 meters, or if the three sections cannot be made roughly equal in length.

The main risk is incorrect commissioning or maintenance. If a bonding point fails to make proper electrical contact, current is forced to find an alternative path, often through the cable armor or external metalwork in unplanned ways. Thermography and sheath voltage testing during commissioning are essential to verify that the cross-bonding scheme is working as designed.