Cable sheath cross-bonding — why long single-core high-voltage cables do not simply earth the sheath at both ends

The guide on single-point versus both-ends earthing of a cable sheath covers the common trade-off between single-point earthing (prevents a ground loop, but offers less high-frequency shielding) and both-ends earthing (best EMC shielding, but can cause a circulating sheath current that lowers the current rating). This article covers a third, specialised technique applied specifically on long single-core high-voltage cable routes to solve this sheath-current problem without accepting the drawbacks of single-point earthing: cross-bonding.

Why both-ends earthing becomes a problem on a long single-core route

In a single-core cable, the AC current in the core generates a magnetic field that, through induction, generates a voltage in the sheath of that same cable — an effect that grows stronger as the cable gets longer. If the sheath is earthed at both ends, this induced voltage closes a loop, and drives a circulating sheath current proportional to the load current in the core. On a short cable route, this sheath current and the associated extra heat generation are typically negligible, but on a long high-voltage route (think kilometres, not metres), the circulating sheath current can reach a significant fraction of the core current, substantially lowering the effective current rating of the cable and causing unwanted extra losses.

Why single-point earthing is not a satisfactory solution for such a route

Single-point earthing (as covered in the guide on cable sheath earthing) does solve the circulating-current problem, but introduces a new problem on a long high-voltage route: at the unearthed end, an inductive sheath voltage builds up over the full length of the cable, which on a long route with high load current can reach a dangerously high level — well above what is safely touchable during maintenance on joints or terminations.

The principle of cross-bonding: cyclic transposition in short sections

Cross-bonding solves both problems at once by dividing the route into short, roughly equal minor sections (typically three minor sections together form one major section) and cyclically swapping the sheath of each phase with that of the next phase at every link box:

Within one major section, the sheath of each phase is thus successively connected to the three minor sections, in a fixed, cyclic order (for example R→Y→B→R).
Because every phase-sheath section thereby "sees" a comparable portion of the total induced voltage from each of the three phases, the induced sheath voltages largely cancel each other out over a complete major section — similar to how transposition of the conductors themselves on an overhead high-voltage line reduces the imbalance between phases.
The sheath is only actually earthed at both ends of a complete major section; at the intermediate link points (the boundaries between the minor sections), the sheath is passed through according to the transposition scheme, not earthed directly.

Link boxes, sectionalising insulators and sheath voltage limiters

The cyclic swapping of the sheath takes place in special link boxes, in which the sheath of each minor section is interrupted via sectionalising insulators and routed to the correct next phase via wired connections. Because, despite the mutual cancellation, a residual (typically much smaller) sheath voltage remains in practice between the ends of a major section — and because a temporary fault in the network can briefly drive this voltage up considerably — a sheath voltage limiter (SVL) is typically fitted at every sectionalising insulator in the link box: a component that stays high-impedance during normal operation, but breaks down at an excessive transient sheath voltage, protecting the insulation of the sectionalising insulator from flashover.

Note: the exact division into minor and major sections, the number of link boxes and the sizing of the sheath voltage limiters follow from the system study for the specific cable route (length, load current, fault level); this article covers the principle, not a ready-made design table for every high-voltage route.

Why this is specifically a technique for long routes

For a short single-core cable route — for example a transformer connection within the same switching station — the induced sheath voltage under single-point earthing is typically already well within safe limits, and the complexity and extra cost of link boxes with cross-bonding is not justified: ordinary single-point or both-ends earthing (see the related guide) is then sufficient. Cross-bonding is applied specifically when the route length is great enough that both the circulating-current derating under both-ends earthing and the built-up sheath voltage under single-point earthing become unacceptable.

Practical relevance

When inspecting or maintaining a long high-voltage cable route with cross-bonding, every link box should be treated as a point where, despite the mutual cancellation, a residual voltage may still be present — before opening a link box, this must be explicitly checked, rather than assuming the sheath is voltage-free there simply because it "appears to be earthed anyway".

Common mistakes

Simply earthing a long single-core high-voltage route at both ends as with a short cable — this causes a circulating sheath current that can significantly reduce the route's current rating.
Earthing a long route at a single point instead of applying cross-bonding — this can produce a dangerously high sheath voltage at the unearthed end.
Opening a link box without checking for residual voltage on the sheath — despite the mutual cancellation between phases, a link box can still hold a touch-dangerous voltage.
Omitting or incorrectly sizing sheath voltage limiters (SVLs) at the sectionalising insulators — without correctly sized SVLs, a temporary overvoltage on the sheath can cause the sectionalising insulators to flash over.