Sheath Bonding for Single-Core Cables: Why Circulating Currents Waste Capacity

A single-core power cable that is rated for 800 A on the datasheet can deliver noticeably less once it is installed, and the reason is often not the cable at all. It is the way the metallic sheath or screen has been earthed. When a single-core cable carries alternating current, the magnetic field around the conductor cuts the metal sheath that surrounds it and induces a voltage along its length. Earth that sheath at both ends and the induced voltage drives a current round the loop: out along one sheath, back through the earth and the other sheaths. That circulating current produces heat in the sheath, sits right next to the insulation, and takes current-carrying capacity away from the conductor. On long runs at high current the loss is large enough to force a bigger conductor size, which is an expensive way to pay for an earthing decision.

Why single-core cables behave differently from three-core

In a three-core cable the three phase conductors sit inside a common screen or armour. The three currents sum to nearly zero, so the net magnetic field outside the cores is small and the induced sheath voltage is negligible. That is one reason three-core XLPE cables up to 33 kV are the default for most feeders: the sheath can be earthed at both ends and nobody has to think about it.

Single-core cables lose that cancellation. Each conductor has its own sheath, and the field from that conductor alone links the sheath fully. The induced voltage per metre rises with current, with the spacing between phases, and with frequency, and it accumulates along the whole route. Lay three single-core cables flat with a gap between them and the outer phases see a larger voltage than the middle one. Lay them touching in trefoil and the induced voltage falls, which is why trefoil is the preferred formation wherever the cable rating allows it. Formation reduces the problem; it does not remove it. The bonding method decides what happens to the voltage that remains.

The three bonding methods

Solid bonding earths the sheath at both ends. It is the simplest arrangement and the safest for a person touching the cable, because the sheath can never stand at more than a few volts above earth. The price is the circulating current. On a short run inside a substation, or on a lightly loaded feeder, the loss is small and solid bonding is a sensible choice. On a long, heavily loaded single-core circuit it is the arrangement most likely to be regretted, because the sheath current can approach the same order as the conductor current, and every ampere of it is turned into heat that the conductor then has to live with.

Single-point bonding earths the sheath at one end only. The far end is left insulated from earth, usually through a sheath voltage limiter, a non-linear device that behaves as an open circuit at normal voltage and conducts only during a fault or a surge. With no closed loop there is no circulating current, and the cable keeps its full rating. Two consequences follow. First, the un-earthed end of the sheath stands at the induced voltage under load, and that standing voltage grows with route length and with fault current, so single-point bonding is confined to runs short enough that the voltage stays within whatever limit the operator has set for touch safety. Second, because the sheath no longer provides a continuous earth return, a separate earth continuity conductor has to be laid alongside the cables, close to them, so that fault current has a low-impedance path back to the source and the protection still trips quickly.

Cross bonding is the answer for long HV routes. The route is divided into three sections of roughly equal length, called minor sections. At each joint between sections the sheaths are transposed: the sheath of phase A in section one is connected to the sheath of phase B in section two, and so on. Over one complete major section of three minor sections the induced voltages of the three phases add to approximately zero, so there is no net driving voltage round the loop and no circulating current, while the sheath is still earthed at the ends of each major section. The cost is in the accessories: link boxes, insulated joints with sectionalised sheaths, and sheath voltage limiters at every transposition point. It is the standard arrangement on 66 kV and higher transmission cable routes and is rarely justified below that.

Armour on single-core cables

The same magnetic field that induces a sheath voltage also acts on any armour. Steel is magnetic, and a steel wire armour round a single-core AC cable would carry heavy eddy and hysteresis losses in addition to the circulating current. That is why single-core cables for AC service are armoured with non-magnetic material, normally aluminium wire, or are supplied unarmoured and given mechanical protection by the installation instead. A specification that calls for galvanised steel wire armour on a single-core AC cable is a mistake and should be queried before the cable is made, not after it is in the ground. LV XLPE single-core cables up to 1000 sq mm are offered in armoured and unarmoured forms for exactly this reason: the armour choice follows the installation, and the installation follows the bonding scheme.

How to decide

The decision follows a short sequence. Use three-core cable wherever the conductor size allows it, and the whole question disappears. Where single-core is required, which in practice means large LV feeders, generator connections, transformer tails and anything above 33 kV, start with trefoil formation and a solid bond, then check the sheath loss for the route length and load. If the loss pushes the required conductor up a size, compare that cost against single-point bonding with an earth continuity conductor. Only when the route is too long for single-point bonding, which is normally the case on HV transmission circuits, does cross bonding earn its complexity.

Two points are easy to miss. The first is that single-point and cross bonding depend on the outer sheath staying intact, because a damaged oversheath earths the metal sheath at an unplanned point and quietly restores the circulating current. Oversheath integrity testing after installation, and again after any excavation near the route, is what keeps the scheme working. The second is that the bonding leads, link boxes and sheath voltage limiters are part of the circuit design, not site details. They belong on the drawings with the cable, and the person sizing the cable should know which scheme the installer intends to use before the rating is fixed.

APAR recommends settling the bonding method at the specification stage, alongside the formation, the armour material and the conductor size, because all four interact. A cable chosen on the datasheet rating and then solidly bonded on a long single-core run will run hotter than intended, and the remedy after energisation is either a derated circuit or a re-laid one. The engineering is well understood and the guidance in IEEE 575 and the cable maker’s installation instructions covers it; what goes wrong is that it is left until the jointer arrives. For assistance with sheath loss calculations on a specific route, the technical team can be reached through the product pages linked above.