What is a Link Box?


By adopting bonded systems with a maximum of 132 kilovolts, sheath breaking may stop circulating currents and restrict the voltage that is induced in the cable sheath.

Link Box

Sheaths are used to provide cable sheath bonding for underground medium and high voltage cable bonding. This helps to limit the amount of circulating sheath current.

For reasons of safety, bonding the cable sheath lowers the voltages that are induced in the cable sheath.

A cable is made up of three major components.

1). Conductor

2). Insulation and

3). Sheath

1). Conductor

The current is carried by the conductor because of its very low resistance.

In order to provide isolation between the conductor and the sheath, insulation is covered all the way around the conductor.


2). Insulation

The insulation is encased in a coating before being wrapped in the sheath.

The extra coating that improves the cable’s protection is provided by the insulation, which is placed over the sheath.


3). Sheath

Sheathing is not necessary for low voltage wires. On the other end, high-voltage and medium-voltage cables both include sheaths that protect them against overvoltage and transients.

Moreover, the sheath serves as a barrier against the transfer of moisture from the external environment into the primary insulation.

Because of this, the usable life of the cable insulation is increased as a result.


What effect does voltage have on the sheath?

Therefore, according to Faraday’s rule of electromagnetic induction, a revolving magnetic field will induce emf (induced emf) on a stationary conductor if it is linked to that conductor in some way.

Alternating current with a high magnitude is used for EHV cables; this creates a connection with the metallic sheath that surrounds the power wire.

This results in emf being produced in the metallic sheath, which in turn causes current to leak out of the cable, leading to losses in the power supply.

Electricity losses of this scale are unsustainable and unaffordable.

Solution for Reducing Sheath Induced Voltage

  • For the purpose of connecting cable and joints/terminations, link boxes feature a structure that is both sealed and waterproof.
  • Make use of link boxes for the following to nullify out the voltage that is produced in the sheath
  • In order to reduce cable losses brought on by the circulating current in the sheath
Schematic of Connectivity in Link Box

Conductor, insulation, and a sheath are all components of the power cable.

It  may prevent the voltage from being induced in the sheaths by switching the positions of the sheaths for all three phases.

A connection may be made from one end of the sheath bonding cable to each of the three sheaths, and another section of the sheath cable can be connected to the grounding earth.

It is possible to reduce the induced voltage to nearly nothing if the sheaths are connected to the ground.

Moreover, there is a significant decrease in the amount of cable loss that may be attributed to sheath losses.

When a power connection is very long, the boxes should be spaced out at equal intervals along the distance from the source end to the load end.

The cable is not severed at any point throughout its whole length; rather, just a portion of the sheath is severed at predetermined intervals of distance.

This results in the cable’s sheath power losses being reduced to their absolute minimum.

Single Bonding Link Box

The most basic kind of special earth bonding involves making arrangements for the cable sheaths of the three high-voltage cables to be linked and bonded at a single point along the length of the circuit.

This method does not include any cable joints and often involves shorter cable circuits.

Single Bonding Link Box 2

At every other point, there will be a voltage that appears from the sheath to the ground, and this voltage will reach its highest value at the place that is the furthest away from the earth bond.

As there is an open sheath circuit (except via the SVL), current does not generally flow longitudinally through the cable sheaths, hence there is no sheath circulation current loss that is incurred.

Therefore, the high voltage cable sheaths need to be suitably insulated from the ground.

Cross Bonding Link Box
  • Cross interconnection of high-voltage(HV) single-core cable metal sheath,
  • Limiting overvoltage applied to cable sheath and both insulated part ends of insulated joint,
  • Controlling induced voltage on metal sheath,
  • Reducing (or) eliminating ring current on cable sheath,
  • Increasing transmission capacity of power cable,
  • Avoiding over-sheath breakdown, and
  • Ensuring a safe operating for per cable.
Cross Bonding Link Box 1
Cross Bonding Link Box 3
  • Reduces the amount of voltage that may build up on the sheath and helps limit as a voltage regulator, sheath circulating current is used

(Limits the lighting and switching surges).

  • Provide convenient access to shield breaks for the sake of testing.
  • They are often utilised in high-voltage (HV) applications ranging from 33 to 440 KV lines.
Applications of Link Box
SVL in a link box