What Happens Inside a Cable Joint During a Lightning Strike?

A lightning strike is one of the most powerful and unpredictable forces in nature. A single bolt can carry up to 200,000 amperes of current, with temperatures reaching 30,000°C—five times hotter than the surface of the sun. When such a strike hits a power line or its vicinity, the energy travels along the conductors, and the cable system—including its joints and terminations—must withstand this immense surge without failing. But what actually happens inside a cable joint during a lightning strike? This article explores the physics, the risks, and the engineering that protects these critical connections.

1. Lightning: A Brief Introduction to the Threat

Lightning is a massive electrostatic discharge that occurs between clouds or between a cloud and the ground. When it strikes a power line or the ground near it, it induces a surge—a sudden, extremely high-voltage and high-current transient—that travels along the conductor at nearly the speed of light.

Key characteristics of a lightning surge:

• Voltage: Up to several million volts (typically 1–10 MV for the main stroke).

• Current: 10,000 to 200,000 amperes (typical: 30,000 A).

• Duration: Very short—a few microseconds to a few hundred microseconds.

• Rise time: Extremely fast—the current rises to its peak in 1–10 microseconds.

This surge travels along the power line, entering substations, transformers, and cable systems. It does not discriminate; it will follow any conductive path it can find, including the conductor of a cable.

2. The Path of the Surge: How Lightning Enters a Cable System

A lightning surge can enter a cable system in several ways:

A. Direct Strike to the Overhead Line
If an overhead line is struck directly, the full lightning current travels along the conductor until it reaches a termination or joint, where it may continue into the underground cable.

B. Induced Surge on an Overhead Line
Even if the line is not struck directly, a nearby strike induces a high-voltage surge on the line through electromagnetic induction.

C. Ground Potential Rise (Step Voltage)
A strike to the ground near a substation or cable termination can raise the ground potential, creating a backflash—a surge that travels from the ground into the equipment.

D. Through Transformer or Switchgear
Lightning arresters at substations divert most of the surge, but some energy may still pass through to the cables.

Once the surge enters the cable, it travels to the joint—and that is where the real action begins.

3. What the Joint Experiences: Voltage, Current, and Stress

When a lightning surge reaches a cable joint, the joint is subjected to three simultaneous stresses:

A. High Voltage (Overvoltage)
The surge voltage can be several times the cable's normal operating voltage. For a 66 kV cable, a surge of 350 kV or more is possible. This extreme voltage creates an intense electric field within the joint—far beyond the normal operating stress.

B. High Current (Surge Current)
The surge current heats the conductor and the connector. In a fraction of a millisecond, the temperature of the conductor can rise significantly (but briefly) due to the massive current.

C. High Rate of Change (di/dt)
The rapid rise of current creates a strong electromagnetic field, which can induce voltages in adjacent components and generate mechanical forces.

The joint's insulation, connector, and sealing must all survive this combined assault.

4. The Vulnerability of Cable Joints

Cable joints are more vulnerable to lightning than the cable itself because:

• They have interfaces – The transition between different materials (insulation, stress control, connector) creates points where the electric field can concentrate.

• They are field-assembled – Installation quality can vary, introducing voids, contamination, or imperfect contact.

• They contain multiple components – Each component (stress cone, connector, insulation body) has its own response to voltage and current.

In a lightning surge, the weakest point in the joint will fail first.

5. The First Microseconds: Field Distortion and Partial Discharge

When the surge enters the joint, the electric field is temporarily distorted. The stress at the shield cut—already the most stressed point—becomes even more concentrated. The voltage may exceed the partial discharge inception voltage (PDIV), and tiny sparks—partial discharges—begin to occur.

What happens during PD:

• Each discharge releases a small amount of energy (heat, UV, ozone).

• The insulation surface may start to track—forming a carbonised path.

• The discharge may propagate, creating a tree-like structure in the insulation.

In a well-designed joint, the stress control system (cone, Hi-K, or NLR) limits this field distortion, preventing PD even under surge conditions. If the stress control is inadequate or mispositioned, the PD can quickly escalate into a flashover.

6. Flashover: The Arc Path

If the field distortion is severe enough, the discharge can become a flashover—an arc that jumps from the conductor to the shield or from one phase to another. A flashover is a complete breakdown of the insulation.

Types of flashover in a joint:

• Internal flashover – The arc occurs within the joint, burning through the insulation.

• External flashover – The arc occurs along the surface of the joint or at the interface.

• Between phases – In a three-phase joint, the arc can jump from one conductor to another.

A flashover releases a large amount of energy in a fraction of a second. The resulting pressure wave can rupture the joint casing, and the arc can vaporise the conductor or connector. The joint is destroyed.

7. Thermal Effects: The Heat of the Surge

Even if there is no flashover, the surge current heats the conductor and the connector. The temperature rise can be calculated using the adiabatic heating formula (assuming no heat is lost during the brief surge). For a short duration (e.g., 20 microseconds), the conductor temperature may rise by several tens of degrees Celsius.

Connector heating is more significant because the connector has a higher resistance than the conductor (due to its material and geometry). If the connector is not properly crimped, it may have a higher resistance, creating a hot spot that can melt or weaken the connection.

Thermal expansion – The rapid heating causes the conductor and connector to expand. This can mechanically stress the connector and the surrounding components, potentially displacing the stress cone or damaging the sealing.

8. Mechanical Effects: The Explosive Force

The rapid heating of the conductor and the arc—if it occurs—creates a pressure wave inside the joint. The joint casing must withstand this pressure without bursting. Most modern joints have a burst-proof casing (e.g., made of glass-fibre reinforced polyester) that can contain the pressure of a flashover.

Mechanical forces – The high current also creates strong electromagnetic forces that try to push the conductors apart. This can stress the connector and the cable.

9. The Role of Surge Arresters

To protect cable joints from lightning, surge arresters (also called lightning arresters) are installed at key points in the network:

• At the substation entrance, to divert the surge to ground.

• At pole-mounted terminations, where the cable connects to an overhead line.

• At joints in exposed locations (e.g., in wind farms or along transmission lines).

A surge arrester is a non-linear resistor that has a high resistance at normal voltage but becomes conductive at high voltage, diverting the surge current to ground. By limiting the voltage that reaches the joint, arresters reduce the risk of flashover and damage.

10. Designing Joints for Lightning Survival

Engineers design cable joints to survive lightning strikes using several strategies:

A. Robust Stress Control
The stress control system (cone, Hi-K, NLR) is designed to withstand surge voltages without initiating PD. This is verified during type testing with impulse voltages.

B. High Dielectric Strength
The insulation materials (silicone, EPDM, XLPE) are selected for their ability to withstand high voltage surges.

C. Mechanical Robustness
The connector and casing are designed to withstand the thermal and mechanical forces of a surge.

D. Sealing and Water Blocking
Sealing prevents moisture ingress, which could reduce the dielectric strength of the insulation and increase the risk of flashover.

E. Type Testing with Impulse
Cable joints are subjected to lightning impulse tests as part of their type testing. The joint is subjected to a voltage waveform that simulates a lightning strike (typically 1.2/50 µs impulse) at a level specified by the relevant standard (e.g., IEC 60840, IEC 62067). A joint that passes this test has demonstrated its ability to survive lightning.

11. The Real World: A Lightning Strike Scenario

Imagine a 66 kV underground cable running from a substation to a wind farm. A lightning strike hits a nearby pole, and a surge travels along the overhead line into the cable. At the first joint (100 m from the pole), the surge arrives.

• 0–2 µs: The voltage at the shield cut rises rapidly. The stress cone does its job, spreading the field.

• 2–10 µs: The voltage peaks at 350 kV. The stress cone and the insulation body withstand the stress—no flashover occurs.

• 10–50 µs: The surge current flows through the connector, heating it slightly. The connector expands, but the spring system or the elastomer's pressure maintains contact.

• 50–200 µs: The surge subsides. The joint returns to normal operation.

If the stress cone were mispositioned or if there were a void in the insulation, a flashover would occur, destroying the joint and causing an outage.

A cable joint during a lightning strike is like a fortress under siege. It must withstand a million‑volt surge, a hundred‑thousand‑ampere current, and the intense heat and mechanical forces that come with them. The joint's stress control system, insulation, connector, and casing all work together to survive this assault.

Through careful design, robust materials, and rigorous testing, cable joints are engineered to survive lightning strikes—ensuring that the power grid remains reliable even in the face of nature's most powerful force. The next time a storm rages and the lights stay on, remember: somewhere in the network, a cable joint has just survived a lightning strike, and it did its job without a sound.