What Happens When Stress Control Fails?
2026-06-29 16:40In every high-voltage cable termination and joint, the most critical component is not the conductor or the insulation—it is the stress control system. This system manages the electric field at the point where the cable shield ends, preventing the field from concentrating and causing damage. When stress control works, the termination or joint operates silently and reliably for decades. But when it fails—or is improperly designed or installed—the consequences can be swift and severe. This article explores what happens when stress control fails, step by step, from the first spark to total breakdown.
1. What Stress Control Is Supposed to Do
To understd failure, one must first understand the purpose of stress control. In a shielded cable, the electric field is radial and uniform. At the end of the cable (in a termination) or at a splice (in a joint), the shield is cut. This creates a sharp edge where the electric field concentrates intensely. The peak stress at the shield cut can be many times higher than the average stress in the cable.
Stress control—whether geometric (stress cone), refractive (Hi-K material), or non-linear resistive (NLR)—smooths out this field. It spreads the voltage drop over a longer distance, reducing the peak stress to a safe level. The goal is to keep the surface stress below the partial discharge inception voltage (PDIV) and the bulk stress below the dielectric strength of the insulation.
When stress control fails, the field is no longer managed. The stage is set for a cascade of destructive events.
2. The First Sign: Partial Discharge Inception
The immediate consequence of failing stress control is partial discharge (PD) . At the shield cut or at any imperfection in the stress control system, the local electric field exceeds the dielectric strength of the surrounding material or air. Tiny sparks—partial discharges—begin to occur.
These discharges are small, often only a few picocoulombs. They produce:
Heat (localised temperature spikes).
UV radiation.
Chemically active gases (ozone, nitric oxides).
At this stage, the termination or joint is still operational. The PD may be detectable with specialised equipment, but it is often invisible to the naked eye. Unless the installer tests for PD, the problem goes unnoticed.
3. Erosion: The Slow Destruction Begins
Over days, weeks, or months, the repeated partial discharges begin to erode the insulation material. The polymer chains break down, creating carbonised tracks and microscopic pits. This process is called tracking or erosion.
The carbonised tracks are conductive, which further concentrates the electric field, making the PD more intense.
The pits enlarge, creating larger voids where more PD can occur.
The surface becomes rough, attracting dirt and moisture, which exacerbate the problem.
The damage is now self-sustaining: each discharge makes the next one more likely and more damaging. The termination or joint is on a path to failure.
4. Surface Tracking: The Carbon Path
If the stress control failure occurs on the external surface of a termination (e.g., because of inadequate creepage distance or pollution), the PD can evolve into surface tracking. A carbonised track forms along the surface of the insulation, connecting the live conductor to the grounded shield.
Once a carbon track is established, it provides a low-resistance path for leakage current. Under wet or polluted conditions, the track can become an electrical arc—a flashover—that completely bypasses the insulation. A flashover is a short circuit that can cause significant damage to the termination and surrounding equipment.
5. Puncture: Breakdown Through the Insulation
If the stress control failure occurs internally (e.g., at a void or interface), the PD can eventually lead to puncture—a complete breakdown of the insulation thickness. This is often the final event.
The puncture is typically a rapid, violent arc from the conductor to the shield or ground. It generates:
Intense heat (melting the conductor and insulation).
An explosive pressure wave (which can rupture the casing).
A loud bang (often described as an explosion).
The cable protection (circuit breaker or fuse) may clear the fault, but the accessory is destroyed. The system experiences an outage.
6. Secondary Damage: Beyond the Accessory
A stress control failure does not just affect the termination or joint. The consequences can ripple through the system:
Adjacent cables – An arc flash can damage neighbouring cables, especially in a congested cable tray or trench.
Equipment – A flashover in a termination connected to a transformer or switchgear can cause damage to bushings or insulators.
Personnel – The explosion or arc can injure anyone nearby. Arc flash hazards are a serious safety concern.
System reliability – The resulting outage may affect critical loads, leading to production losses or public inconvenience.
In submarine or underground networks, the cost of repairing a failed accessory can be enormous—far exceeding the cost of the accessory itself.
7. Common Causes of Stress Control Failure
Understanding why stress control fails is key to preventing it. The most common causes include:
| Cause | Description |
|---|---|
| Installation error | Stress cone mispositioned, wrong stripping dimensions, contamination. |
| Wrong product selection | Using a termination with inadequate voltage rating or incorrect bore size for the cable. |
| Material degradation | Aging of Hi-K or NLR materials; loss of permittivity or non-linear properties. |
| Mechanical damage | Crushing or bending that displaces the stress control element. |
| Thermal overload | Excessive temperature that degrades the stress control material. |
| Pollution (outdoor) | Salt, dust, or moisture on the surface that changes the field distribution. |
| Over-voltage | Lightning or switching surges that temporarily exceed the stress control's capability. |
8. Detecting Failure Before It Completes
The good news is that impending stress control failure can be detected before it causes a breakdown. The key is to monitor for partial discharge:
On-line PD monitoring – Permanent sensors (HFCT, UHF) continuously monitor critical accessories.
Off-line PD testing – Periodic testing during maintenance outages.
Thermal imaging – Hot spots may indicate a developing issue.
Visual inspection – Checking for surface tracking, discolouration, or cracks.
If PD is detected early, the accessory can be replaced or repaired before a catastrophic failure occurs.
9. How to Prevent Stress Control Failure
Prevention starts at the design and installation stages:
Choose the right product – Ensure the termination or joint is correctly rated for the system voltage, cable size, and environmental conditions.
Follow installation instructions meticulously – Especially regarding stripping dimensions, cleaning, and stress control positioning.
Use cold-shrink or pre-molded accessories – These have factory-molded stress control elements, reducing the risk of field errors.
Test after installation – Perform PD testing and withstand voltage tests to verify the installation.
For outdoor terminations, keep weather sheds clean and consider applying anti-tracking silicone grease.
Regular maintenance and periodic PD testing are essential for long-term reliability.
Stress control is the guardian of every high-voltage cable accessory. When it works, the termination or joint operates silently, safely, and reliably for decades. When it fails, the consequences can be catastrophic—partial discharge, tracking, puncture, flashover, and system outages. The failure is often progressive, starting with a few tiny sparks that grow into a destructive arc.
Understanding what happens when stress control fails is the first step to preventing it. Careful design, meticulous installation, and regular monitoring are the keys to ensuring that the guardian never sleeps. In the world of high-voltage engineering, stress control is not just a feature—it is a necessity.