Using Proper Stress Cones in Medium Voltage Terminations

1. Introduction & Context

Terminating a 120V or 480V cable is straightforward: strip the insulation, crimp the lug, and bolt it down. Terminating a 4160V, 13.8kV, or 35kV medium-voltage (MV) cable requires an entirely different discipline. At these voltage levels, the electrical field doesn’t just stay inside the wire; it attempts to escape. If the cable is not properly terminated using a “stress cone” or stress relief termination kit, the cable will literally tear itself apart.

2. The Core Issue

Medium-voltage cables are constructed with a semiconductor layer (semi-con) over the insulation and a grounded metallic shield over that. This construction confines the intense electrical field evenly within the insulation.

• The Cutback Point: When you strip a cable to terminate it, you must cut back the grounded shield and semi-con layer. At the exact edge where the semi-con ends, the electrical field is no longer confined uniformly.
• Electrical Field Concentration: Instead of radiating straight out, the lines of electrical flux crowd tightly together at the edge of the semi-con cutback, creating massive localized voltage stress.
• Corona and Tracking: Without a stress cone, this concentrated voltage stress ionizes the surrounding air (corona discharge), creating ozone which degrades the insulation. Micro-arcing begins to burn a conductive carbon path (tracking) along the surface of the insulation towards the grounded shield.
• The Explosive Failure: Once the carbon track bridges the gap between the high-voltage lug and the grounded shield, a catastrophic phase-to-ground arc flash occurs, blowing the termination box apart.

A stress cone (often a pre-molded rubber component or cold-shrink tube) is engineered with specific geometry and materials to gradually flare out and dissipate the electrical field, preventing this deadly concentration of stress.

3. Actionable Takeaways

• Never Treat MV Like LV: Never allow an electrician to terminate medium-voltage cable unless they hold specialized, up-to-date MV splicing and termination certifications.
• Absolute Cleanliness: When installing a stress cone, the insulation must be meticulously sanded and cleaned with approved solvents. A single speck of conductive dust left on the insulation under the stress cone will initiate tracking.
• Follow the Instructions Exactly: Cutback dimensions for the semi-con and insulation must be measured to the millimeter according to the termination kit manufacturer’s instructions. A mismeasured cutback defeats the stress cone geometry entirely.