Medium Voltage Cable Termination and Splicing — Field Procedures

What Makes Medium Voltage Different

At voltages above 2 kV, the electric field around the conductor is strong enough to cause partial discharge (corona) in any air gap or void in the insulation. This is why MV cables have a semiconducting shield layer that must be handled correctly during termination and splicing. A 480 V termination is forgiving — strip, crimp, tape. A 15 kV termination done wrong will fail in months or weeks, often catastrophically.

MV Cable Construction

A typical MV cable (5–35 kV) has these layers from inside out:

1. Conductor — copper or aluminum, stranded
2. Conductor shield (strand shield) — semiconducting layer that smooths the electric field at the conductor surface
3. Insulation — cross-linked polyethylene (XLPE) or ethylene propylene rubber (EPR)
4. Insulation shield — semiconducting layer over the insulation, controls the electric field at the outer surface
5. Metallic shield — copper tape or wire braid, provides a ground path and shields the cable
6. Jacket — PVC or polyethylene outer protection

Every termination and splice must manage the interface between these layers — especially the transition from shielded to unshielded insulation at the termination point.

Termination Methods

MV terminations use stress cones or stress control tubing to manage the electric field at the point where the insulation shield is cut back:

• Heat-shrink terminations — tubes that shrink over the cable with a heat gun, incorporating stress relief. Most common for indoor and outdoor use up to 35 kV.
• Cold-shrink terminations — pre-expanded EPDM rubber tubes that contract when a support core is removed. No heat required — faster and eliminates heat-damage risk.
• Porcelain or polymer terminations — used for outdoor riser poles and switchgear connections where weather and UV protection are critical.

Splicing

MV splices rejoin two cable ends underground or in a vault. The splice kit must replicate every layer of the original cable construction:

1. Strip and prepare both cable ends to the manufacturer's dimensions (critical — every measurement matters)
2. Install the connector (compression or mechanical) on the conductors
3. Apply semiconducting tape or tubes over the connector and strand shield transition
4. Apply insulation buildup (tape or molded body)
5. Apply semiconducting shield restoration
6. Reconnect the metallic shield (braid-to-braid bonding)
7. Apply outer jacket restoration

Each step must be clean, smooth, and free of air voids. A single trapped air bubble at 15 kV will erode the insulation through partial discharge until the cable fails.

Hi-Pot Testing

After termination or splicing, a high-potential (hi-pot) test verifies insulation integrity. DC hi-pot testing applies a voltage above the cable's rated operating voltage for a set time and monitors leakage current. VLF (very low frequency) testing is increasingly preferred because it stresses the insulation more uniformly than DC and is less likely to cause damage to XLPE insulation.

Typical test voltages for acceptance testing per IEEE 400:

• 15 kV class cable: VLF test at 17–20 kV for 30–60 minutes
• DC test (if used): 3× rated voltage for 15 minutes

Common Failure Causes

• Moisture in the termination — water tracking along the insulation surface under the stress cone. Cure: clean, dry workspace; follow manufacturer prep instructions exactly.
• Nicked insulation shield — a knife cut into the insulation during shield removal creates a stress point. Cure: use only the manufacturer's recommended stripping tools.
• Incorrect shield cutback length — too short and the stress cone does not control the field. Too long and exposed insulation is stressed. Cure: measure twice with the template provided in the kit.
• Cold-weather installation — XLPE cable and heat-shrink materials become stiff below 50°F. Cure: warm the cable and materials in a heated enclosure before installation.

Medium voltage cable work should only be performed by experienced industrial electricians trained on the specific termination and splice kits being used. Commissioning procedures must include hi-pot testing on every MV circuit before energization.