Medium Voltage Cable Termination and Splicing — Field Procedures

What Makes Medium Voltage Different

On shielded MV cable, the electric field in the insulation is high enough that air voids, knife nicks, or incorrect shield cutback create partial-discharge sites that grow into failures. Semiconducting conductor and insulation shields keep the field smooth — and those shields must be restored or terminated with listed stress-control accessories. A 480 V lug is forgiving. A 15 kV class termination installed wrong often fails in months (sometimes sooner), sometimes violently.

NEC Article 315 covers medium-voltage conductors and cable rated 2001 V through 35 000 V AC (and a limited DC range). Common North American cable classes are 5, 15, 25, and 35 kV. Always follow the specific kit instructions and the owner’s test specification.

MV Cable Construction

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

1. Conductor — copper or aluminum, usually stranded
2. Conductor shield (strand shield) — semiconducting layer that smooths the field at the conductor surface
3. Insulation — XLPE or EPR
4. Insulation shield — semiconducting layer over the insulation
5. Metallic shield / neutral — copper tape, concentric wire, LC shield, or similar — return/ground path and electrostatic shield
6. Jacket — PVC, PE, or other outer covering (armor/sheath variants exist)

Every termination and joint must manage the transition where the insulation shield ends — that is what stress cones / geometric or refractive stress-control tubing are for. Product type tests for accessories are covered by standards such as IEEE 48 (terminations) and IEEE 404 (joints/splices); field craft still makes or breaks the installation.

Termination Methods

MV terminations use stress cones or stress-control tubing at the insulation-shield cutback:

• Heat-shrink terminations — polymeric tubes shrunk with controlled heat; common indoor/outdoor kits through 35 kV class when installed per instructions
• Cold-shrink terminations — pre-expanded silicone or EPDM bodies that contract when the support core is removed; no torch heat on the cable
• Porcelain or polymeric outdoor terminations — used where creepage, weather, and UV demand a different profile (risers, some switchgear interfaces). Separable insulated connectors (elbows) are a different product family (IEEE 386) and are not covered by IEEE 48

Splicing

MV joints rejoin cable ends (or transition constructions) in vaults, manholes, or direct-buried applications. Prefabricated cold- or heat-shrink kits still require precise prep:

1. Strip and prepare both ends to the manufacturer’s dimensions (templates matter)
2. Install the connector (compression or mechanical) on the conductors
3. Restore semiconducting continuity over the connector / strand-shield transition as the kit requires
4. Apply the joint insulation body (molded cold-shrink, heat-shrink, or tape build-up per design)
5. Restore the outer semiconducting shield
6. Bond/continue the metallic shield or concentric neutral per the kit and grounding design
7. Restore the jacket / environmental seal

Keep surfaces clean and dry. Contaminants and voids at the insulation interface are classic PD initiation sites on 15 kV class systems.

Field Testing After Termination or Splice

After work, verify the cable system before energization. For extruded shielded cable (XLPE/EPR), industry practice favors VLF AC withstand per IEEE 400.2 over DC hi-pot. DC stress can leave space charge in aged extruded insulation and is widely discouraged as a maintenance test on XLPE; laminated (e.g. PILC) systems are a different IEEE 400.1 context. A simple VLF withstand is pass/fail — it does not by itself “detect PD.” Partial-discharge assessment is a separate diagnostic (IEEE 400.3 and related methods); tan δ / tip-up can be run with suitable VLF equipment under IEEE 400.2 diagnostic guidance.

Example IEEE 400.2 sinusoidal VLF levels for 15 kV class cable systems (phase-to-ground, rms; confirm waveform type and the edition/table your procedure cites):

• Acceptance / installation: about 21 kV rms (acceptance) / 19 kV rms (installation) for ~60 minutes at 0.1 Hz on new circuits
• Maintenance: about 16 kV rms for a typical ~30-minute withstand on aged circuits (owner may extend time on critical feeders)

Do not substitute a generic “17–20 kV for 30–60 minutes” or “DC = 3× rated for 15 minutes” rule of thumb for the governing IEEE table, manufacturer limits, and owner specification. Elbows and terminations may need additional insulation or guards during the test to prevent flashover.

Common Failure Causes

• Moisture / contamination — tracking under stress control. Cure: clean, dry prep; follow kit cleaning steps.
• Nicked insulation — knife damage while removing the insulation shield. Cure: manufacturer stripping tools and practice cuts.
• Wrong shield cutback — stress control cannot work if dimensions are short or long. Cure: kit template; measure twice.
• Cold-weather installation — stiff XLPE and heat-shrink compounds below manufacturer limits (often cited near 50°F / 10°C for some materials). Cure: warm cable and kits per instructions before install.

MV terminations and splices should be done by craft trained on the specific kits in use. Industrial electrical / I&E field services and commissioning should include the owner-approved cable-system test (typically IEEE 400.2 VLF for extruded MV) before energization.