MV Cable Accessories Selection by Voltage
How to select the right MV cable termination, joint and stress-control kit by voltage class. Covers U0/U matching, shielding cut-off positions, stress-control tube placement, and termination requirements for 0.6/1 kV, 8.7/15 kV and 26/35 kV systems.
1. U0/U and Voltage Class Boundaries
The first rule of MV accessory selection: match the kit to the cable's rated voltage U0/U. U0 is the rated voltage between conductor and earth (or neutral), and U is the rated voltage between phases. Using a kit from a lower voltage class is the most common cause of premature failure. The insulation wall and stress-control geometry are sized for a lower electric field.
Below is a practical boundary table for the three classes stocked by Wellele:
| System Voltage | Cable U0/U | Kit Voltage Class | Standards | Cores Available |
|---|---|---|---|---|
| 0.6/1 kV | U0=600V, U=1000V | 0.6/1 kV | IEC 60502 | 1, 3, 4, 5-core |
| 8.7/15 kV | U0=8.7kV, U=15kV | 8.7/15 kV | IEC 60502, IEEE 386 | 1, 3-core |
| 26/35 kV | U0=26kV, U=35kV | 26/35 kV | IEEE 386 | 1, 3-core; cold shrink 10-year warranty |
A procurement engineer admitted: "We ordered the 0.6/1 kV kit for an 8.7/15 kV feeder." The result is a joint that passes initial commissioning but fails under sustained operating voltage. Partial discharge at the screen edge eats the insulation from the inside.
2. Stress-Control Tube Placement
At 8.7/15 kV and 26/35 kV, the stress-control tube is the single most critical component. It must land exactly on the cable's shield cut-back line. The boundary where the semiconducting screen ends and the accessory takes over the field control.
| Voltage | Critical Placement | Verification |
|---|---|---|
| 0.6/1 kV | Critical | Visual + insulation resistance |
| 8.7/15 kV | Exact on cut-back | Partial discharge test recommended |
| 26/35 kV | Exact on cut-back | PD test + voltage withstand |
Mark the screen cut-back, position the stress-control tube to that line, and recover it fully and evenly before moving on. On 8.7/15 kV three-core feeders, the three individual tubes must each align. A shift on any one core creates a localised field concentration.
3. Shielding and Semicon Cut-Off
The screen (shield) and armour continuity through the joint is what maintains the cable's electrical and mechanical integrity. If the connections are not carried through, the joint breaks the shielding path, inviting EMI and creating a safety gap.
- Strip the semicon layer back flush to the stress-control tube's designed edge, not beyond.
- Use the kit's screen and armour continuity components to reconnect any broken shield path.
- Verify continuity before closing up. A joint that breaks the screen invites EMI and a safety gap.
- For armoured cables, connect the armour to the kit's continuity components and test.
This is especially critical at 26/35 kV where any discontinuity in the shielding generates high field gradients. A cold-shrink 26/35 kV kit's pre-expanded silicone recovers uniformly and is therefore more forgiving of field conditions than heat shrink, which is one reason 26/35 kV cold shrink kits typically carry a 10-year warranty.
4. Termination vs Joint: Environmental Conditions
The same voltage class kit may be specified for either a termination (live-end connection) or a joint (splice). The installation environment is the deciding factor:
| Environment | Termination | Joint | Preferred Technology |
|---|---|---|---|
| Indoor switchgear | Stress control + shed design | Mastic + seal at both ends | Heat shrink (hard crush fit) |
| Outdoor pole top | Weatherproof shed | Full environmental seal | Cold shrink (no flame, easier access) |
| Underground / direct burial | N/A | Water-tight seal mandatory | Cold shrink or adhesive-lined heat shrink with mastic |
| Joint bay / pit, no power | N/A | Seal + shield continuity | Cold shrink (no power needed) |
| Shop floor, quantity | Bulk ordering | Bench-installed | Heat shrink (calibrated gun) |
5. Installation Conditions That Change the Choice
Beyond voltage, the installation conditions determine whether heat shrink or cold shrink is the safer, more reliable choice:
- Flammable or hazardous zones (gas stations, mining, refineries): cold shrink removes the flame risk entirely. Confirm site permit rules before selecting heat shrink.
- Confined cubicles: heat shrink needs radial clearance for the heat gun to sweep around the full circumference. Cold shrink needs only axial slide-on length.
- Pole-top or remote joints: no power means no heat gun. Cold shrink's pull-and-done install wins.
- Underground / wet routes: the seal is the joint, not the outer wrap. Either technology works if sealed correctly with mastic or adhesive lining.
6. Spec Checklist Before You Order
- Confirm the cable's rated voltage U0/U and verify your kit matches with margin for the system test voltage.
- Confirm the operating temperature range covers your environment, not just the ambient.
- Check the standard on the data sheet (IEC 60502, IEEE 386) against your project spec.
- Confirm the screen and armour continuity kit matches your cable construction (wire screen, tape screen, armoured or not).
- Send the cable or component outer diameter, the voltage, and the quantity. We return a sized recommendation and a cut-length that allows for recovery shrinkage.
Related: 0.6/1 kV Heat Shrink · 8.7/15 kV Heat Shrink · 26/35 kV Heat Shrink · 0.6/1 kV Cold Shrink · 8.7/15 kV Cold Shrink · 26/35 kV Cold Shrink · Heat vs Cold Shrink Comparison
Selection and pre-order sequence
- Match the kit to the cable's rated voltage U0/U: 0.6/1 kV, 8.7/15 kV or 26/35 kV, with margin for the system test voltage.
- Position the stress-control tube exactly on the shield cut-back line, and recover it fully and evenly before moving on.
- Strip the semicon layer back flush to the stress-control tube's designed edge, not beyond.
- Reconnect screen and armour continuity with the kit's components, and verify continuity before closing up.
- Choose termination vs joint by environmental conditions, not by voltage class alone.
- Send the pre-order pack: rated voltage U0/U, operating temperature range, the standard on the datasheet (IEC 60502, IEEE 386), the screen and armour construction, and the cable OD.
Frequently asked questions
What does U0/U mean for cable accessories?
U0 is the rated voltage between conductor and earth or neutral, and U is the rated voltage between phases. The accessories kit must match your cable's U0/U rating with adequate margin for the system test voltage.
Where should the stress-control tube sit on a 26/35kV termination?
The stress-control tube must land exactly on the cable's shield cut-back line. Any shift leaves the electric field concentrated at the screen edge and risks partial discharge. Mark the cut-back, position precisely, and verify with a PD test.
How do shielding and semicon layers affect 8.7/15kV joints?
At 8.7/15kV the semicon layer must be stripped back to the stress-control tube's edge and reconnected through the kit's continuity components. A broken shield path invites EMI and a safety gap.
Do I need armour continuity through the joint?
Yes, if your cable is armoured. Use the kit's armour continuity components and verify continuity before closing up.
Why does 26/35kV cold shrink have a longer warranty?
Cold shrink at 26/35kV typically carries a 10-year warranty because the silicone recovery is more tolerant of field conditions and the kit is less sensitive to heat-application errors.
Need the right termination kit for your voltage?
Send the cable OD, voltage class, and environment. We return a sized recommendation and a quote.
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