Cable joint tubing is not the same as general-purpose heat shrink tubing. A medium voltage cable joint must restore three functions of the cable: conductor connection, insulation, and moisture protection. Using the wrong tubing — or general-purpose tubing instead of joint-specific tubing — leads to partial discharge, insulation breakdown, and eventual failure.
Joint Tubing vs General Tubing
A cable joint kit contains several tubes, each with a specific function:
Stress control tube. Placed over the area where the cable screen is cut back, this tube manages the electric field concentration at the screen termination. Without proper stress control, partial discharge occurs at this point even if the insulation is otherwise adequate.
Insulation tube. The main body of the joint, this restores the cable's insulation thickness. It must be rated for the system voltage and compatible with the cable's XLPE insulation.
Outer sheath tube. This restores the cable's outer protective layer and provides mechanical and moisture protection. In adhesive-lined versions, it creates a waterproof seal for direct burial.
General-purpose tubing cannot replace these components because it lacks the semiconductive stress control layer and is not qualified for the voltage class.
The Four Selection Inputs
To select the right joint kit, gather four pieces of information:
1. System voltage. 1 kV, 11 kV, 33 kV, or 35 kV. The joint's insulation and stress control are designed for a specific voltage class. Using a lower-voltage joint on a higher-voltage system will fail during the first power frequency test.
2. Conductor cross-section. In mm² or AWG. The joint's conductor connector must match the cable's conductor size. Most kits are available in ranges (e.g., 95-185 mm², 185-400 mm²).
3. Number of cores. Single-core or three-core. Three-core joints are larger and include additional components to manage the core separation and re-formation.
4. Installation environment. Indoor, outdoor, underground, or subsea. This determines whether you need heat shrink or cold shrink, and whether additional moisture protection is required.
Kit vs Individual Components
Complete joint kits include all necessary tubes, connectors, and materials in one box with installation instructions. They're the recommended choice for most installations because all components are matched and tested together.
Individual tubes (stress tubes, insulation tubes, sheath tubes) are available for utility stockrooms and maintenance programs. This approach requires more expertise to assemble correctly but allows flexible inventory management.
Selection Table
| Voltage | Cable Type | Recommended Kit | Cores |
|---|---|---|---|
| 1 kV | XLPE/PVC | TRSY-1 / JRSY-1 | 1-5 |
| 10 kV | XLPE | JRSY-10 | 1 or 3 |
| 35 kV | XLPE | JRSY-35 | 1 or 3 |
| Any | XLPE, no-heat sites | JLS cold shrink | 1 or 3 |
FAQ
Can I use LV tubing on an MV joint if I add more layers?
No. LV tubing lacks the semiconductive stress control layer required at MV voltage levels. Layering LV tubes does not replicate this function. Partial discharge will occur at the screen cut, leading to insulation erosion and failure. Always use voltage-rated joint kits.
What is the difference between a straight joint and a branch joint?
A straight joint connects two cable ends in-line. A branch joint (T-joint) connects a main cable to a branch cable. Branch joints are more complex and require a different kit design. Our standard kits are for straight joints; contact us for branch joint requirements.
Do I need to test the joint after installation?
For MV joints, a DC withstand test or partial discharge test after installation is recommended, especially for critical circuits. For LV joints, visual inspection and insulation resistance measurement are typically sufficient.
Written by Long, Sales Engineer at ZZ Heat Shrink. For MV cable joint specifications, see JRSY series. For cold shrink joints, see JLS series. For a quotation, contact our engineers.