Stress Control Tube Field Grading Guide
A stress control tube reshapes the electric field where a medium-voltage cable screen ends. It is not there to add insulation thickness. It spreads field concentration away from the sharp screen cut so partial discharge, tracking and early failure are far less likely. This guide covers how the grading works, how to size and position the tube, how to install it, and how it compares with a stress cone, a semi-conductive tube and stress control tape.
What a stress control tube does
A heat shrinkable stress control tube is a sleeve made from an electrically engineered material, normally co-extruded from a stress-grading layer and an insulating layer, then cross-linked and expanded. Under heat it recovers like other cross-linked polyolefin parts and conforms to the prepared cable core. Its job is electrical: the grading layer has a controlled resistivity that redistributes the field at the screen discontinuity, replacing a sharp peak with a gradual voltage drop along the tube.
The tube is one component inside a complete termination or joint kit. It sits over the screen cutback before the outer insulation tubes are shrunk, and it works together with stress-control tape, mastic, insulating tubing, anti-tracking tubing, breakout boots, sealing tubes, rain sheds and earthing parts.
Why the field concentrates at the screen cutback
Along the normal cable length the field stays fairly even because the cable is a coaxial structure. At a termination or joint the semiconductive screen must stop. That stop creates a geometric break where the field lines crowd toward the sharp screen edge.
Without a graded transition, the local field can drive partial discharge, surface tracking, electrical treeing, insulation erosion, flashover and a shorter service life. A stress control tube spreads the stress over a longer region of insulation and relieves the cutback.
Field-grading methods
| Method | How it works | Typical form |
|---|---|---|
| Geometric stress cone | Changes the termination shape so the field spreads over a larger area | Pre-moulded stress cone or shaped body |
| Capacitive grading | Uses high dielectric constant material to shift the capacitive field | High-K tube, high-K tape, silicone compound |
| Resistive grading | Uses semi-conductive or non-linear resistive material to control the gradient | Non-linear stress control tube or co-extruded layer |
| Combined system | Uses two or more methods together | Tube, tape, mastic, insulating body and outer protection |
Medium-voltage accessories often blend geometric, capacitive and resistive principles in one design. See MV termination partial discharge prevention for the interface work that decides whether the grading holds.
Stress control tube vs normal heat shrink tubing
| Dimension | Stress control tube | Normal heat shrink tubing |
|---|---|---|
| Primary function | Field grading at the screen cutback | Insulation, abrasion, marking, bundling |
| Where it sits | Over the screen cutback | Around wires, terminals, splices, jackets |
| Electrical behavior | High-K, semi-conductive or non-linear resistive | Usually insulating polymer |
| Role in MV termination | Designed for the field-control system | Not automatically suitable |
| Cutback coverage | Required and dimension-controlled | Not a design feature |
| Voltage range | Matched to the MV class of the kit | Often low-voltage or general purpose |
Do not substitute ordinary insulation tubing for a verified stress-control material in a medium-voltage accessory. The two parts solve different problems.
Stress control tube vs stress cone
| Dimension | Stress control tube | Stress cone |
|---|---|---|
| Physical form | Tubular sleeve | Shaped or pre-moulded field-control structure |
| Installation | Slides over insulation and cutback | Fitted at the cable-end stress region |
| Grading method | Capacitive, resistive or non-linear | Often geometric; may add high-K or resistive |
| Typical system | Heat-shrink or cold-shrink kits | Pre-moulded terminations, separable connectors |
| Used together | Yes | Yes |
A tube can act as part of the stress-cone system rather than replace every cone function. The full comparison is in stress control tube vs stress cone.
Stress control tube vs semi-conductive tube
| Dimension | Stress control tube | Semi-conductive tube |
|---|---|---|
| Main role | Reduces field concentration at the cutback | Restores or extends a semiconductive shield |
| Material behavior | High-K, non-linear resistive or engineered grading | Conductive or semi-conductive |
| Position | Covers the cutback and runs onto insulation | Over insulation where a screen is required |
| Controls voltage gradient | Yes | Not the primary purpose |
A termination may use both: the semi-conductive layer controls the shielding interface, while the stress-control layer manages the field where the screen ends.
Stress control tube vs stress control tape
| Dimension | Stress control tube | Stress control tape |
|---|---|---|
| Form | Pre-manufactured tube | Stretchable tape |
| Installation | Slide into position, then heat or cold shrink | Wrap by hand with controlled tension |
| Thickness | Generally consistent | Depends on wrapping skill and overlap |
| Irregular shapes | More limited | Excellent |
| Repeatability | Higher with the right size | More operator-dependent |
| Used together | Yes | Yes |
Heat-shrink systems often use tape, mastic and tubing together. Tape and mastic fill uneven areas; the tube gives a repeatable grading layer over the prepared insulation. Practical wrapping is covered in how to wrap a cable joint with self-amalgamating tape.
High-K vs resistive stress control
| Property | High-K / capacitive | Resistive / non-linear |
|---|---|---|
| Main mechanism | Shifts capacitive field with high dielectric constant | Controls gradient through resistive or non-linear behavior |
| Typical material | High-K silicone, high-K compound, high-K tape | Semi-conductive compound, non-linear polymer, ZnO-based material |
| Common type | Cold-shrink field-control tube, pre-moulded termination | Heat-shrink stress-control tube, co-extruded layer |
| Main advantage | Intimate contact through elastomeric compression | Controlled grading in compact heat-shrink builds |
Typical voltage classes
Stress control tubing appears in medium-voltage accessory systems. Published product families quote a range of voltage classes, but the exact rating must always drive selection. Wellele supplies the tube as part of termination kits matched to the cable voltage class, for example the 8.7/15 kV heat shrink cable accessories and the 26/35 kV heat shrink cable accessories; cold shrink equivalents cover 8.7/15 kV and 26/35 kV.
| Example class | Common published range | Selection note |
|---|---|---|
| General MV tube | 6.6 kV to 36 kV | Confirm cable OD and system voltage for the exact SKU |
| Heat shrink tube | up to 24 kV | Product-specific limit |
| Heat shrink termination family | 8 kV, 15 kV or 35 kV variants | Pick the correct family |
| Cold shrink MV products | about 5 kV to 46 kV | Kit selection required |
How to choose the correct stress control tube
Selection follows the electrical system, the cable dimensions, the accessory type and the environment. Match the tube to the cable insulation diameter range as well as the conductor size. Two cables with the same conductor can have different insulation diameters and screen builds. Start from MV cable accessories by voltage.
- Electrical data. System voltage class, cable insulation type (XLPE, EPR, PVC, PILC), termination or joint type, indoor or outdoor site, required standard (IEC, IEEE, CENELEC or utility), and the partial-discharge, impulse, dielectric and tracking performance needed.
- Cable data. Conductor size and diameter, main insulation diameter, diameter over the semiconductive screen, metallic screen type, jacket diameter, cutback position and connector dimensions.
- Product data. Supplied and recovered ID, recovered wall, recommended overlap length, recovery method, grading material system, and the tape, mastic or compound the kit needs.
How to position the tube
- One end overlaps the semiconductive or metallic screen cutback.
- The rest runs over clean, undamaged insulation.
- The tube covers the required stress-control tape, mastic or transition layer.
- Position it before recovery so alignment holds after longitudinal shrink.
- Never install it over contamination, sharp edges, damaged insulation or wrong preparation.
A published example overlaps the screen cutback by about 20 to 30 mm and extends onto the insulation. The real length follows the kit instruction.
How to install heat shrink stress control tubing
Safety: medium-voltage termination and joint work needs trained and authorized staff, plus isolation, lockout, test-for-dead, earthing, PPE and commissioning procedures.
- 1. Confirm kit and cable compatibility Verify the voltage class, cable insulation type, insulation diameter, construction and accessory kit size range before you open the package.
- 2. Prepare the cable to the drawing Remove jacket, metallic screen, semiconductive screen and insulation only as far as the installation manual states. Do not score the main insulation.
- 3. Clean and protect the insulation Wipe off semicon residue, grease, dust and moisture with an approved cleaner and lint-free cloth. Keep the surface dry and undamaged.
- 4. Apply the required stress-control materials Lay down stress-control tape, mastic, filling or dielectric compound exactly as the kit specifies.
- 5. Position the stress control tube Slide the tube so it overlaps the screen cutback and runs onto clean insulation by the specified length.
- 6. Recover with controlled heat Use the approved heat source, keep it moving around the full circumference, and follow the prescribed shrink direction.
- 7. Inspect after cooling Check for full recovery, correct position, no air gap, no wrinkle, no scorch and complete cutback coverage.
- 8. Complete the remaining layers Fit the insulation tube, anti-tracking tube, sealing parts, rain sheds, earthing and breakout boot, then run the required tests.
For the step-by-step kit view, see stress control tube installation steps and the general heat shrink installation guide.
How much heat is needed
The correct heating process follows the specific tube. Products differ in recovery start temperature, full recovery temperature, wall thickness, material system, approved heat source, shrink direction and ambient limits. One manufacturer quotes a minimum full-recovery temperature near 120°C for a specific heat-shrink termination; that is not a setting for every tube.
- Use the exact technical datasheet and installation manual.
- Use a controlled heat source and keep it moving.
- Heat evenly around the full circumference and follow the shrink direction.
- Do not force fast recovery with localized high heat.
- Confirm finished geometry and coverage after cooling.
Avoiding voids, wrinkles and tracking failure
| Problem | Typical cause | Prevention |
|---|---|---|
| Tube wrinkles | Oversized tubing, uneven heating, wrong position | Pick the correct size and heat uniformly |
| Air voids | Moisture, contamination, poor mastic use, wrong overlap | Clean and dry the insulation; apply materials correctly |
| Poor stress control | Tube misses the screen cutback | Mark the location and follow kit dimensions |
| Damaged insulation | Semicon removal scratches the insulation | Use approved tools and inspect before installation |
| Local overheating | Heat source too close or stationary | Keep the heat moving and controlled |
| Early flashover | Contamination, wrong materials, incomplete grading | Use the complete system and the full procedure |
| Partial discharge | Void, damaged insulation, wrong position | Keep interfaces clean and test as required |
| Surface tracking | Wrong outdoor system, weak anti-tracking layer | Use the right outdoor design and clean build |
Most stress-control failures are system failures, not tube-only failures. Cable preparation, cutback geometry, material position, insulation cleanliness, earthing, sealing and testing all matter. The symptom list for misapplied tubes is in stress control tube problems.
Where the tube fits in a complete kit
A stress control tube is one functional part of a qualified accessory system. On its own it is not a complete medium-voltage termination or joint. Selection and installation must cover the full set: tube, stress-control tape and mastic, insulation restoration, screen continuity, earthing, sealing, anti-tracking outer insulation and weatherproof parts. For joint work the bigger picture is in cable joint cold shrink vs heat shrink and the cable joint solution.
Frequently asked questions
What is a stress control tube?
A stress control tube is a heat-shrinkable or cold-shrinkable sleeve used in medium-voltage cable terminations and joints. It lowers field concentration at the end of the semiconductive or metallic screen by spreading the electric field along its length.
Why is stress control needed at a cable screen cutback?
The screen ends in a sharp cut that concentrates the field. Grading spreads that stress over a longer distance and lowers the risk of partial discharge, tracking, insulation aging and failure.
What is the difference between stress control tubing and normal heat shrink tubing?
Stress control tubing uses an engineered grading material at the cutback. Normal heat shrink tubing is made for insulation, marking, abrasion and mechanical cover. It should not stand in for a stress control product in a medium-voltage termination or joint.
What is a cable stress cone?
A stress cone is a field-control structure that reduces field concentration where the screen ends. It can be geometric, pre-moulded, high-dielectric or resistive, and a stress control tube can form part of a stress-cone system.
What voltage range is stress control tubing used for?
It is common in medium-voltage accessory systems such as 6.6 kV, 11 kV, 15 kV, 24 kV and 35 kV. The actual rating depends on the product, the cable design, the insulation diameter, the accessory system and the applicable standard.
Can stress control tubing be used for 11 kV cable?
Yes, if the product or termination kit is rated for 11 kV and matches the cable insulation diameter, screen construction, insulation type and environment.
Can stress control tubing be used for 33 kV or 35 kV cable?
Yes, but only with products built for the 26/35 kV or 35 kV class. Do not use 11 kV or 15 kV tubing on 33 kV or 35 kV cable unless the manufacturer approves it.
Can stress control tubing be used for cable joints?
Yes, as part of a medium-voltage joint system around connector transitions and screen cutbacks. A complete joint also needs insulation restoration, screen continuity, earthing, sealing and outer protection.
Can stress control tubing be used outdoors?
Yes, as part of a complete outdoor-rated termination. The full system may need anti-tracking tubing, rain sheds, UV-resistant materials, enough creepage distance, moisture sealing and correct earthing.
Can stress control tubing be used with XLPE and EPR cables?
Yes. Stress control systems exist for both XLPE and EPR. Match the cable insulation diameter, screen type, voltage class, core count and environment to the exact accessory kit.
Can I use normal heat shrink tubing instead of a stress control tube?
No. Normal heat shrink tubing does not give the field-grading behavior required at a medium-voltage screen cutback. Use the correct stress-control material inside a complete, tested accessory system.
How do I install heat shrink stress control tubing?
Prepare the cable to the kit drawing, clean the insulation, apply any required stress-control tape or mastic, position the tube so it overlaps the screen cutback, recover it with even heat, inspect the result, then fit the remaining termination or joint components.
References
- TE Connectivity heat-shrink whitepaper
- TE Raychem HVT-Z heat-shrink terminations
- 11 kV heat shrink termination instruction (cablejoints.co.uk)
- Electric stress control in cable joints and terminations (Electrical Technology)
- Heat shrink cable terminations references (Power and Cables)
- Heat shrink stress control tubing product notes (VP TPL)