Heat Shrinkable Stress Control Tube
Heat Shrinkable Stress Control Tube is a semi-conductive/insulating composite tube used at medium-voltage cable terminations to even out the electric field where the cable screen ends. Without stress grading, field concentration at the screen cut-back point drives partial discharge and premature failure; the stress control tube spreads that field along its length. The tube matches the cable voltage rating and recovers with the same hot-air process as the rest of the termination kit. Send the cable voltage class and cross-section and we return the matched tube.
Specification
| Material | Semi-conductive / insulating composite |
|---|---|
| Voltage rating | Matches cable rating |
| Note | Evens cable-termination E-field, relieves stress concentration |
What it is and how it works
The tube is co-extruded from a stress-grading layer and an insulating layer, then cross-linked and expanded. Its function is electrical, not mechanical: the stress-grading material has a controlled impedance that redistributes the electric field at the screen discontinuity, replacing a sharp field concentration with a gradual voltage drop along the tube. Under heat the tube recovers like other cross-linked polyolefin components, conforming tightly to the cable core. It is normally one component inside an MV termination or joint kit, positioned over the screen cut-back before the outer insulation tubes are shrunk.
Where Heat Shrinkable Stress Control Tube is used
Primary applications: Cable accessory stress-cone area.
How to select the right size
Selection is fixed by the cable it serves:
- Voltage class. The tube's grading characteristics are designed for a specific voltage range : state the system voltage (8.7/15kV, 12/20kV, 26/35kV).
- Cable cross-section. The recovered ID must close on the cable core over the insulation; the cross-section decides the tube size.
- Kit or standalone. If the tube is part of a termination project, tell us the kit type : we supply it as a kit component or as a standalone replacement.
Common Failure Modes & How to Prevent Them
These are the issues most often seen when installing or specifying Heat Shrinkable Stress Control Tube, with the root cause and the prevention or fix for each.
It shrinks but still slips on the wire
Root cause: The supplied diameter was too large for the substrate. Heat shrink only contracts so far; a 2:1 tube shrinks to half its original size and then stops. Below about 20% compression it has nothing to bite on.
Fix: Size to a recovered ID roughly 15-25% smaller than the part you cover. On connectors or mixed-diameter sections, move to 3:1 or 4:1 so one size clears the bulge and still grips the wire. As a rule, design to about 75% of the rated shrink capacity and keep the rest as margin.
One side looks burnt, the other still thick
Root cause: An open flame or a heat gun held in one spot creates hot spots. Polyolefin degrades past its threshold and loses its elastic memory in those patches; thermal cycling later opens cracks from there.
Fix: Use a temperature-controlled heat gun at 90-120 C, keep it moving, and work from the centre outward. Never use a lighter or torch on production work - it also lays down carbon that hurts dielectric performance.
Looks shrunk but it never sealed
Root cause: Underheating. The tube didn't reach full recovery, so the wall stayed soft and the ends didn't close.
Fix: Hold even heat until the tube fully conforms - usually 10-15 seconds of sustained heat at the right temperature. If you see gaps or a puffy look, go back over it.
It cracked along the axis after a few months
Root cause: Wrong material for the environment. PVC softens around 80 C; cross-linked polyolefin is rated to 125 C. The tubing performed as specified - the specification didn't match the real heat.
Fix: Confirm the material on the datasheet and, for incoming batches, a spot FTIR check. Match the continuous temperature rating to the actual environment, not the ambient guess.
Water got in and the copper went green
Root cause: Single-wall tubing tightens around the wire but does not seal axially. Moisture wicks in along the strands and sits against the conductor.
Fix: For anything wet, outdoor, or marine, use dual-wall adhesive-lined tubing. The inner melt adhesive fills the gap and stops capillary wicking. Single-wall is fine only in dry, indoor builds.
After shrinking, every wire looks the same
Root cause: The marking under the insulation is hidden once the tube recovers.
Fix: Print or tag the wire, or use coloured sleeving, before you apply heat. Six months later, tracing an unlabelled harness wastes more time than the labels cost.
The joint was exposed after heating
Root cause: Polyolefin also shortens 5-15% in length during recovery. A piece sized to the bare minimum ends up short.
Fix: Add 10-15% to the length and keep at least 10 mm overlap past the connection on each side (25 mm where a seal matters). We print the expected longitudinal shrink on our datasheets so the cut length is right the first time.
Request a quote for Heat Shrinkable Stress Control Tube
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Size range & specification notes
Used at cable terminations to even out the electric field and relieve stress concentration at the screen cut-back.
Frequently asked questions
What does a stress-control tube actually do?
At cable terminations the insulation shield ends in a sharp cut, creating a field concentration that initiates breakdown. Heat Shrinkable Stress Control Tube is a field-grading tube whose controlled dielectric constant spreads that stress over a longer distance, preventing corona and partial discharge at the shield cut.
Where exactly does it sit and how much overlap is needed?
It is placed over the shield cutoff of the termination. The controlled-resistivity layer must overlap the insulation shield by at least 20 mm and extend onto the conductor/insulation as specified, with no air gap: we normally fill the overlap with silicone grease to kill voids.
Will it perform at the rated voltage and temperature?
The tube is a semi-conductive / insulating composite, with service temperature matched to the cable system, supplied to match the voltage rating of the termination kits (it evens the cable-termination E-field and relieves stress concentration). It is part of a coordinated accessory set, not a generic sleeve, so the grading stays consistent with the stress-cone design.
Can I install it without creating a void?
Yes, if you clean the insulation, apply grease at the overlap, and shrink from one end so air is pushed out. A trapped bubble is the usual cause of PD, so we give the exact overlap and wrap direction in the kit sheet.
Is it compatible with heat-shrink and cold-shrink terminations?
It is supplied as the stress-control element of the matching Matches cable rating termination kit (heat-shrink or cold-shrink per your choice). Ordering the full set guarantees the tube, insulation and stress cone are dimensionally coordinated.
What documentation supports the bid?
We provide the kit's compliance data () and batch certificates; for utility approvals we align the paperwork to your market and can supply type-test references.
Why does my heat shrink still slip after shrinking?
Usually the supplied diameter was too large, so recovery stops before it grips. Size to a recovered ID about 15-25% smaller than the part, and use 3:1 or 4:1 on connectors.
Do I need adhesive-lined tubing for an indoor panel?
Not if it stays dry. Single-wall polyolefin is fine for indoor, dry insulation. Reach for dual-wall only where moisture, spray, or outdoor exposure exists.
How much longer should I cut the tube?
Add 10-15% for longitudinal shrink, plus at least 10 mm overlap past the joint on each side (25 mm where a seal matters).
Can I use a lighter instead of a heat gun?
Only as a field emergency. Open flame gives uneven heat, soot, and carbon that hurts dielectric performance. Use a temperature-controlled heat gun.