Stress Control Tube Positioning: Why 10 mm
A stress control tube is the one component in a medium-voltage termination whose position is a specification, not a preference. Move it off the screen cut-back and the electric field stops being graded. The termination can pass its commissioning test and still discharge years later.
What the tube is doing electrically
The tube is co-extruded from a stress grading layer and an insulating layer, then cross-linked and expanded. Where the cable screen ends, the field concentrates. The grading layer has a controlled impedance that spreads that concentration along its length, replacing a sharp gradient with a gradual voltage drop. Heat shrinkable stress control tube is normally one component inside an MV termination or joint kit, positioned over the screen cut-back before the outer insulation tubes are recovered.
None of that is mechanical. The tube does not hold anything together and it does not seal. Its whole function is the field, and the field depends on where the tube sits.
Positioning, step by step
- Strip the semiconductive layer back to the dimension the kit drawing gives for the cable type and voltage class.
- Smooth the cut edge of the semicon. A step or ridge left at the cut is the start point for discharge.
- Mark the cut-back line on the insulation with a marker that leaves no conductive residue, or use the marking band supplied with the kit.
- Slide the tube on before the joint is made, not after.
- Land the tube on the marked line so it covers the semicon cut by 10 mm to 15 mm and extends past it along the insulation.
- Check the extension against the data sheet for the voltage class. On 26/35 kV builds it is longer than on a 12/20 kV build.
- Recover evenly from the centre outward, then re-check the position. Heat moves the tube, so confirm it after recovery and not before.
The dimensions that decide it
| Dimension | Value used | Where it comes from |
|---|---|---|
| Coverage over the semicon cut | 10 mm to 15 mm | Kit drawing |
| Extension past the cut along the insulation | Set by voltage class, often 100 mm to 200 mm | Data sheet |
| Overlap at each end of the graded zone | 15 mm | Kit drawing |
| Air void tolerated at the cut | None. A 0.5 mm pocket is enough to discharge at 26/35 kV | Field experience |
| System voltage classes | 0.6/1 kV, 8.7/15 kV, 26/35 kV | Product range |
What goes wrong, and why
| Symptom | Cause | Prevention |
|---|---|---|
| Tube slips after recovery | Inner diameter sized on cable OD instead of the prepared insulation diameter | Size on the prepared core and confirm the recovered ID |
| Discharge at the cut | Tube positioned off the cut-back line | Mark the line and re-check after recovery |
| Discharge along the tube | Semicon step left at the cut | Smooth the cut before the tube goes on |
| Wrinkles in the recovered wall | Heat applied at a point instead of swept | Sweep the heat source from the centre outward |
| Water in the termination | Ends not sealed | Follow the kit sealing sequence |
The failure mechanisms behind these symptoms are covered in stress control tube problems, and the voltage class table sits in MV cable accessories by voltage.
What to send
Give us the cable voltage class, the insulation diameter after stripping, the shield construction and the kit type. Send those and we return the matched tube with its position dimension.
Frequently asked questions
Why does 10 mm of movement matter on a stress control tube?
Because the tube grades the field only where it sits. Shift it off the cut-back line and the concentration at the screen end is no longer spread along the graded length, so discharge starts at the wrong point.
Do I need to mark the cut-back line?
Yes. Without a mark, the position after recovery is a matter of eye. Mark the line, land the tube on it and re-check once the tube has cooled.
The tube covers the cut. Is that enough?
Coverage is half of it. The tube also has to extend past the cut along the insulation by the distance on the data sheet for that voltage class.
Why did my tube move during heating?
Heat makes the wall soft before it recovers. Sweeping from the centre outward holds the tube in place better than heating from one end, and the position still needs a check afterwards.
Which dimension do I size on?
The prepared insulation diameter, not the cable OD. Sizing on the cable OD is the usual reason a recovered tube does not bite the core.