Stress Control Tube Problems: Why 10-35 kV Terminations Fail
Most 10-35 kV termination faults do not start inside the cable. They start at the stress-control tube, at the point where the insulation shield is cut back. This page walks through the failures we see in returned terminations and the field data that tells you what actually went wrong.
1. What the Stress Control Tube Actually Does
A medium-voltage termination ends the insulation shield at a sharp cut. At that edge, the electric field concentrates many times above the average stress. If nothing redistributes it, corona starts at the cut and erodes the insulation until breakdown. The heat-shrink stress control tube is a field-grading layer with a controlled dielectric constant (typically εr 12-30 for resistive grading, or 6-8 for capacitive grading) that spreads the stress over a longer distance along the insulation surface.
Two things must both be right for it to work: the tube must sit exactly over the shield cut-back, and there must be no air at the interface. Almost every field failure we analyse fails one of these two.
2. Symptom → Root Cause → Fix
| Symptom on site | Most likely root cause | Field fix |
|---|---|---|
| Tube slid off the cut, bare insulation visible | Supplied ID too large; tube recovered but did not bite the semicon | Re-size on prepared cable OD; tube must recover onto semicon edge with 10-15 mm overlap |
| Tube too short, gap at the top | Longitudinal shrink cut the piece short | Add 10-15% to cut length; verify recovered length on a sample |
| Rough, pitted surface under the tube after removal | Air void ionised under PD over months | Smooth semicon edge, clean surface, heat centre-out to expel air |
| Discharge tracking along the tube surface | Contamination or moisture on the insulation surface | Wipe with approved solvent, control humidity, avoid touching the surface after cleaning |
| Corona at the cut edge despite correct position | Semicon cut not clean; loose strands left | Cut shield with a sharp blade, remove every loose strand, sand any burr |
These are the patterns, but the numbers matter. A 26/35 kV termination that runs clean at 30 kV with a good stress-control interface will begin to discharge as soon as even a 0.5 mm air pocket sits at the cut — the stress magnification is that sharp.
3. Positioning Is a Measurement Job, Not a Visual Job
The most common error is placing the tube "by eye" over where the installer thinks the shield ends. Do this instead:
- Mark the shield cut-back with a visible pen before sliding anything on.
- Confirm the tube's supplied ID fits the prepared insulation without force but with contact — a loose fit means re-size.
- Position so the tube covers the semicon cut by 10-15 mm and extends past it along the insulation by the distance on the data sheet (often 100-200 mm depending on voltage class).
- Heat from the centre outward so the tube recovers first at the middle, then sweeps to both ends — this drives trapped air out instead of sealing it in.
On critical feeders, photograph the marked position before and after recovery so the QA record shows the cut-back line matched the tube edge.
4. Air Voids, Semicon Edges and Surface Cleanliness
An air gap between the semicon, the tube and the insulation is a miniature capacitor with a very low breakdown voltage. Under normal service voltage the gap ionises, and the local discharge erodes the insulation polymer. The erosion is slow — months, not minutes — which is why the fault appears long after commissioning and gets blamed on the cable.
Three habits remove the risk:
- Cut the semicon with a single sharp blade and lightly sand the edge so it steps down smoothly instead of standing proud.
- Clean the whole stress zone with the solvent specified by the kit supplier, then do not touch it — skin grease is enough to seed tracking.
- Heat centre-out and inspect for trapped bubbles by the colour change; a second, cooler pass over any suspected spot closes the interface.
5. Which Test Catches It
An AC or DC withstand test will often pass even when a stress-control interface is marginal, because a short test does not sustain the PD long enough to fail. The test that separates good from bad is a partial-discharge measurement at power-frequency voltage. It detects the pico-coulomb-level discharges at the cut that precede breakdown. For critical 26/35 kV switchgear feeders, specify a PD test at 1.5 U0 and log the pC reading.
Related: Heat Shrinkable Stress Control Tube · MV Termination Partial Discharge Prevention · MV Accessories by Voltage
Frequently asked questions
Why does my stress control tube keep sliding off the shield cut-back?
Either the supplied inner diameter was sized too large, or the cut-back mark was placed past the tube's designed recovery point. Re-size on prepared cable OD and confirm recovery with 15 mm overlap each side.
How much does longitudinal shrink matter for a stress control tube?
Polyolefin field-grading tubes shorten 5-15% during recovery. Cut to the bare minimum and the tube pulls back off the cut, leaving a bare gap where corona starts. Add 10-15% to the cut length.
Can an air void under the stress control tube cause partial discharge?
Yes. An air pocket at the semicon/tube/insulation interface concentrates stress and ionises, producing PD that erodes the insulation. Smooth the semicon, clean the surface, heat centre-out.
What test catches a mispositioned stress control tube?
A partial-discharge test at power-frequency voltage, with the cut-back marker verified, reveals discharge at the interface. A DC or AC withstand test alone may pass while PD is already present.
Specifying a stress control tube for a 10-35 kV termination?
Send the cable OD, insulation diameter, shield construction and voltage class. We return a sized recommendation with the correct cut length.
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