MV Cable Termination Partial Discharge Prevention
Partial discharge is the early warning of most medium-voltage termination failures, and it is caused, almost always, by a small mistake at the shield cut-back. This guide is the engineering checklist for keeping PD out of an 8.7/15 kV or 26/35 kV termination. Position, preparation, cleanliness and the test that actually catches it.
1. Where PD Starts
At the point where the insulation shield ends in a cut, the electric field concentrates to many times the average stress. A field-grading stress-control tube normally redistributes this, but if the tube is mispositioned, the semicon edge is dirty or rough, an air void sits at the interface, or surface contamination is present, the local stress ionises the air. That discharge is PD, and over months it erodes the insulation until breakdown. PD is not a random event. It is the visible start of a defined chain of errors.
2. The Termination PD Prevention Checklist
- Stress-control tube position: place it exactly over the cut-back mark, covering the semicon edge by the designed overlap and extending along the insulation to the printed distance.
- Shield cut-back: cut the shield to the mark with a sharp blade; no loose strands or burrs at the edge.
- Semicon preparation: step and smooth the semicon edge so it tapers rather than standing proud; remove every speck of residue.
- Cleanliness: clean the whole stress zone with the specified solvent and keep hands off it afterwards.
- Air-free interface: heat centre-out and inspect for trapped bubbles by the recover/colour change.
- Dimensional tolerance: confirm tube supplied ID fits, and the assembly's phase/earth (repeater) clearances meet the data sheet.
3. The Semicon Edge Is a Precision Part, Not a Rough Cut
Even a small rough spot on the semicon cut acts as a micro-electrode that concentrates the field. Treat the cut as a precision step: single clean blade pass, then light sand to a smooth taper, then clean. Installers who rush this step are the ones whose terminations show PD on the first test.
4. Why Cleanliness and Air-Free Assembly Matter More Than People Expect
Skin grease, dust and moisture at the stress zone seed tracking, and an air pocket at the interface ionises at normal service voltage. Both are invisible at install and fatal months later. The assembly discipline, clean, don't touch, heat centre-out, is what separates a termination that holds PD-margin from one that does not.
5. The Test That Catches It: PD Measurement
A DC or AC withstand test is not a PD test. It applies a short high stress and a termination with marginal PD will often pass it. The correct check is a partial-discharge measurement at power-frequency voltage (typically at 1.5 U0 for type tests, lower for routine), logged in pico-coulombs (pC). If the pC reading is above the contract limit, do not commission. Re-inspect the stress-control interface before it fails in service. No specific pC pass value is quoted here, because it must come from your project's test specification.
Related: Heat Shrinkable Stress Control Tube · Stress Control Tube Problems · MV Accessories by Voltage
Frequently asked questions
What causes partial discharge in a cable termination?
PD starts at a stress concentration: a mispositioned stress-control tube, a dirty or rough semicon edge, an air void at the interface, or surface contamination. The field at the shield cut-back is far above average stress, so any small imperfection there ionises and erodes insulation over time.
How critical is the stress-control tube position?
It must sit exactly over the shield cut-back, covering the semicon edge by its designed overlap and extending along the insulation to the specified distance. Even a few millimetres off leaves an ungraded field at the cut, where PD begins.
What dimensional checks prevent PD?
Confirm the cut-back mark matches the kit step, the tube supplied ID fits the prepared insulation, the semicon edge is clean and stepped, and the assembly phase/earth clearances meet the data sheet. Measure, do not estimate.
Does a withstand test guarantee the termination has no PD?
No. A short AC or DC withstand test will often pass a termination that already has marginal PD. A dedicated partial-discharge measurement at power-frequency voltage, logged in pC, is the test that catches it.
Planning a critical MV termination and need PD-margin certainty?
Send the voltage class, cable OD, shield construction and the PD test limit from your spec. We size the stress-control kit and confirm the assembly steps.
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