What Happens When You Cut the Screen
Inside an intact medium voltage cable the electric field is uniform and radial. The conductor is at line voltage the metallic screen is at earth and the field runs cleanly between them through the insulation.
The moment the screen is cut the field has nowhere tidy to go. It concentrates at the cut edge to a level far above the average stress in the insulation. Left alone that concentration starts partial discharge which slowly erodes the insulation from the surface inward until the termination breaks down.
Everything else in a termination exists to manage that one problem.
How Stress Control Fixes It
A stress control layer is a high permittivity material applied over the screen edge and onto the insulation. Because its dielectric constant is far higher than the cable insulation it redistributes the equipotential lines and spreads the field over a longer distance instead of concentrating it at one edge.
The grade matters. A 24 kV grade has a dielectric constant of 10 to 25. A 36 kV grade runs 25 to 50 because the field at the higher voltage class needs to be spread further.
The Three Preparation Errors That Cause Most Failures
In our experience of field failures three preparation errors account for the large majority of early terminations failures and all three are avoidable.
- Semiconductive residue left on the insulation surface. Any trace becomes a discharge site. Wipe in one direction away from the screen so contamination is not dragged back over cleaned insulation.
- A ragged screen edge. A burred or torn screen edge concentrates the field again even under a stress control layer. Dress the edge square before applying anything.
- Insufficient overlap. The stress control layer must overlap both the metallic screen and the insulation by the distance in the kit drawing. Too short an overlap means the field is still concentrated at the end of the layer.
What the Test Regime Is Checking for
Partial discharge measurement at 1.73 U0 is the direct check on whether the field control is working. The limit in the IEC 60502-4 sequence is 10 pC maximum measured both at ambient temperature and at maximum conductor operating temperature.
The reason it is measured hot as well as cold is that materials change with temperature. A termination that measures clean cold and dirty hot has a field control problem that will show up in service.
If a Termination Has Already Failed
Cut it back and inspect. Tracking marks on the insulation surface point to a cleaning failure. Erosion concentrated at one point on the screen edge points to a preparation or overlap error. Surface tracking on the outer layer points to a pollution or creepage problem which is a different fix entirely.
If the failure pattern is surface tracking on the outside rather than internal discharge the answer is more creepage not better stress control.
