Low-Voltage Versus Medium-Voltage Construction: What Changes Above 1 kV

Side by side

Comparing the manufacturer's construction statements for the two ranges directly, using the same YJV family:

0.6/1 kV — Compact stranded copper conductor Class 2 to IEC 60228 / XLPE insulation rated 90 °C / PVC or FR-PVC type ST sheath to IEC 60502, black.

3.6/6 kV and above — Compact stranded copper conductor Class 2 to IEC 60228 / semiconducting conductor screen / XLPE insulation rated 90 °C / semiconducting insulation screen / copper tape screen / PVC or FR-PVC type ST sheath to IEC 60502, black.

What is common: the conductor class, the insulation material and its 90 °C rating, and the sheath specification. What is added: three layers, of which only one is metallic.

Why the screens are not extra insulation

The two semiconducting screens are the parts most often misread in a specification. They are semiconducting, so they do not add dielectric strength — they cannot, since they conduct.

Their function is geometric. A stranded conductor presents an uneven surface at the microscopic level, and at medium voltage the resulting field concentration at the strand interstices can initiate partial discharge that erodes the insulation from the inside out. The conductor screen provides a smooth equipotential surface so the field inside the insulation is uniform; the insulation screen does the same at the outer boundary. Without them, increasing insulation thickness alone does not solve the problem, because the discharge starts at the surface irregularity rather than in the bulk of the insulation.

Practical consequences

The extra layers change how the cable is handled and terminated, not just how it is specified.

Termination becomes a layered procedure: the metallic screen has to be terminated and earthed, and the semiconducting insulation screen has to be removed cleanly to a defined distance and the stress at that cut managed. A medium-voltage termination done with low-voltage practice is a common source of early failure.

The metallic screen also brings the magnetic-duct restriction into play — single-core cables must not be laid in magnetic ducts because induced circulating current in the duct adds losses and heating. Neither consideration arises on a 0.6/1 kV installation.

Choosing between them

The choice is made by system voltage, not by preference. A 0.6/1 kV cable is not a cheaper medium-voltage cable, and a medium-voltage construction is not a more robust low-voltage one — they are built to different standards, IEC 60502-1 and IEC 60502-2 respectively, with different insulation thickness rules and test voltages.

Where a project steps voltage through a transformer, the cable schedule should follow the voltage on each side rather than standardising on one construction. Standardising on the higher-voltage build across a whole site adds termination cost and complexity to every low-voltage circuit without any corresponding benefit.