Medium-Voltage Cable Buying Guide

Which voltage rating and standard apply to the order?

The rated voltage must be written in the notation required by the governing standard. For extruded-insulation cables, IEC 60502-2 covers voltages from 6 kV (Um 7.2 kV) to 30 kV (Um 36 kV). Above 30 kV up to 150 kV (Um 170 kV), IEC 60840 applies.

Confirm the maximum system voltage as well as the nominal voltage, because the insulation must be rated for the maximum. Also confirm the grounding method. The grounding arrangement changes the voltage the insulation sees during an earth fault, so the cable specification and the system study must agree.

What does each layer of an MV cable do?

A typical XLPE-insulated MV cable has a semiconductive conductor screen, XLPE insulation, a semiconductive insulation screen and a metallic screen. XLPE is commonly used for the insulation. Its maximum conductor temperature is commonly cited as 90 °C in normal operation and 250 °C under short circuit for up to 5 s (commonly cited from IEC 60502-1 and manufacturer data).

• Conductor and insulation screens are semiconductive layers that control the electric field at the conductor and insulation surfaces. Their construction should be stated in the specification.
• Metallic screen: copper wires or tape provide screening and may carry earth-fault current. Their area and bonding arrangement require calculation, not estimation.
• Armour: single-core AC cables use non-magnetic armour, typically aluminium wire armour (AWA), because steel armour around a single AC conductor causes magnetic losses and heating.

Water protection depends on the route. Water-blocking layers and sheath choice should be selected for the actual installation conditions, as described in the radial and longitudinal water-blocking guide.

How should the metallic screen and sheath bonding be specified?

Single-core MV cables are bonded in one of three ways: both-ends (solid) bonding, single-point bonding or cross-bonding. Each method changes the sheath voltage and the earth path, so the choice belongs to the system designer and the utility.

• Both-ends (solid) bonding connects the sheath to earth at both ends.
• Single-point bonding earths the sheath at one end only. The other end is isolated through a sheath voltage limiter (SVL), and a parallel earth continuity conductor is needed.
• Cross-bonding transposes the sheaths at link boxes.

Standing-voltage limits vary by utility and country, so this guide does not give a single figure. Take the limit that applies to your project from IEEE 575 Annex and the utility's own rules.

What must the specification list for each item?

Use the table below as a checklist when writing the inquiry. The basis column shows where each value should come from; it does not replace the governing standard.

Values marked as commonly cited should be confirmed against the governing standard edition and the manufacturer's datasheet before ordering.

What test evidence and accessories should you request?

Ask for routine and type test evidence, and state which tests the supplier will perform on each drum. Routine partial-discharge testing is commonly cited as no discharge above 10 pC at 1.73 U0 for IEC 60502-2 cables. The partial discharge testing guide explains factory and site testing. For the routine AC withstand test, take the test voltage and duration from the datasheet for the cable and standard edition you are buying.

Accessories are part of the cable system. Joints and terminations must match the conductor size, insulation diameter, screen construction and equipment interface. An MV cable can pass factory tests yet fail early if the termination does not match the insulation diameter or screen construction. Ask for approved accessory data, confirm the installer's competence and include after-installation testing in the procurement plan.

How should drums and cable ends be handled?

Agree drum lengths and pulling limits in the order, and take the minimum bending radius from the datasheet. Sources give different values for similar cables, so no figure is quoted here. The installation guide covers bending and pulling in more detail.

• Keep all cable ends sealed at all times, including during storage.
• Store and handle drums upright, never on a flange. Lift with a mandrel through the centre hole or with forks under both flanges, not on the lagging.
• Chock drums with wedges, keep them off flood-prone ground, and avoid impact or bending damage during unloading and rolling.

Frequently asked questions