What are the layers of an MV XLPE cable?
Every layer in an MV cable has a job. Working from the centre out:
- Conductor: compacted stranded copper or aluminium to IEC 60228 class 2. It may be water-blocked.
- Conductor screen: a thin extruded semiconducting layer. It smooths the electric field around the strands so there are no sharp points.
- XLPE insulation: cross-linked polyethylene, rated for 90 °C continuous and 250 °C in a short circuit.
- Insulation screen: another semiconducting layer, bonded or strippable. It gives the field an even outer boundary.
- Metallic screen: copper wires and/or tape. It carries the charging current and the earth fault current, and keeps the outside of the cable at earth potential.
- Oversheath: PE for buried cables, PVC, or LSZH where fire performance in buildings matters.
- Armour (optional): aluminium wire (AWA) on single-core cables, so it does not heat up in the magnetic field, or steel wire (SWA) on three-core cables.
PractitionerConductor screen, insulation and insulation screen are extruded together in one pass (triple extrusion) and dry cured. This avoids voids and contamination at the interfaces, which is where partial discharge starts.
How do you size an MV cable?
- Design current: I = S / (√3 × V). For ring cables, use the full ring transfer load, not just one substation.
- Current rating: rate the cable for how it is really laid, including soil thermal resistivity, ground temperature, depth, grouping, ducts and the bonding method. IEC 60287 is the reference method. Manufacturers' tables are based on it, with correction factors.
- Short circuit: A ≥ I × √t / k. For XLPE (90 °C to 250 °C), k = 143 for copper and 94 for aluminium. Also check the metallic screen against the earth fault current and duration.
- Voltage drop: usually small at MV, but check long feeders and rural lines.
- Practical checks: bending radius, pulling tension, termination size in the RMU, and the utility's standard sizes.
ExpertFor the most onerous section of a route (a road crossing in ducts, a section near a hot pipe, or a riser shaft), rate that section separately. The hottest section sets the rating for the whole circuit. Where the soil may dry out around loaded cables, IEC 60287-2-1 lets you model a dried zone with a higher resistivity, or you can specify thermally stable backfill with a tested resistivity.
Why do soil resistivity and ground temperature matter so much?
Almost all the heat from a buried cable must flow out through the soil, so the soil's thermal resistance (T4) is usually the largest term in the rating. Moist soil might be 1.0 K·m/W, while dry desert sand can be 2.5 to 3.0 K·m/W. Ground temperature at depth in the GCC can be 35 °C or more, which leaves less temperature headroom before the 90 °C limit. Use the calculator above and change these two values to see the effect.
Sheath bonding: both ends, single point or cross bonded?
- Both ends bonded: simple and safe, with no standing voltage. But the magnetic field induces circulating currents in the screens, which add heat. The effect grows with conductor size, screen area and cable spacing.
- Single-point bonded: earthed at one end only, so no circulating current flows. A standing voltage builds up at the open end in proportion to load and length, so sheath voltage limiters (SVLs) and often a parallel earth continuity conductor are fitted.
- Cross bonded: the route is split into three minor sections with screen transpositions at link boxes. The induced voltages cancel over each major section, which suits long routes.
Many utilities set a limit for standing voltage under full load, often around 50 to 65 V. Check your utility's rule.
How is an MV cable terminated?
A termination has to do three things: control the electric field where the screen stops, insulate the exposed length of insulation, and seal the cable against moisture. The usual types are heat-shrink, cold-shrink and pre-moulded (including the screened separable connectors used in RMUs). Preparation matters more than the kit itself: a square, clean cut of the insulation screen, no nicks in the insulation, clean surfaces and correct dimensions from the kit instructions.
Regional notes
GCC
Cables are usually to IEC 60502-2 or BS 6622, with LSZH sheaths (BS 7835) inside buildings. Design for high ground temperatures and dry sand, and use thermally stable backfill where the utility specifies it. Only use cable makes and sizes the utility has approved (DEWA, AADC/ADDC, SEWA, KAHRAMAA, MEW, SEC and others).
Australia and New Zealand
MV cables are to AS/NZS 1429.1. AS/NZS 3008 covers LV cables, so MV ratings come from IEC 60287 or manufacturers' data. Network operators publish their own standard cable sizes, laying details and bonding rules.
Asia
IEC 60502-2 is the common base, with national versions such as IS 7098-2 in India. Utilities such as TNB and SP Group set their own standard sizes and accessory types.