XLPE Insulation and Sheath Compounds: Where Each Is Used

XLPE as insulation

Cross-linked polyethylene is the insulation material across the manufacturer's entire XLPE power cable range, at both low and medium voltage. Its advantage over thermoplastic polyethylene is that cross-linking turns the material into a thermoset: it keeps its shape and mechanical strength at temperatures that would soften an uncross-linked compound.

The manufacturer rates the insulation at 90 °C continuous conductor temperature, with 130 °C permitted under emergency overload and 250 °C for up to five seconds under short circuit. Those limits belong to the insulation system as a whole, not to the sheath.

Choosing the sheath compound

The sheath is a mechanical and environmental layer, and it is where the fire-performance choice is actually made. Three compounds appear across the range:

• PVC type ST to IEC 60502 — the standard sheath, supplied black. General indoor and outdoor use.
• FR-PVC — the flame-retardant grade used in the ZR- variants, where flame propagation is a specified requirement.
• LSOH — low-smoke zero-halogen, used in the WDZAN-YJY construction, where smoke density and acid gas are the governing concern.

Because the conductor and insulation are unchanged between them, moving from PVC to FR-PVC or LSOH alters the fire behaviour of the cable without altering its current-carrying characteristics. That is often the least disruptive way to meet a fire specification on an existing design.

Where halogen-free is actually required

The reason to specify LSOH is the environment the cable runs in, not a preference for a newer material. In a fire, PVC compounds release hydrogen chloride, which is both toxic and corrosive to equipment. In a confined or occupied space — a metro tunnel, a data centre, a hospital, a high-rise shaft — the smoke and acid gas are frequently a greater hazard than the fire itself.

The manufacturer's LSOH build is CU/MICA/XLPE/LSOH: standard Class 2 copper conductor, mica tape for circuit integrity, XLPE insulation and an LSOH sheath. Where the specification is driven by evacuation time or by protecting sensitive equipment from corrosion, this is the construction to name. Where the cable is buried in open ground with no occupancy risk, the case for it is much weaker and the standard PVC sheath remains appropriate.

Material claims and what verifies them

Material choices are only meaningful if they are tested. Flame propagation is measured by the IEC 60332 series, halogen acid gas evolution by IEC 60754 and smoke density by IEC 61034 — three separate standards measuring three separate properties.

A cable described as flame-retardant, or as halogen-free, should be supported by the corresponding test evidence for the actual construction offered. Where a project requires certified fire performance, ask for the test report against the specific type rather than accepting a material name as a substitute.