ArticlesTechnically Speaking

Water treeing in Middle Voltage cables

The excellent electrical performance of XLPE is the reason it is now used almost exclusively for insulating low voltage, distribution and transmission cables. While XLPE has far better resistance to moisture than paper insulation or PVC, moisture can affect long term performance of XLPE under the influence of high electrical stress. All low voltage and distribution cables can be installed without precautions, but some moisture protection is advisable for medium voltage cables and cables for sub-transmission. The use of metal sheaths for transmission cables is considered essential.

There were serious cable failures when XLPE was first introduced in the late 1960s in the USA. This was due to incompatible semi-conductive materials and lack of triple extrusion, but also water ingress and treeing in steam cured XLPE. The cable industry has since made significant developments in XLPE materials, cable design and processing, so that 500kV XLPE cables are now in service and XLPE is the most practical insulation material for the range of cables used by the electricity supply industry worldwide. However, while this material is highly moisture resistant, moisture can still cause electrical failure under the influence of high electrical stress. Moisture mode of failure The first XLPE cables were produced on steam cure CV (Continuous Vulcanisation) lines where moisture saturated the insulation due to the temperature and pressure of the steam (~200°C, 200 psi). Many of these early cables are now showing signs of water trees.

For more than 10 years, all XLPE insulated cables produced in Australia have been processed in a ‘dry cure’ manner that ensures water absorption is less than 200 ppm. Very few of these cables have significant water treeing. While water trees may not cause breakdown, in cases where there is a high stress point or a space charge developed during DC testing, such trees may develop into electrical trees which subsequently cause premature breakdown.

Protection/prevention modes :

  • Radial protection – using a metal sheath on the cable to provide an impervious barrier to entry.
  • Longitudinal protection – using solid fillers to fill all voids where water may enter or use of water swellable tapes, which fill the spaces only when the water is present and thereby prevent the longitudinal progress of any further water.
  • Moisture resistant insulation material – such as Tree Retardant XLPE – TR-XLPE.


Low voltage cables Cables designed to function at voltage levels up to 1,000V have insulation thicknesses well above those required for purely electrical requirements. The voltage stresses in the insulation are therefore small and inadequate to promote the growth of water trees that lead to electrical failure of the cable. Thus, no protection is required from the effects of water or moisture.

Various forms of distribution cable protection and their respective cable life expectancies are outlined below:

  • Cable without protection and affected by water/moisture >15-25 years
  • Use of water swellable tapes >20-30 years
  • Use of insulation materials i.e. TR-XLPE >30-35 years
  • TR-XLPE plus water swellable tapes >35-40 years
  • Metal moisture barrier (i.e. lead sheath) 60 years


A metal moisture barrier such as an extruded lead sheath or a welded corrosion resistant sheath is the most secure design option for transmission cable and will ensure a life expectancy of 50-60 years.

Other factors reducing cable life:

  • Life expectancy is reduced when insulation is subjected to “over voltage” in the form of surges and impulses.
  • Changes in the environment, for example, depth of cover, adjacent services and microbiological effects in the soil, can increase operating temperatures and reduce cable lifetime (cables are designed for a maximum operating temperature of 90°C with limited overload periods as defined in the relevant standard).
  • Poor supervision/management and adverse installation conditions may cause cable damage.





From the article: Designs and reliability of underground cables and systems By Hilary Marazzato and Ken Barber, Olex Cables, Abstract of the second article in a three-part series on underground cables published in Australian Power Transmission & Distribution magazine