Water blocking in cables - the what, why and how

The primary objective of water blocking in cables is to prevent the entry and migration of moisture or water throughout the cable.

When moisture is introduced into areas of installation it can cause premature failure of the cable, accessories or electrical equipment. There are two ways moisture or water may ingress a medium voltage (MV) cable:

Radial ingress - In the case of radial ingress, moisture or water enters the cable either by permeation through the protective layers (sheaths) or through any breach of these sheaths (mechanical damage). Once water has entered the cable it then travels longitudinally through it.

Longitudinal ingress - In the case of longitudinal ingress, moisture or water enters the cable through ineffective end capping or poorly made joints or terminations (especially if jointing pits etc are flooded).

Anatomy of Australian MV cables

There are two types of MV cables used in the Australian market, specifically three-core and single-core constructions, that are manufactured in accordance with AS/NZS 1429.1.

The cross sectional drawings below show the respective constructional make-up of both types.

Typical constructions used by the Australian utility market (not to scale)

Three core.

                     
                                                                                               

Single core                        

Observations

It can be seen from these cross-sectional drawings that both the three- and single-core cable constructions have areas internally where water may progress through the length of the cable.

These areas are:

For three-core cables: The gaps between the screen wires over each core; the central void; the filled interstices (or outer voids, filled with non-hygroscopic polypropylene fillers); and the compacted conductor.

For single-core cables: The gaps between the screen wires over the core; and the compacted conductor.

Longitudinal water blocking

Three-core cables are more difficult to longitudinally water block than single-core cables. This is due to the fact that the large interstitial areas in the cable cannot be effectively blocked using fibrous filling materials. There are constructions utilising extruded filling compounds but these have proven cost prohibitive and are not in general manufacture or use in the Australian market.

Generally for three-core cable constructions, the accepted level of longitudinal water blocking in the Australian market is semi-conductive water swellable tapes (SCWST) under the core screen wires. Additionally, the inclusion of water-blocked conductors may be considered. Refer to AS/NZS 1429.1 clause 2.14 Water-blocking (optional) for further guidance on this matter.

Single-core cables lend themselves to more efficient longitudinal water blocking than their three-core counterparts. The gaps between the screen wires in the single-core cables can be effectively blocked using water swellable tapes (WSTs). WSTs effectively prevent water ingress into cables. The standard format for this process being that SCWST is first applied over the insulation screen and, following this, a non-conductive water swellable tape (NCWST) then applied over the screen wires. Again, the inclusion of water-blocked conductors may be considered as a countermeasure.

Radial water blocking

In the case of radial water blocking, there are many preventive methods available to cable manufacturers. These methods can be utilised for both single- and three-core cable constructions, which include:

• Extruded non-ferromagnetic sheaths - lead, stainless steel, aluminium or copper being the most commonly used.
• Hermetically sealed tapes applied longitudinally under the sheath layer(s).
• Low-permeability polymeric sheaths such as HDPE.

Metallic non-ferromagnetic sheaths and barrier layers

The traditional non-ferromagnetic sheath found in cables, lead, has been in use for over 100 years in cable production. Lead was the favoured sheath for paper-insulated cables because it is an impervious barrier to both water and hydrocarbons, although corrosion can occur when in direct contact with alkaline soil.

Over the last two decades, alternative designs for cables have been developed using extruded aluminium, copper sheaths and longitudinally applied foil tapes. These alternative designs have the advantage over lead in that they do not suffer the same OHS and environmental pollution concerns as lead.

Low-permeability polymeric sheath compounds

Cables sheathed with the correct grade of HDPE demonstrate a high resilience to radial water penetration. The HDPE compounds used in modern cable construction are pipe grade with excellent mechanical properties. Three-core cables sheathed with HDPE (with and without WSTs in the individual screens) are used for direct buried application by many Australian power utilities.

Test for effectiveness of water blocking

AS/NZS 1429.1:2006 Appendix C defines the water penetration test required to be met for all cables claiming compliance to that standard. In the test, a prepared cable sample is subjected to the exposure of a 1000 mm head of water for 24 hours, after which time 10 heating cycles (to a maximum of 100°C) shall be applied over an 8-hour period with the water head maintained at 1000 mm.

The cable is deemed effectively water blocked if there is no leakage at the cable ends at the conclusion of the test.

Summary

Although modern cables are supported well by compound and technological development, paramount to the effective exclusion of water and moisture in any cable are the pre- and post-installation techniques applied by the installer. Ineffective protection of cable ends in exposed situations should be avoided. Every effort should be made to ensure cable pits and conduits are not flooded and that at time of pulling, effective mastic-filled end caps should be used to avoid force flooding the cable with water. Cable ends should be secured above any high water level in the installation pending final works.