Water blocking options for Medium Voltage cables.

• Radial Ingress

In the case of radial ingress moisture 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, 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

Fundamentally there are two MV cable types used in the Australian Market being three core and single core constructions manufactured in accordance with AS/NZS 1429.1, the cross sectional drawings below show their respective constructional makeup.

Observations

It can be seen from these cross sectional drawings that both cable constructions have areas internally where water may progress through the cable. These areas are;

For three core cables;

1. The gaps between the screen wires over each core
2. The central void
3. The filled interstices (or outer voids, filled with non- hygroscopic polypropylene fillers)
4. The compacted conductor

For single core cables;

1. The gaps between the screen wires over the core
2. The compacted conductor

Longitudinal water blocking

Three core cables are more difficult to longitudinally water block than single core cables. This is because the large interstitial areas can not be effectively blocked using fibrous filling materials. (There are constructions utilizing extruded filling compounds but these have proven cost prohibitive and are not in general manufacture / 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. The gaps between the screen wires can be effectively blocked using water swellable tapes (WST). The standard format for such being semi-conductive water swellable tape (SCWST) applied over the insulation screen and a non-conductive water swellable tape (NCWST) applied over the screen wires. Again the inclusion of water blocked conductors may be considered.

Water swellable tapes are highly absorbent and when activated on exposure to water, swell to effectively block its ingress along the cable peripherals.

Radial water blocking

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

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

Metallic Non ferro magnetic sheaths and barrier layers

The traditional non ferro magnetic sheath used in cables is lead, having 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 when in direct contact with alkaline soils corrosion can occur.

Over the last two decades alternate designs have been developed using extruded aluminium and copper sheaths and longitudinally applied foil tapes. These designs have advantage over lead in that they do not suffer the same OH&S 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 water swellable tapes 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 exposure to a 1000mm 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 1000mm.

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 is 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.