High voltage materials when buried underground generate considerable amounts of heat, which must be dissipated by using a quality thermal backfill. The TR results obtained are less than 0.90 which has taken several months of complex evaluation and development.
Not taking this into account when burying electrical cable can result in intense overheating and inevitably, power failures as experienced in the 1999 electricity crisis in Auckland. It appeared that many electricity cables were operating with a smaller safety margin than had been assumed by their operators. One of the problems identified was the relatively low conductivity of the surrounding trench fill, compared to the assumed design value. This was exacerbated by factors such as variable backfill material and inconsistent compaction of the fill material in the trench.
Buried electrical cable requires thermal backfill to surround it in the cable trench in order to dissipate the heat from high voltage underground cabling. The design of the cabling systems assumes the thermal properties of the backfill material and the surrounding ground. Thermal backfills do not require compaction as they flow in a manner that is similar to concrete.
Fluidised Thermal Backfill
In recent years fluidised thermal backfills (FTB) have become more common. They have been developed to meet thermal resistivity, thermal stability strength and flow criteria. For hard-to-access areas, such as congested areas with numerous underground services, narrow trenches and small diameter tunnels, FTB is ideal.
The use of FTB overcomes the problems of relatively low conductivity of surrounding trench fill, as well as factors such as variable backfill material and inconsistent compaction of the fill material in the trench. FTB provides an ideal way of providing a high-quality cable backfill. FTB possesses excellent heat dissipation properties and a consistent quality. The higher costs of FTB are offset by lower installation costs.
Benefits of fluidised thermal backfill:
- TR value lower than 0.90
- Does not require compaction, therefore there is no risk of damaging the cable jacket.
- Flows in a similar manner to concrete
- Provides stable substrate for follow-on road repairs.
- Can alleviate thermal instability issues.
- Is generally supplied from concrete trucks.
- Can be used on flat or hilly ground.
- Can be pumped or poured.
- Rarely requires special bulk heading or shoring.
- Flows readily to fill all spaces, without vibration.
- Solidifies to uniform density by consolidation. Excess water seeps to the top.
- Hardens quickly. Ground surface can be put to use the following day.
- Can be broken up with a back hoe due to low strength (100 to 250 psi [0.7 – 1.8 MPa])