|
|
|
Thermal soil properties
Around underground power cables
|
|
|
The ampacity (current rating) of an underground cable circuit is determined for more than 50% by the thermal characteristics of its surrounding area and of the laying depth. For directly buried cables and for cables in ducts, the thermal soil properties are really decisive. It is highly important to determine these properties for both new and service-aged circuits.
|
|
|
Ampacity of underground cables
The ampacity is determined by the depth of installation and by the thermal soil properties, see Figure 1. Figure 2 gives an example of the influence of thermal resistance on ampacity. The so-called g-factor in Km/W represents the thermal property. For sand this factor varies from 0.3 – 4.5 Km/w, for clay it ranges between 0.6 – 4.0 Km/w and for peat between 2.0 – 20 Km/w. This g-factor depends on the size of the grains and on humidity.
|
|
|
Figure 1. Soil properties and installation depth of a cable
circuit
|
|
|
Figure 2. Example of ampacity for different soil characteristics
(g-values) and different cross-sections of cable
conductors
|
|
|
Determination of thermal properties of the soil
A sample of the soil has to be taken at the selected locations. This sample will be analyzed for type of particles, size of grains, humidity and so on. Calculation will result in the g-factor for the relevant sample.
|
|
|
Ampacity
The engineer will use the determined g-factor in the engineering
of the new cable circuit or for the improvement of an existing
circuit.
|
|
|
|
|
|
|
|
