Cable conditions of use
2019-06-24
When a large number of cables are laid in groups, the current carrying capacity of the cables is reduced due to the mutual heating effect. Large specifications of the cable sometimes need to consider the use of two or more smaller specifications of the parallel cable to replace, because the large cross-section cable due to skin effect and proximity effect makes the unit section of the flow reduction. On the other hand, the ratio of the surface area of the large cross-section cable to the cross-sectional area is reduced so that the heat dissipation capacity of the large cable is poor. If multiple cables are used in parallel, the relative position of each cable should be taken into account to reduce the uneven distribution of cable carrying capacity.
For cables laid in underground pipes, when using the load factor, the heat capacity of the average heat loss of the pipe and its surrounding soil should be taken into account. The temperature of the subsurface changes with the change of the average heat loss, which allows the higher short-term load factor to be the ratio of the average load to the peak load, usually measured on the basis of the daily average load. And the peak load is generally referred to within 24h, 0.5 ~ 1h during the maximum load of the average. For buried cables, the average surface temperature can be limited to 0 ~ 60 ℃ according to soil conditions, to prevent the loss of soil moisture and cable thermal breakdown. When the cable is close to other load cables or heat sources, or when the ambient temperature exceeds the ambient temperature of the specified cable carrying capacity, the rated load capacity of the cable must be reduced. The normal ambient temperature of the cable unit is the temperature at which the cable is installed without the load. In order to properly determine the cable specifications required for a given load, this temperature should be thoroughly understood. For example, in the air with other cables separated from the laying of the cable, the ambient temperature is the cable with the load before the temperature. For cables in the air, it is also assumed that there is enough space around the cable to distribute the heat generated by the cable and does not raise the temperature of the entire room. If the above correct conditions are specified, the following environmental conditions can be used to calculate the current carrying capacity of the cable.
City high and medium voltage distribution lines should use the following cable lines. Urban areas and areas where there are special requirements for urban planning and city environment; technically difficult to deal with serious corrosive areas; key scenic tourist area; vulnerable to salt or tropical storm invasion of the main coastal areas The city's important power supply section; the other for the power grid structure and operational safety needs of the lot. Urban low-voltage distribution lines should be used in the following circumstances should be used cable area load density of urban centers; large construction area of the new residential and high-rise residential district; according to the plan should not be through the overhead lines of the streets or regions and crowded areas; Through the technical and economic comparison of electricity county group level line more appropriate in other cases. For the use of cable lines and underground conditions do not have, you can use insulated cable overhead laying.
Indoor low-voltage cable, the national electrical regulations on the current meter is based on the ambient temperature of 30 ℃. However, most of the summer months of the region, at least some parts of the building 40 ℃ is appropriate. In determining the cable carrying capacity must be considered near the cable the most unfavorable heat source. The local overheating of the cable may be caused by a steam pipe or a heat source near the cable, or it may be caused by a cable passing through a boiler room or other high temperature. In order to avoid such problems, may need to change the line.
Outdoors For cables installed at the shade, the maximum ambient temperature is generally 40 ° C, and for cables installed in the sun, the maximum ambient temperature is generally 50 ° C. When using these ambient temperatures, it is assumed that the maximum load occurs just at the specified ambient temperature. In the hottest day of the day, or when the sun is sunny, some loops are not running at full capacity. Under such conditions, the use of ambient temperature of 40 ℃ for outdoor cables from the security side, is more reasonable.
Underground conditions in a different area of the country, the ambient temperature for the underground cable is subject to change. China's northern region, the ambient temperature often take 20 ℃, the central region, is commonly used 25 ℃; and the southernmost and southwest, the ambient temperature may take 30 ℃. The geographical boundaries of these ambient temperatures are not precisely delineated. The maximum ambient temperature can be measured at a point away from the heat source at the depth of the buried cable. Changes in soil ambient temperature will lag behind air temperature for several weeks.
The thermal performance of the media around the cable is an important parameter when determining the carrying capacity of the cable. The type of soil that embeds cable or cable tubing has a significant effect on the cable carrying capacity. Porous loose soils, such as gravel and ash backfill, usually have a higher temperature and lower current carrying capacity than sand or clay. Therefore, before calculating the cable specifications, you should know the type of soil and soil thermal resistivity. The water content of the soil also has an important effect on the current carrying capacity of the cable. In dry areas, in order to compensate for the increase in thermal resistance due to lack of moisture, it is necessary to reduce the rated carrying capacity of the cable, or take other precautions. On the other hand, in areas where frequent damp underground or tidal waters are affected, the cable can pass through a larger current than normal current. For places where frequent dampness or dampness and dryness occur, there is a need for shielding even for high-voltage lines where there is a transition from dry cables to "natural shielded" wet cables. Because in these places will produce voltage gradient stress mutation, unless specifically designed for this non-shielded cable.
Direct burial in the permafrost region, should be buried below the permafrost layer, when not buried in the soil can be a good dry soil in the dry soil or backfill buried, but also take other measures. Directly laying the cable, is strictly prohibited in the underground pipe above or below. Separated by a separator to 0.25m; with a cable piercing can be 0.1m; special circumstances can be reduced. When buried in non-permafrost areas, the cable embedding depth shall not be less than 0.3 m for the cable sheath to the base of the underground structure. Directly laying cables and railways, roads or streets crossed, should wear protective tube, and the protection beyond the roadbed, street pavement and drains side of 0.5m or more. Directly laying the cable into the structure, through the wall should be set at the protection of the tube, and the implementation of water blocking block. Directly laying the cable connector configuration, connector and adjacent cable clearance, not less than 0.25m. The position of the parallel cable connector should be staggered and not less than 0.5m. Slope top of the joints placed, should be horizontal. For the important circuit of the cable connector, it should be about 100mm on both sides of the beginning of the local section, according to reserve the way to lay the cable. Directly laying the cable in the special replacement of backfill, the backfill of the soil to deal with the outer jacket of the cable without corrosion.
