The rated values of the cable are mainly temperature, voltage and current. The rating of the cable determines the parameters in which the cable can be used safely.
The temperature rating is divided into the maximum temperature rating of the conductor and the minimum installation temperature.
The maximum temperature rating of power cable conductor is -20 ℃ -90 ℃
The long-term allowable working temperature of the overhead conductor is + 70 ℃.
The minimum laying temperature of the cable is not lower than 0 ℃. It must be pre-heated when laying below 0 ℃.
Voltage is usually defined by the following: Uo = RMS value between any insulated conductor and earth or metal coating U = RMS value between any phase conductor and another phase conductor or single insulated conductor system (Um) = maximum system voltage
Among them, low-voltage cables are 0.6k-1kV, medium-voltage cables 3-35kV, and high-voltage cables 35-110kv, and ultra-high voltage cables 110-175kV.
The rated current is usually defined in the following terms:-normal maximum continuous current rating-short circuit current rating
Cable ampacity refers to the amount of current that a cable line passes when transmitting electrical energy. Under thermally stable conditions, the cable ampacity when the cable conductor reaches the long-term allowable working temperature is called the long-term allowable ampacity of the cable.
The current-carrying capacity of the cable is related to the cross-section of the wire, as well as the material, type, laying method, and ambient temperature of the wire. There are many factors that affect the calculation and the calculation is more complicated.
Internal factors affecting cable current carrying capacity
The properties of the wire itself are internal factors that affect the current carrying capacity of the cable. Increasing the core area, using highly conductive materials, using insulating materials with good heat resistance and thermal conductivity, and reducing contact resistance can increase the current carrying capacity of the cable.
- Increase the core area to increase the current carrying capacity of the cable
The core area (conductor cross-sectional area) is positively correlated with the current-carrying capacity. Generally, the safe current carrying capacity is 5 ~ 8A / mm2 for copper wires and 3 ~ 5A / mm2 for aluminum wires.
- Use of highly conductive materials to increase the current carrying capacity of the cable
If the copper wire is used instead of the aluminum wire, the current-carrying capacity can be increased by 30% under the same specifications. In some high-demand occasions, a silver wire is even used.
- Insulation material with high-temperature resistance and thermal conductivity is used to increase the current-carrying capacity of the cable