Cable specifications

1, rated voltage

The rated voltage depends on the phase voltage of the cable system, the type of system, the time to troubleshoot the protection equipment, and so on. In a non-grounded system, the single-phase earth fault of the cable will run for a long time. But will produce an inter-line voltage gradient between the two other ungrounded conductor insulation, requiring the insulation to be thicker. No fault between the two phases can not be applied for a long time between all the line voltage, if the protection equipment can be removed within 1min failure, then in this grounding system can choose 100% of the rated voltage cable. For ungrounded systems, a 133% rated voltage cable is required when the 1 minute clearance time specified for the 100% rated voltage level can not be met but the clearance is guaranteed within 1 h. When the time for clearing the ground fault is quite long, use a 173% rated voltage level for insulation.

2, the choice of wire

Selection of wire specifications should be considered: national electrical specifications, thermal effects of load current, mutual heating effect, loss of electromagnetic induction, dielectric loss, load current related indicators, emergency overload indicators, voltage drop limits and fault current The

3, load current indicators

The current meter lists the minimum specifications for the required wires. However, the choice of cables in the project tends to be conservative, which is taking into account the load growth, voltage drop and short circuit current heating and other factors.

4, emergency overload indicators

The normal load limit for insulated wires and cables is based on practical experience and represents the aging speed of the cable. This aging rate is expected to allow the effective life of the cable for 20 to 30 years. The average failure rate will be doubled and the insulation life of the cable will be reduced by half for every 8 ° C rise in normal daily load. It is an extraordinary practice for the cable to continue operating at temperatures above the maximum rated or rated current. The increase in temperature is proportional to the loss of the wire, and the loss increases with the square of the current, and the larger voltage drop may present an unpredictable danger to the continuity of the equipment and the power supply. The cable shall operate at the maximum emergency overload temperature and shall not exceed 100 h per year, and the overload period of 100 h shall not exceed five times of the cable life. All kinds of insulated cables have a short overload overload factor. The overload factor of the insulated cable can be obtained by multiplying the overload factor by the nominal rated current of the cable.

5, the voltage drop indicators

If the cross section of the supply line is not large enough, an excessive voltage drop will occur in the circuit. The voltage drop is proportional to the line length. Taking into account the normal start and operation of the motor, lighting equipment and other load with great impact current, the specification provides that the steady-state voltage drop of the electric, electric or lighting feeder should not exceed 3%, including the total length of the feeder line and feeder line The voltage drop should not exceed 5%. In the case of a short circuit, the temperature of the wire rises rapidly. However, due to the thermal characteristics of cable insulation, sheathing, coating material, etc., after the short circuit is removed, the cooling process of the wire is slow. Do not pay attention to the thermal stability of the cable will be due to deterioration of the insulation material caused by permanent damage to the cable insulation, which may be associated with flue gas and combustible gases. If there is enough heat, these gases will ignite the fire, causing a serious fire. Even if the degree is not so serious, may also make the cable insulation or sheath expansion, resulting in gaps, resulting in possible failure. For high voltage cables, this is particularly serious. In addition to thermal stress, thermal expansion also produces mechanical stresses in the cable. Due to the abrupt heating, these stresses may cause unwanted cable movement. However, the new cable strengthens the bundles and sheaths, significantly reducing the impact of this stress. When selecting and using cables in a predetermined temperature range, it is not necessary to pay attention to their mechanical characteristics unless the cable is very old or the lead cable. In the event of a short circuit or a large impact current, single-core cable will withstand the mutual repulsion or attraction between the cables. In order to prevent damage to the cable due to this movement, the cable laid on the cable support or cable tray should be secured.