Selection and Application of Control Cable

Although the wire and cable industry is only a supporting industry, it accounts for 1/4 of the output value of China's electrical industry. A wide range of products, a wide range of applications, involving power, construction, communications, manufacturing and other industries, and is closely related to various sectors of the national economy. In real life, people always confuse the power line and the control line, and it is difficult to distinguish which is the power line and which is the control line.

KVV is used for industrial and mining enterprises, energy and transportation sector control and protection of the line of the connection, commonly known as the control cable. Mainly used for cable trench, pipeline, shaft and other mechanical tension larger fixed occasions. In this article, Asia Cable will talk to you about the selection and use of control cables.

Overview of 1. Control Cable

The cables that connect from the control center to various systems to transmit signals or control operating functions are collectively called control cables. The early functions of the control cable are relatively simple, including: indicator light display, instrument indication, relay and switch device operation, alarm interlock system, etc. In recent years, due to the wide application of weak current and computer network, new functions have put forward higher requirements for the selection and application of control cables. In this paper, some new problems in the selection and use of control cables in recent years are discussed for reference.

Main series of 2. control cables

The main products of today's control cables are: PVC insulated control cables, natural styrene butadiene rubber insulated control cables and polyethylene insulated control cables. In addition, there are cross-linked polyethylene insulation and ethylene propylene rubber insulation products. The oil-impregnated paper-insulated lead-sheathed control cable produced in the early years has been phased out.

The rated voltage of the control cable is indicated by U0/U. China's national standard promulgated in 1998 stipulates that the rated voltage of plastic insulated control cables is 450/750V, and foreign countries have proposed 600/1000V products as the conventional product series of control cables. At present, China can also produce 600/1000V plastic insulated control cables. The rated voltage of rubber insulated control cables is specified as 300/500V.

The control cable core is copper core, with nominal section of 2.5mm and below, 2~61 cores; 4~6mm,2~14 cores; 10mm,2~10 cores. Control cable working temperature: rubber insulation 65 ℃, PVC insulation 70 ℃ and 105 ℃. Control cables used in computer systems are generally insulated from PVC, polyethylene, cross-linked polyethylene and fluoroplastics.

Measures 3. ensure the normal operation of the control cable and prevent interference

In order to ensure that the control cable can reduce the scope of influence in the event of insulation breakdown, mechanical damage or fire, the national standard GB50217-91 "Code for Design of Electric Power Engineering Cables" stipulates: current, voltage, DC power supply and trip control circuit. Separate control cables shall be used for both systems requiring enhanced reliability.

After the control cable is put into operation, there is electrical interference between different cores of the same cable and between cables laid in parallel. The main causes of electrical interference are:

(1) Electrostatic interference caused by capacitive coupling between cores due to applied voltage;

(2) electromagnetic induction interference caused by the current. Generally speaking, when there is a high-voltage and high-current interference source nearby, the electrical interference is more serious. Since the core wire spacing of the same cable is small, the degree of interference is much greater than that of adjacent parallel cables.

For example, in the control circuit of a split-phase operation circuit breaker in an ultra-high voltage substation, the three phases share a single cable. There was an accident like this. The pulse of split phase operation causes the thyristors of other phases to be triggered, resulting in three-phase false linkage. Using a separate cable, no more misoperation accidents. Another example is a computer monitoring system for a power plant. Because the analog low-level signal line and the power line of the transmitter are combined with a four-core cable, an interference voltage of 70V has been generated on the signal line, measured in millivolts. Low level signal loop will obviously affect the normal operation.

4. measures to prevent or mitigate electrical interference are mainly in the following three areas

1. One spare core of the control cable is grounded

Practice has proved that when a spare core in the control cable is grounded, the amplitude of the interference voltage can be reduced to 25% ~ 50%, and the implementation is simple, while the cost of the cable increases little.

2. A control cable is not suitable for circuits that will have serious consequences when electrical interference occurs, including:(1) weak current signal control circuit and strong signal control circuit;

(2) The loop between the low-level signal and the high-level signal;

(3) Each phase weak current control circuit of AC circuit breaker shall not use the same control cable. However, for each pair of round-trip conductors in a weak current circuit, if they belong to different control cables, a circular arrangement may be formed during the laying process, and a potential will be induced under the cross-linking of the magnet wires. Similar to the power supply, its value may affect the weak current circuit. Low-level parameter interference has a large impact, so it is recommended to use the control cable as the round-trip conductor.

3. Metal shielding and shielding layer grounding

Metal shielding is an important measure to reduce and prevent electrical interference, including the overall shielding of the iron core, sub-shielding and double-layer overall shielding. The metal shielding type of control cable shall be selected according to the strength of possible electrical interference, and comprehensive anti-interference measures shall be taken to meet the requirements of reducing interference and overvoltage. The higher the requirements for the anti-interference effect, the greater the corresponding investment. When using steel tape armor and steel wire braid overall shield, the cable price will increase by about 10% to 20%.