The wire and cable industry accounts for 1/4 of the output value of China's electrical industry. It has a wide variety of products and a wide range of applications, involving power, construction, communications, manufacturing and other industries, and is closely related to all sectors of the national economy. In real life, people always confuse power cables and control cables, and it is difficult to distinguish which is the power cable and which is the control cable.
1. Overview of control cables
Cables that connect from the control center to each system to transmit signals or control operational functions are collectively referred to as control cables. The early functions of the control cable are relatively simple, including: indicator light display, instrument indication, operation of relays and switchgear, alarm interlocking system, etc. In recent years, due to the wide application of weak current and computer networks, new functions and higher requirements have been put forward for the selection and application of control cables.
Second, the main series of varieties of 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-clad control cables produced in the early years have been eliminated.
The rated voltage of the control cable is represented by U0/U. my country's national standard issued 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, my country can also produce 600/1000V plastic insulated control cables. The rated voltage of the rubber insulated control cable is specified as 300/500V.
The wire core of the control cable is copper core, the nominal cross section is 2.5mm² and below, 2~61 cores; 4~6mm², 2~14 cores; 10mm², 2~10 cores. The working temperature of the control cable: rubber insulation is 65°C, and PVC insulation is 70°C and 105°C. The control cables used in computer systems generally use PVC, polyethylene, cross-linked polyethylene and fluoroplastic insulation products.
3. Measures to ensure the normal operation of the control cable
In order to ensure that the control cable can reduce the scope of influence in the event of insulation breakdown, mechanical damage or fire, GB 50217-2018 "Code for Design of Electric Power Engineering Cables" stipulates that the current, voltage, and DC power supply and trip control loop of dual protection are required. Two sets of systems to enhance reliability should use independent control cables.
After the control cable is put into operation, there are electrical interference problems between different cores of the same cable and between cables laid in parallel. The main reasons for the electrical interference are:
(1) Electrostatic interference caused by the capacitive coupling between the cores due to the applied voltage;
(2) Electromagnetic induction interference due to energized current. In general, when there is a high-voltage and high-current interference source nearby, the electrical interference is more serious. Since the distance between the cores of the same cable is small, the degree of interference is much greater than that of the adjacent cables laid in parallel.
4. Measures to prevent or mitigate electrical interference
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% to 50%, and the implementation is simple and the cost of the cable increases very little.
2. For circuits with serious consequences in the event of electrical interference, one control cable should not be used, including:
(1) Weak electric signal control loop and strong electric signal control loop;
(2) The loop between the low-level signal and the high-level signal;
(3) The same control cable should not be used for each phase weak current control circuit operated by the AC circuit breaker in separate phases. However, for each pair of round-trip wires in the weak current circuit, if they belong to different control cables, a ring arrangement may be formed during laying, and an electric potential will be induced under the cross-link of the electromagnetic wires of the similar power supply, and its value may affect the weak current circuit. Low-level parameter interference has a greater impact, so it is advisable to use a control cable for the round-trip wire.
3. Metal shield and shield grounding
Metal shielding is an important measure to reduce and prevent electrical interference, including overall shielding, sub-shielding and double-layer overall shielding of the core. The selection of the metal shielding type of the control cable should be included in the comprehensive interference suppression measures according to the strength of the possible electrical interference, so as to meet the requirements of reducing interference and overvoltage. The higher the requirements for the anti-interference effect, the greater the corresponding investment. When steel tape armored and steel wire braided overall shielding are used, the price of the cable will increase by about 10% to 20%.
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