The role of shielded cables in the integrated wiring system

1. Development of integrated wiring technology

At present, the integrated wiring technology has been widely used in my country, and various information technologies and corresponding transmission methods have developed rapidly. Following this, various local area networks applied to intelligent buildings emerge in an endless stream to better meet the needs of various types of voice and data. , image and other communication services in the transmission needs of intelligent buildings. The information transmission rate of the integrated wiring system has been increased from 10Mbit to 100Mbit to more than 1000Mbit, and the cable materials used have also been correspondingly developed from Category 5 and Category 5e twisted pair to Category 6, Category 6e, Category 7 twisted pair and leather. Cables and other types.

2. The role of shielded cables in the integrated wiring system

One of the advantages of the integrated wiring system is that it has strong compatibility. The wiring of different networks such as voice, data and image can be planned and designed in a unified way, and different transmission media, information sockets, cross-linking equipment, adapters, etc. can be integrated into a standard wiring system to realize the information inside the building. The modular management method of the transmission system can significantly improve the flexibility of the network.

Except for the optical fiber cable, cables of different properties are generally laid along the weak current channel. Due to the serious electromagnetic interference (especially co-frequency interference) between cables, the electrical signal will generate an electromagnetic field during the transmission process, within the range of the electromagnetic field. The other cables will generate induction signals, and the strength of the induction signals depends on factors such as the strength of the magnetic field, the radiation source, the distance of the cable being sensed, and the length of the induction cable. The stronger the radiation source signal and the higher the frequency, the stronger the electromagnetic field generated; the closer the cable is to the radiation source, the longer the cable, the stronger the induction signal; when the radiation line is laid in parallel with the induction line, the induction intensity reaches its peak value.

Considering economic investment and other reasons, some networks with lower transmission quality requirements can choose unshielded twisted pair cables. For some networks with high anti-interference performance requirements, shielded twisted pair cables should be used to ensure the shielding performance between various types of cables.

The shielding performance is directly related to the security and stability of the network, ensuring good electromagnetic compatibility (EMC) performance of the integrated wiring system.

First, it can effectively suppress the phenomenon of noise, crosstalk, and data packet loss between cables caused by electromagnetic interference, thereby improving the stability of transmission services.

The second is to prevent the leakage of electromagnetic signals. At present, advanced electromagnetic detection equipment can receive electromagnetic waves radiated from computer networks and communication voice lines from kilometers away, and steal relevant information through technical means such as restoration and reproduction, thereby endangering individuals, enterprises (units) and even countries. information security. A shielded cable with good performance can block electromagnetic radiation generated by high-frequency signals in the transmission cable, block the leakage of electromagnetic signals, and avoid information leakage caused by coupling the carried electromagnetic signals to adjacent cables in the same weak current channel.

Integrated wiring system

3. Application of shielded cable

A cable with a shielding layer between the outer sheath of the integrated wiring and the core wire is called a shielded cable. The shielding layer (single layer or double layer) is mainly made of non-magnetic metal materials such as copper and aluminum, and its thickness is much smaller than its skin depth. At present, the most widely used is the shielded twisted pair (STP, Shielded Twisted Pair), which takes into account the balance of the twisted pair and the shielding of the shielding layer, and adds one or two layers of aluminum foil on the outside of the four pairs of twisted pairs. The principle of reflection, absorption and skin effect of electromagnetic waves by the shielding layer, combined with the transmission performance of the twisted pair to balance and cancel the crosstalk, can effectively prevent external electromagnetic interference from entering the cable and prevent internal signal leakage.

The so-called skin effect is that when there is a current passing through the conductor, a magnetic field will be generated around it, and the magnetic field will generate an induced current in the conductor. High, the smaller the skin depth, that is, the higher the frequency, the weaker the penetration ability of electromagnetic waves.

4. Grounding of shielding layer

7.0.4 of "GB50311-2007 Integrated Cabling System Engineering Design Specification" stipulates that the integrated wiring system should use a grounding system with shared grounding. If the grounding body is set separately, the grounding resistance should not be greater than 4Ω. For example, when there are two different grounding bodies in the grounding system of the wiring system, the grounding potential difference should not be greater than 1Vr.m.s.

The use of shielded cables must ensure good independent grounding, so as to effectively suppress electromagnetic interference, but if the grounding method is improper, it will directly affect the shielding effect and cannot solve the interference problem. When grounding the shielding layer, it must also be noted that the shielding layer should be connected with special connectors (such as metal clamps) to avoid the so-called "pig tail" effect Azimuth overlap, resulting in increased high frequency overlap impedance, increased electromagnetic interference coupling voltage, and difficulty in discharging electrostatic current).

(1) For different transmission signal frequencies, the shielding grounding methods are also different, and the grounding can be classified according to the low and high frequency conditions. In a low-frequency circuit with an operating frequency of less than 1MHz, the inductance between the wiring and the device has less influence, while the circulating current formed by the grounding circuit has a greater impact on the interference, so a single-point grounding should be used; when the signal operating frequency is greater than 10MHz, the impedance of the ground wire changes significantly. If it is too large, it is necessary to reduce the ground wire impedance as much as possible, so multi-point grounding should be used; when the signal operating frequency is 1~10MHz, single-point grounding should be used when the longest trace or grounding lead wire length is less than 1/20 of the wavelength, otherwise the application will be more point to ground.

(2) From the perspective of the circuit reference point, if the shielding layer is grounded in the whole process, the effect is the best, but it is difficult to achieve in reality. Therefore, the grounding of the shielding layer is mainly divided into single-end grounding, two-end grounding and floating ground (insulated from the earth, used in special cases). The shielding layer connection of the integrated wiring must be complete and through. When single-end grounding, the grounding point should be connected to the joint grounding body of the building through the through shielding layer (the shielded wires between the weak currents on each floor should be assembled in parallel as much as possible); when the two ends are grounded, they can be connected to the protective ground of each floor. , such as the protective ground wire row, bridge, information point bottom box in the cabinet, etc., through the protection of the joint grounding body connected to the building on the ground. Whether it is single-ended or both-ended grounding, there are certain contradictions in the coexistence of pros and cons. We should have a clear understanding of this.

"GB50217-1994 Code for Design of Electric Power Engineering Cables" - 3.6.8 The grounding method of the metal shield of the control cable shall meet the following requirements:

(1) The shielding layer of the analog signal loop control cable of the computer monitoring system shall not constitute two or more points of grounding, and a centralized one-point grounding should be used.

(2)

(2) For the shielding layer of the control cable except for the case of item (1) that requires one point grounding, when the interference of electromagnetic induction is large, two points of grounding should be used; the interference of electrostatic induction is large, and one point of grounding can be used. For double shielding or composite overall shielding, one point should be used for the inner and outer shielding, and two points should be grounded.

Single-ended grounding is the most commonly used grounding method. The shielding layer is grounded with equipotential single-ended, and the other end is suspended. There is no ground loop on the shielding layer, and there is no ground potential difference, which effectively avoids the ground loop current. Therefore, it has an anti-electromagnetic interference capability of up to 70dB attenuation, and single-ended grounding has been widely used in integrated wiring weak current systems. The disadvantage of single-ended grounding is that there is no loop current on the shielding layer, and the shielding layer has the ability to resist electric field interference, but cannot prevent the voltage induced by changes in magnetic field strength, so it lacks the ability to suppress magnetic field coupling interference.

Grounding both ends can induce a current in the shielding layer in the opposite direction to the external interference current through the magnetic field generated by the external interference current, and this current plays the role of offsetting and reducing the interference current. When the two ends are grounded, the shielding layer, the ground wire and the ground will form a closed loop. Due to the inconsistency of the ground potentials at both ends, there is a ground potential difference (except for those with a potential difference less than 1Vr.m.s), which will generate an additional in the shielding layer. The ground loop current will cause coupling interference to the twisted pair in the cable, and this current will cause the shielding layer to generate a secondary magnetic field, which will interfere with the twisted pair in the cable, and the stray circulating current in the signal loop will cause current unbalanced. Due to the interference of the electromagnetic field, the ability of the shielding decoupling of the twisted pair is reduced.

For circuits with high input or output impedance, especially in a high static environment, it is necessary to use a double-layer shielded cable, and the two layers of shielding should be insulated and isolated from each other. The outer shield must be grounded at both ends, mainly to reduce the intensity of electromagnetic interference. Due to the existence of a potential difference, a current is induced, so a magnetic flux that reduces the strength of the source magnetic field is generated, thereby basically canceling the voltage induced when there is no shield layer. The inner shielding layer must be grounded at a single point of equipotentiality, because the external strength has been reduced, and the purpose of the inner layer is to discharge as soon as possible and eliminate interference. In reality, unless there are special requirements for confidentiality protection, double-shielded cables are rarely used in the integrated wiring system. Generally, the special bridge and threading steel pipe occupied by the shielded cable are used as the outer shielding layer, and the shielded cable is used. The shielding layer is used as the inner shielding layer.

5. Precautions for the installation of the bridge in the integrated wiring system

"GB50311-2007 Engineering Design Specification for Integrated Wiring System"--7.0.7 When the cables of integrated wiring are laid with metal trunking or steel pipe, the trunking or steel pipe should maintain continuous electrical connection, and there should be no less than two points of good ground.

The metal bridges and conduits in the integrated wiring system are important infrastructures and can also play a certain auxiliary role in electromagnetic shielding. Only the network cables of the same service attribute can be laid on the dedicated shielded tray, and it is strictly forbidden to lay them together with cables of other services in the same shielded tray. Different cables are laid in parallel, and electromagnetic coupling will occur with each other. The longer the parallel direction, the stronger the coupling induction. Therefore, the bridge enters the corridor from the weak current room, and the metal pipe from the corridor bridge to the information socket must be connected with other cables. The distance between them meets the relevant requirements of network construction standards.

Since the wiring bridge is a complete set of structures formed by metal straight grooves, elbows, tees, and four-way connections through connecting plates, if the bridge is made of surface-sprayed or painted products, its interconnection is insulated, only relying on The jumper wire is used for electrical connection; there is an insulating layer between the cover plate of the bridge frame and the tank body, and the shielding and grounding effects obtained cannot be guaranteed. Therefore, the bridge frame and threading steel pipe with galvanized anti-rust treatment on the surface should be selected, which has the advantages of good electrical conductivity and low grounding resistance, which ensures the overall shielding effect of the fully enclosed trunking. The bridge frame and threading steel pipe joints must be connected by wiring to ensure that the whole process of shielding can be achieved. The shielding information point requires that the metal bottom box be embedded (the metal bottom box can be grounded if conditions permit). The special shielded bridge should be directly connected to the joint grounding body of the building, and the grounding resistance should be less than 4Ω. Also, other facilities cannot use the bridge as a ground wire.

It should also be noted that although the bridge has a shielding effect, its shielding index is uncertain. At present, there is no national standard for the shielding performance of the bridge and the trunking. Therefore, in practical applications, it is impossible to rely on the bridge as a cable. shield. As a shielding material, the bridge can only be used as a non-standardized auxiliary means to strengthen the shielding and isolation effect to improve the shielding effect.