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What are the precautions for shielded wiring systems?

2022-08-22
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How much do you know about shielded cabling systems?

1. What are the precautions for shielded wiring systems?

1. Difference in transmission performance between F/UTP cable and S/FTP cable

From the electromagnetic compatibility point of view, the S/FTP cable can completely eliminate the alien crosstalk ( AXT). In addition, from the point of view of channel transmission capacity, in the case of the same bandwidth, S/FTP cable can provide higher signal-to-noise ratio (SNR), which can provide higher channel transmission capacity, so S/FTP cable is a high-speed network The more ideal solution in the application.

 

2. Anti-interference ability of shielded cable

From the analysis of the coupling attenuation index of the wiring system, the anti-electromagnetic interference ability of the twisted-pair cable of the twisted-pair cable and various cable shielding methods is increased by about 20-30dB. Among them: the aluminum foil shielded twisted pair cable (F/UTP) is 85dB, the wire mesh/aluminum foil double shielded twisted pair cable (SF/UTP) is 90dB, and the wire mesh total shielded/aluminum foil pair shielded twisted pair cable (S/FTP) is 95dB.

 

3. Bit error rate information of shielded cables

According to relevant information, the bit error rate test of aluminum foil shielded twisted pair cable (F/UTP) has been done abroad. If the shielding layer is not grounded or not well grounded, under certain conditions, the probability of bit error rate in the information transmitted by the network will be increased. The result will be about 30%.

 

4. When the wiring system adopts the shielded Category 6A system, there is no need to test the alien crosstalk of the line.

The shielding system uses a metal shielding layer to completely eliminate alien crosstalk (AXT), that is, to provide a sufficiently high alien crosstalk (AXT) margin. Crosstalk (AXT), only the electrical performance parameters within the test channel are required.

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Due to the complex testing process, long testing time and expensive instrumentation, it is recommended to test only when troubleshooting or when it is very necessary.

 

5. The manufacturing process of the unshielded twisted pair cable can achieve balanced transmission, but it is not enough to suppress external interference

When the pair transmission bandwidth of the twisted pair cable exceeds 30MHZ, the unshielded twisted pair cable is susceptible to external electromagnetic interference and information leakage. The above phenomenon will be more serious if the construction operation of the cable is not standardized during the installation process, and the physical structure of the cable pair is changed, such as being stretched or flattened.

 

6. Information leakage still needs to be considered in commercial buildings

Integrated wiring systems are used for high-speed transmission. Due to hardware reasons such as the balance tolerance of the twisted pair cable, it may also cause the transmission signal to radiate into space, and there is a possibility that information may be leaked unconsciously. Within the same building, there may well be different units or departments.

 

Therefore, the design should be based on user requirements, in addition to considering electromagnetic interference, the requirements for preventing electromagnetic radiation should also be considered. These are two sides of the same problem, and with shielding, both can be resolved.

 

7. The grounding of the shielded wiring system only needs to be grounded at the end of the patch panel

Equipotential bonding should be made to the wiring box or wiring cabinet in the telecommunications room, and the shielded information socket itself in the work area should not be grounded. However, in order to achieve grounding at both ends of the shield, the shielding layer of the shielding information socket and the shielding jumper wire communicates with the shielding layer of the network equipment in the working area. When a device is plugged into a power outlet, it is naturally grounded through the protective conductor (PE).

 

8. If the shielded wiring system is not grounded, the anti-interference ability is still better than that of the unshielded wiring system

According to the test data of the third-party test laboratory, the shielding system is 100-1000 times more resistant to external coupling noise than the unshielded system under normal and good grounding conditions. The system resistance can still be more than 10 times that of the unshielded cabling system.

 

9. When the wiring system has been grounded, it is still necessary to meet the equipotential bonding requirements

There is only grounding, and it is unsafe to have no equipotential bonding. The grounding system must have equipotential bonding. Equipotential bonding is to reduce the voltage difference between different grounding systems. Especially for electronic information networks, it can improve electromagnetic compatibility performance. Equipotential bonding is suitable for all types of wiring systems.

 

10. "Grounding" in shielded wiring engineering does not specifically refer to releasing interference signals to the "earth"

"Grounding" in shielded wiring engineering, if the purpose is to improve electromagnetic compatibility, should be understood as incorporating the shielding layer into the equipotential bonding system; if it involves lightning protection, it should be understood as connecting to the earth itself.

 

Just imagine that the internal wiring systems of airplanes, rockets, satellites, space stations, etc. in flight cannot be connected to the earth, but they all have good electromagnetic compatibility, because they have reliable equipotential bonding inside.

 

11. If the channel test of the shielded wiring system passes the connectivity check, it does not mean that the shielding layer is not faulty

 

2. How to detect the shielding layer and grounding of the wiring system

"Code for Engineering Design of Integrated Cabling System (GB 50311-2007)" (3.5) stipulates the use conditions of shielded wiring, and emphasizes that "the continuity of the shielding layer should be maintained (3.5.4)." Correspondingly, in the engineering acceptance specification (GB 50312-2007), "connection of cable shielding layer" is a must-test item for each cable.

 

The problem is: because the shielding layer of the data cable is a braided mesh (or braided mesh + aluminum foil) structure, simply testing the connectivity can only detect the complete breakage of the conductor of the shielding layer, but cannot detect partial defects, partial open circuits, virtual connections, and severe deformation. and other defects.

 

1. Multimeter and Clamp Earth Resistance Tester

1.1 Multimeter

Conventional (analog or digital) multimeter resistance gear can measure the DC resistance of the shielding layer of the wiring system. After confirming that the shielding layer of the cable on the side of the patch panel has been reliably terminated, use the test pen to contact the shielding cover of the two RJ45 sockets in the work area, and the meter reading is the sum of the DC resistances of the shielding layers of the two cables. However, if the distance between the two information sockets exceeds the length of the multimeter test line, the test line needs to be extended, which is inconvenient to operate; when the resistance is too large, the specific location of the high resistance point cannot be confirmed.

 

1.2 Clamp Earth Resistance Tester

Although the meter is called "earth resistance meter", it can measure the impedance of any conductor that forms a closed loop (using an AC signal to test, "conductor impedance" is more accurate than "conductor resistance".), its The test method is also called "voltage injection, current detection method".

 

2. Time domain reflection

2.1 Working principle

The integrated wiring certification tester has a "time domain reflection (TDR)" function, which can assist in locating the location of the fault point in the line. A single electrical pulse signal transmitted in a pair of conductors will produce echo reflections when it encounters impedance fluctuation points.

 

By calculating the time when the echo signal appears, you can locate the position of the open circuit, short circuit or abnormal impedance point.

 

2.2 Test the transmission impedance of the shielded cable

The time domain reflection test must be carried out in a pair of conductors with impedance relationship, so all or one of the cores of the shielded twisted pair must be regarded as one conductor, and the shielding layer must be tested as the other conductor. The actual test is Transmission impedance of shielded cables.

 

3. Estimate shielding isolation

Shield isolation (or coupling attenuation) is a more accurate indicator of shield performance, but the problem is that such tests can only be performed under laboratory conditions. There is no instrument suitable for use at the engineering installation site, which does not mean that there is no way to "estimate" the shielding isolation degree at the construction site.

 

Relevant standards have added the "alien crosstalk" test in Category 6A wiring. Although from the perspective of wiring certification testing, the shielded wiring system does not need to test for "alien crosstalk", but if this test method is used for shielded wiring, although it is not strictly The test results actually reflect the "shielding isolation" index, so it can be used to estimate the performance of the shielding layer.

 

4. Joint test of shielding and grounding impedance

4.1 Grounding method of shielded wiring system

To give full play to the anti-interference advantages of shielded wiring, in addition to ensuring the continuity of the shielding layer, it is also necessary to ensure that the shielding layer is well grounded. In shielded wiring, one end of the shielding layer of the cable is grounded through the wiring equipment in the wiring room, and the other end is connected to the power protection ground through the terminal equipment in the working area to form a complete grounding.

 

The "Code for Engineering Design of Integrated Cabling System (GB 50311-2007)" stipulates in the description of Article 7.0.6 involving grounding practices: "In order to ensure the entire shielding effect, the shielding metal cover of the terminal equipment can be connected with the TN-S system in a corresponding way. The PE wire is grounded.”

 

Correspondingly, it is stipulated in 6.1.2: "1. Each work area should be equipped with at least one 220V AC power socket. 2. The power socket of the work area should be a single-phase power socket with protective grounding, and the protective ground and the neutral line should be should be strictly separated."

 

4.2 TN-S power distribution system and power socket wiring

The so-called TN-S system is a form of low-voltage power distribution. The neutral line (zero line N) and the protective ground line (PE line) in the system have only one common point, and the other parts are strictly separated. The neutral wire carries the power current of the electrical equipment; the protective ground wire is an equipotential connection, which is the discharge path of the fault current or interference current, and is not charged under normal circumstances.

 

The wiring of the power socket in the work area should meet the mandatory provisions of "GB50303-2002 Code for Acceptance of Construction Quality of Building Electrical Engineering", which stipulates in 22.1.2 of this standard: "...the right hole of a single-phase three-hole socket is connected to the phase line, and the left hole is connected to Neutral connection; ... ground (PE) wire connected to upper hole"

 

4.3 Test of power protection ground

Since the grounding of the work area involves strong electricity, it is necessary to test the wiring of the power socket before the joint test of the shielding and grounding impedance to eliminate the faults such as live ground wire, open ground wire, zero/ground reverse connection, and poor ground wire contact. Keep people and equipment safe while preparing for subsequent testing.

 

Especially for the protective ground wire, if the impedance can be quantitatively tested, it is of great significance to judge the grounding effect of the shielded wiring system.

 

4.4 Joint test of shielding layer and grounding

 

4.4.1 Field test shielding and grounding before installing terminal equipment

 

After confirming that the wiring of the power socket is correct, use the resistance gear of the multimeter to directly test the resistance between the shielding cover of the information socket and the protective ground wire (PE wire) of the power socket

 

4.4.2 Test shielding and grounding after installation of terminal equipment

 

Terminal equipment equipped with shielded network card needs to be connected to the shielded wiring system through a shielded jumper, and connected to the TN-S power supply system through a 3-core power cable.

 

This test method can more accurately obtain the grounding resistance (impedance) value of the shielding system. When the test result is unqualified, it is necessary to use corresponding means to locate the fault and eliminate it.

 

The above are several methods for "quantitatively testing" the connectivity of the shielding layer of the wiring system based on the analysis of the functions of the instruments commonly used in the current wiring engineering. We hope to give you some help.

 

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