Electromagnetic interference and Protection of Integrated Wiring System

With the rapid development of the information age, various high-frequency communication facilities are constantly emerging, and the influence of electromagnetic radiation and electromagnetic interference between each other is becoming increasingly serious. In foreign countries, electromagnetic influences have been viewed as an environmental pollution. Special organizations have been established to manage telecommunication and electronic products, and electromagnetic radiation Limitation Standards have been formulated and controlled.

In fact, in the surrounding environment of the Integrated Wiring System, interference sources such as fluorescent lights, argon lamps, electronic starters, and ac-sensing devices are inevitable, elevator, transformer, radio transmitting equipment, switching power supply, electromagnetic induction furnace, radar equipment and power line and power equipment with a voltage below 500 V. Among them, the greatest harm is electromagnetic interference and electromagnetic radiation.

Electromagnetic interference is the parasitic energy emitted by the Electronic System. The electronic system here also includes cables. This parasitic electrical energy can affect the normal operation of the Integrated Wiring System on other nearby cables or systems, reduce the reliability of data transmission, increase the bit error rate, distort the image, and control the misoperation of the signal. Electromagnetic radiation is related to the security issue of information being stolen by irrelevant persons during normal transmission, or may cause electromagnetic pollution.

Protection measures must be taken to suppress electromagnetic interference.

The author believes that the protection against electromagnetic interference should be strengthened in the following two aspects.

1. Enhance the anti-interference of the internal structure and materials of the cabling system.

For computer equipment, communication equipment, electronic equipment, etc., the product should be made of metal materials in the shape structure of the box, box, Cabinet, Rack, make it the first place in the form of law, in addition, the grounding terminal is well grounded to enhance the anti-interference and anti-radiation capabilities of the equipment to a certain extent.

The material and performance selection of cables in the Integrated Wiring System should be based on user requirements and the surrounding environment conditions of the building. Anti-interference capability and transmission performance are generally used, followed by economic factors. Currently, the reference values for anti-interference ability of various commonly used cables and wiring equipment are as follows: UTP cable (unshielded layer) 40dB FTP cable (longitudinal aluminum foil) 85dB SFTP cable (longitudinal aluminum foil, copper braided mesh) 90dB STP cable (No. Aluminum Foil for Core Cables and cables, copper Braided Cables) 98dB wiring equipment deteriorated after insertion ≤ 30dB

For the link of the Integrated Wiring System, twisted pair wires are usually used. Twisted Pair wires can absorb and transmit electromagnetic fields. The test shows that if the twist distance of the twisted pair is smaller than the wavelength of the electromagnetic wave, we can think that the current generated in the first twist joint is the same as the current generated in the second twist joint, in this way, the impact of electromagnetic fields on twisted pair wires can be offset.

On the other hand, the current in the cable produces an electromagnetic field. According to the principle of electromagnetic induction, we can determine the direction of the electromagnetic field. The electromagnetic field generated by the cable in the first twist joint is the same as that produced in the second twist joint, with the opposite direction and Zero addition. However, this situation can only happen in the ideal balance cable, but in fact the ideal balance cable does not exist.

First, bending will lead to loose twisting. On the other hand, any metal object near the cable will form a capacitor coupling with the twisted pair, so that the electromagnetic field in the adjacent twisted joint is no longer in the opposite direction. If the above situation occurs, the cable will emit electromagnetic waves.

Therefore, when the interference field intensity of the surrounding environment or the noise level of the Integrated Wiring System is higher than the CECS72: 97 section 12.2.1 3rd of the engineering design specifications for the Integrated Wiring System of buildings and buildings: "When the interference source signal or computer network signal frequency is greater than or equal to 30 MHz, it shall exceed the standard magnitude, select different shield cable systems and shield wiring devices, such as FTP, SFTP, and STP ".

The shielding layer of the shielded cable is usually wrapped in metal foil of a certain thickness. It has the following three factors:

(1) reflection loss: some electromagnetic interference is reflected by the outer layer of the shielding layer; the other part is injected into the shielding layer.

(2) absorption loss: Electromagnetic Interference injected into the shielding layer is absorbed due to transmission loss.

(3) Skin Effect: electromagnetic interference is transmitted along the outer layer of the screen due to the skin effect.

Therefore, using a shielding system for cables and wiring equipment can enhance anti-interference and anti-radiation capabilities.

For shielding systems, it is not enough to have only one layer of metal shielding layer. More importantly, it is necessary to have a correct and good grounding system, in addition, the distribution cabinets of each part should be separately routed to the grounding body using appropriate cross-section wires. All shielding of the Integrated Wiring System should be continuity and the two ends should be grounded. If there are two grounding bodies, the Grounding Potential Difference should not be greater than 1Vr. m. S (valid value ).

2. Pay attention to the influence of the device transmission line distance from different interference sources.

In addition to shielding and grounding, the Integrated Wiring System must stay away from interference sources as much as possible to reduce the impact on the normal operation of the system.

The following are two experiments:

(1) UTP network. The walkie talkie is used to interfere with the transmission cable. If the walkie talkie is opened 0.5 metres away from the cable, the file will be transmitted at 1 minute 45 seconds, and the screen will be distorted, but the result will be normal. Using the FTP network, the same lab cable is not disturbed. The file is uploaded within one minute, and the image is not distorted. When the UTP cable is used and the walkie talkie is 2 meters away, there is no interference.

(2) When the UTP cable is used in parallel with the power line, the error rate is much smaller when the 0.2 m is left, and the error rate is very little when the 1 m or more is left.

From the above experimental results, we can see that the transmission cable is different from the interference source, and its transmission signal distortion will also be different.

Therefore, in the construction and installation process, we should pay attention to the influence of the wiring system and the spacing between different interference sources, strictly design and construct the cables in accordance with the specifications, test and archive the electrical performance, and strengthen the awareness of prevention.