A brief discussion on electromagnetic interference and protection of integrated wiring system

With the rapid development of the information age, various high-frequency communication facilities continue to emerge, and the electromagnetic radiation and electromagnetic interference between them are becoming increasingly serious. In foreign countries, electromagnetic impact has been regarded as a kind of environmental pollution, and special institutions have been established to manage telecommunications and electronic products, formulate electromagnetic radiation limit standards, and control them.

In fact, in the surrounding environment of the integrated wiring system, there are inevitably interference sources of one kind or another, such as: fluorescent lamps, argon lamps, electronic starters or sympathetic equipment, elevators, transformers, radio transmission equipment, switching power supplies, electromagnetic induction furnaces, radar equipment, and power lines and power equipment with a voltage below 500 volts. Among them, the most harmful are electromagnetic interference and electromagnetic radiation. Electromagnetic interference is the parasitic electric energy radiated by electronic systems, and the electronic systems here also include cables. This parasitic electric 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 malfunction the control signal. Electromagnetic radiation involves the security issue of information being stolen by unrelated personnel during normal transmission, or causing electromagnetic pollution. In order to suppress electromagnetic interference, protective measures must be taken.

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

1. Strengthen the anti-interference ability of the internal structure and materials of the wiring system.

For computer equipment, communications, electronic and other equipment, the product appearance and structure should be made of metal boxes, cabinets, and racks to make them into a legal form, and the grounding terminals should be used for good grounding, which to a certain extent enhances the equipment's anti-interference and anti-radiation capabilities.

The selection of materials and performance of cables for the integrated wiring system should be based on user requirements and the surrounding environment of the building. Generally, the principle of anti-interference ability and transmission performance is taken as the main consideration, and economic factors are secondary. The reference values ​​of anti-interference ability of various commonly used cables and wiring equipment are as follows: UTP cable (no shielding layer) 40dB FTP cable (longitudinal aluminum foil) 85dB SFTP cable (longitudinal aluminum foil, plus copper braid) 90dB STP cable (the first pair of core wires and cable wires are wrapped with aluminum foil and copper braid) 98dB Deterioration after wiring equipment is inserted ≤30dB

Twisted pair cables are usually used for the links of the integrated wiring system. Twisted pair cables have the ability to absorb and emit electromagnetic fields. Tests show that if the twist pitch of the twisted pair cable is very small compared to the wavelength of the electromagnetic wave, we can assume that the current generated by the electromagnetic field in the first twisted node is the same as the current generated in the second twisted node. In this way, the influence of the electromagnetic field on the twisted pair cable can be offset. On the other hand, the current in the cable generates 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 twisted node is equal in magnitude to the electromagnetic field generated in the second twisted node, but in opposite directions, and the sum is zero. However, this situation can only occur in an ideal balanced cable. In fact, an ideal balanced cable does not exist. First, bending will cause the twisted node to loosen. On the other hand, any metal object near the cable will form a capacitive coupling with the twisted pair cable, so that the directions of the electromagnetic fields in adjacent twisted nodes are no longer completely opposite. 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 provisions of Article 12.2.1, Paragraph 3 of the "Engineering Design Specifications for Integrated Wiring Systems of Buildings and Building Complexes" CECS72:97: "When the frequency of the interference source signal or computer network signal is greater than or equal to 30MHZ, different shielded cable systems and shielded wiring equipment such as FTP, SFTP, STP, etc. should be selected according to the magnitude of the excess over the standard."

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

(1) Reflection loss: Part of the electromagnetic interference is reflected by the outer layer of the shielding layer; the other part of the electromagnetic interference that enters the shielding layer is reflected by the inner layer of the shielding layer.

(2) Absorption loss: The electromagnetic interference injected into the shielding layer is absorbed due to propagation loss.

(3) Skin effect: Electromagnetic interference will be conducted along the outer layer of the shielding layer due to the skin effect.

Therefore, using a shielding system for cables and wiring equipment can enhance the ability to resist interference and radiation.

For the shielding system, it is not enough to have a metal shielding layer alone. What is more important is to have a correct and good grounding system, and the wiring cabinet in each part should be wired separately to the grounding body with wires of appropriate cross-section; all shields of the integrated wiring system should maintain continuity and be grounded at both ends. If there are two grounding bodies, the grounding potential difference should not be greater than 1Vr.mS (effective value).

2. Pay attention to the impact of the distance between the equipment transmission line and different interference sources.

In addition to using shielding and grounding to improve its anti-interference and anti-radiation capabilities, the integrated wiring system must also be kept as far away from interference sources as possible to reduce their impact on the normal operation of the system.

Let’s do two experiments:

(1) Use UTP network. Use a walkie-talkie to interfere with the transmission cable. Turn on the walkie-talkie 0.5 meters away from the cable. A 1-minute file takes 1 minute and 45 seconds to transfer, and the screen is distorted, but the result is normal. Use FTP network to do the same experiment. The cable is not interfered. The file is transferred in 1 minute and the image is not distorted. When using UTP cable, there is no interference when the walkie-talkie is 2 meters away.

(2) Use UTP cable in parallel with the power line. The bit error rate is much smaller when the distance is 0.2 meters, and it is extremely small when the distance is more than 1 meter.

From the above experimental results, it can be seen that the distortion of the transmission signal will be different if the transmission cable is at different distances from the interference source.

Therefore, during construction and installation, attention should be paid to the influence of the distance between the wiring system and different interference sources, design and construction should be strictly in accordance with the specifications, electrical performance testing and archiving should be done well, and awareness of prevention should be strengthened.