Three conditions must be met to produce interference: interference source, interference channel and easily interfered equipment. Interference sources are divided into internal and external. The interior is mainly the principle of the device and the quality of the product. The external factors are mainly determined by the use conditions and environmental factors, such as the surge voltage, strong electric field or strong magnetic field and electromagnetic wave radiation caused by the switching operation and weather influence of the DC circuit of the working power supply.
There are three kinds of interference channels: conduction coupling, common impedance coupling and electromagnetic coupling. The external is mainly transmitted to the internal through the electromagnetic coupling of distributed capacitance, while the internal has three kinds. Due to the unreasonable selection and structural layout of the sensitive elements used in the equipment, the anti-interference ability of the equipment itself is poor. To suppress the interference, reduce its amplitude and influence is to improve the external environment.
Inductive lightning can be generated by electrostatic induction or electromagnetic induction, which has a high probability of forming inductive lightning voltage and poses a great threat to the electronic equipment in buildings. The focus of lightning protection work of home network system is to prevent the invasion of inductive lightning. There are three ways of lightning over-voltage and over-current invading home network system
1) It is intruded by the AC 220 V power supply line; the power supply of the smart home system is input into the room by the power line, and the power line may suffer from direct lightning and inductive lightning. In addition, the low-voltage line may also be struck by direct lightning or induced lightning overvoltage. The average Lightning Overvoltage on 220 V power line can reach 10000 V, which can cause devastating damage to home network system. One of the reasons for the complexity of power supply interference is that it contains many variable factors. Power supply interference can exist in the form of “common mode” or “differential mode”.
“Common mode” interference refers to the potential difference between the power line and the earth, or between the neutral line and the earth. The “differential mode” interference exists between the phase line and the neutral line of the power supply. For three-phase power supply, it also exists between phase lines. The second reason for the complexity of power supply interference is that the interference situation can change from short-lived peak interference to total power loss. The types of power supply interference are shown in Table 1: the ways of power supply interference entering the equipment: electromagnetic coupling, capacitive coupling and direct entering.
2) It can be divided into three cases: the intrusion of smart home system transmission line
① When the ground protrusion is hit by direct lightning, the strong lightning voltage will break down the adjacent soil, and the lightning current will directly invade the cable sheath, and then break down the sheath, making the high voltage invade the line.
② When thunder cloud discharges to the ground, over-voltage of several kilovolts is induced on the line, which damages the electrical equipment connected with the line and invades the communication line through the equipment connection. This kind of intrusion spreads along the communication line, involving a wide range and causing great harm.
③ If a multi-core cable is used to connect wires from different sources or multiple cables are laid in parallel, when a wire is struck by lightning, overvoltage will be induced in adjacent wires, damaging low-voltage electronic equipment.
3) Ground potential counterattack voltage invades through the grounding body; when lightning strikes, the strong lightning current is discharged into the earth through down lead and grounding body, and the radioactive potential distribution near the grounding body will generate high voltage ground potential counterattack if other grounding bodies connected with electronic equipment are close, and the invasion voltage can reach tens of thousands of volts. The lightning protection of buildings against direct lightning introduces a strong lightning current into the ground through the down lead, which produces a strong electromagnetic field change in the nearby space, and will induce Lightning Overvoltage on the adjacent wires (including power line and signal line). Therefore, the building lightning protection system can not protect the computer, but may introduce lightning. The integrated wire chip of home network system and other equipment has very weak withstand voltage, usually below 100 V, so it is necessary to establish a multi-level computer lightning protection system to ensure the safety of the computer, especially the computer network system.