A few years ago, people mentioned building automation system, which mainly refers to the automatic control system of HVAC equipment in buildings. In recent years, it has covered all controllable electrical equipment in buildings, and electrical automation has become an indispensable basic link of building automation system. In the building automation system, the design of electrical automation system occupies an important position. Here, I only talk about my own views and opinions on electrical grounding and electrical protection in electrical automation.
1 electrical grounding
In the design of building power supply and distribution, the design of grounding system is very important, because it is related to the reliability and safety of power supply system. Especially in recent years, the emergence of a large number of intelligent buildings has put forward many new contents for the design of grounding system.
At present, there are two main ways of electrical grounding:
1. 1 TN-S-s system. TN-S is a three-phase four-wire plus PE grounding system. Usually, when there is an independent substation in the building, the system is used for incoming lines. The characteristic of TN-S system is that neutral line N and protective grounding line PE are grounded at the neutral point of transformer, and there is no electrical connection between them. The neutral line n is charged and the PE line is not charged. The grounding system has a safe and reliable reference potential. As long as the same technical measures as TN-C-S grounding system are taken, TN-S system can be used as the grounding system of intelligent buildings. If there are no special requirements for electronic equipment such as computers, this grounding system is generally adopted.
In intelligent buildings, there are many single-phase electrical equipment, and the single-phase load accounts for a large proportion, and the three-phase load is usually unbalanced, so there is random current on neutral line N. In addition, because a large number of fluorescent lamps are used for lighting, the third harmonic generated by them is superimposed on line N, which increases the current on line N. If the N wire is connected to the equipment shell, it will cause electric shock or fire accident; In TN-S system, if N line and PE line are connected together and then connected to the equipment shell, the danger is greater, and all the equipment shells connected with PE line are charged; Will expand the scope of electric shock accidents; If the N-wire, PE-wire and DC grounding wire are all connected together, in addition to the above dangers, electronic equipment will be disturbed and unable to work. Therefore, intelligent buildings should have the DC grounding, AC working grounding, safety protection grounding and lightning protection grounding of electronic equipment that ordinary buildings should also have. In addition, there are many program-controlled switchboard rooms, computer rooms, fire control and fire alarm monitoring rooms, and a large number of precision electronic instruments and equipment that are easily disturbed by electromagnetic waves in intelligent buildings, so the requirements of anti-static grounding and shielding grounding should also be considered in the design and construction of intelligent buildings.
1.2tn-c-s system. TN-C-S system consists of two grounding systems. The first part is TN-C system, and the second part is TN-S system. The interface is at the connection point of N line and PE line. This system is generally used in the place where the building power supply is led out from the regional substation. TN-C system is used before entering the house, and repeated grounding is done at the entrance, and it becomes TN-S system after entering the house. Tn-c system has been analyzed before. The characteristic of TN-S system is that the neutral line N and the protective grounding line PE can't be electrically connected when entering the house. In this system, the neutral line N is often charged, and the protective grounding line PE has no power supply. When the system is in normal operation, the equipment shell and metal components connected by PE line will never be electrified, so TN-S grounding system obviously improves the safety of people and things. At the same time, TN-C-S system can be used as the grounding system of intelligent buildings as long as we take measures such as grounding leads, each lead is led out from the grounding body, and the correct grounding resistance value is selected, so that electronic equipment can get an equipotential reference point.
2 electrical protection
2. 1 AC working grounding: working grounding mainly refers to the grounding of neutral point or neutral line (N line) of transformer. N-wire must be insulated with copper core. There are auxiliary equipotential terminals in power distribution, which are generally in the cabinet. It must be noted that the terminal cannot be exposed; Can not be mixed with other grounding systems, such as DC grounding, shielding grounding, anti-static grounding, etc. You can't connect it with PE line. In high voltage system, neutral grounding can make grounding relay protection work accurately and eliminate single-phase arc grounding overvoltage. Neutral grounding can prevent zero-sequence voltage deviation and keep the three-phase voltage basically balanced, which is very meaningful for low-voltage systems and can facilitate the use of single-phase power supply.
2.2 Safety protection grounding: Safety protection grounding is to connect the uncharged metal parts of electrical equipment with the grounding body. That is, the electrical equipment in the building and some metal components near the equipment are connected with PE wires, but it is forbidden to connect PE wires with N wires.
In modern buildings, there are many devices that need safety protection and grounding, such as high-voltage equipment, weak-current equipment and some uncharged conductive equipment and components, and safety protection and grounding measures must be taken. When the insulation of electrical equipment without safety protection grounding is damaged, its shell may be charged. If the human body touches the shell of this electrical equipment, it may be electrocuted or life-threatening. We know that in parallel circuit, the current value flowing through each branch is inversely proportional to the resistance, that is, the smaller the grounding resistance, the smaller the current flowing through the human body. Usually, the resistance of human body is several hundred times larger than the grounding resistance, and the current passing through human body is several hundred times smaller than the current flowing through the grounding body. When the grounding resistance is extremely small, the current flowing through the human body is almost equal to zero. In fact, because the grounding resistance is very small, the voltage drop caused by grounding short-circuit current is very small, so the voltage of the equipment shell to the ground is not high. When people stand on the ground and touch the shell of the equipment, the voltage borne by the human body is very low and there is no danger. Installing protective grounding device and reducing its grounding resistance is not only an effective measure to ensure the safe and effective operation of electrical system in intelligent buildings, but also a necessary means to ensure the safety of equipment and personnel in non-intelligent buildings.
2.3 Shielded grounding and anti-static grounding: In modern buildings, shielding and its correct grounding are the best protection methods to prevent electromagnetic interference. PE wire can be used to connect the equipment shell; The shielded grounding of conductor requires that both ends of shielded pipeline be reliably connected with PE line; Indoor shielding should also be reliably connected with PE line at multiple points. Anti-static interference is also very important. In a clean and dry room, people will generate a lot of static electricity when walking and moving equipment rub against each other. For example, in an environment with relative humidity of 10 ~ 20%, people can accumulate 35,000 volts of electrostatic voltage when walking. If there is no good grounding, it will not only interfere with electronic equipment, but even damage the equipment chip. Electrostatic objects or objects that may generate static electricity (non-insulators) form a circuit grounding with the earth through electrostatic conductors, which is called anti-static grounding. Anti-static grounding requires that all equipment enclosures and indoor (including ground) facilities must be reliably connected to PE lines in a clean and dry environment. The smaller the grounding resistance of grounding devices in intelligent buildings, the better. Independent lightning protection grounding resistance should be ≤10Ω. The grounding resistance of independent safety protection shall be ≤ 4Ω; Independent AC working grounding resistance should be ≤ 4Ω; Independent DC working grounding resistance should be ≤ 4Ω; Anti-static grounding resistance generally requires ≤100Ω.
2.4 DC grounding: In an intelligent building, there are a large number of computers, communication equipment and building automation equipment with computers. In these electronic devices, a series of processes, such as inputting information, transmitting information, converting energy, amplifying signals, logical actions, outputting information, etc. All of them are carried out quickly through micro-potential or micro-current, and the equipment often works through the Internet. Therefore, in order to make it accurate and stable, it is necessary to have a stable reference potential in addition to a stable power supply. The insulated copper core wire with large cross section can be used as a lead-out, one end of which is directly connected to the reference potential, and the other end is used for DC grounding of electronic equipment. This lead should not be connected with PE wire, and it is forbidden to connect with N wire.
2.5 Lightning protection and grounding: There are a large number of electronic equipment and wiring systems in intelligent buildings, such as communication automation system, fire alarm and fire linkage control system, building automation system, security monitoring system, office automation system, closed-circuit television system and their corresponding wiring systems. These electronic devices and wiring systems generally belong to parts with low withstand voltage level, high anti-interference requirements and fear of being struck by lightning. Whether it is direct attack, serial attack or counterattack, electronic equipment will be damaged or seriously interfered to varying degrees. Therefore, all functional grounding of intelligent buildings must be based on lightning protection grounding system, and a strict and complete lightning protection structure must be established.
Intelligent buildings mostly belong to a class of load, and should be designed according to the protection measures of a class of lightning protection buildings. The lightning arrester adopts pin-belt combined lightning arrester, and the lightning protection belt adopts 25×4(mm) galvanized flat steel to form a grid ≤ 10× 10(m) on the roof. The grid is electrically connected with the roof metal components and the steel bars in the column head of the building, and the down lead uses the steel bars in the column head. This can not only effectively prevent lightning from damaging the equipment in the building, but also prevent external electromagnetic interference.