Design of grounding system for the hottest intelli

2022-09-23
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Grounding system design of intelligent buildings

source: network

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in the design of building power supply and distribution, grounding design plays an important role, because it is related to the reliability and safety of distribution system. After entering the 1990s, the emergence of a large number of intelligent buildings has put forward many new requirements for the design of grounding system. Then, among the commonly used grounding types, which one can be suitable for intelligent buildings? How can the grounding requirements of weak current equipment and lines of intelligent system be considered as a whole with the grounding of strong current equipment and lines? Let's explore further

in the design of building power supply and distribution, grounding design plays an important role, because it is related to the reliability and safety of distribution system. After entering the 1990s, the emergence of a large number of intelligent buildings has put forward many new requirements for the design of grounding system. Then, among the commonly used grounding types, which one can be suitable for intelligent buildings? How can the grounding requirements of weak current equipment and lines of intelligent system be considered as a whole with the grounding of strong current equipment and lines? Let's explore further

design of grounding system for intelligent buildings

system

i means that the power end is not grounded, or it is grounded through high impedance. T means that the exposed conductive part of the electrical equipment on the load side is directly grounded. The biggest advantage of IT system is that when a single-phase ground fault occurs, the fault current is very small, and the fault line can not be cut off. In order to ensure personal safety, it is required to send a signal when a ground fault occurs. The contact voltage of the equipment is not greater than 50V, and its action current should meet the requirements of the following formula:

RA • ID ≤ 50V

where: ra - contact resistance of exposed conductive parts( ω)

id - the first short-circuit fault current between the phase line and the exposed conductive part (a)

in order to meet this requirement, the ground capacitance of the distribution system should be reduced, such as limiting the total length of the equipment line. The disadvantage of IT system is that it is not suitable to allocate neutral line n, and some safety measures must be added. It is not suitable for the low-voltage distribution system of intelligent buildings with a large number of single-phase equipment. However, it system can be used for power supply after lightning proof and interference proof isolation transformers are set for the important host room equipment and terminal equipment on each floor of the intelligent system

system

the first symbol t indicates that there is a point of direct grounding at the power end; The second symbol t indicates that the exposed conductive part of the electrical device is directly grounded, which is electrically independent of the grounding point at the power end

2.1 TT system is characterized by no electrical connection between neutral line N and protective grounding line PE, that is, neutral point grounding and PE line grounding are separated. When the system is in normal operation, when the three-phase load municipal synthetic leather chamber of commerce actively orders the industry to set off a new upsurge of raw material revolution in the next 35 years, the PE line will not be charged when the neutral line n is charged

2.2 when the metal shell of electrical equipment is charged (the phase line touches the shell or the equipment insulation is damaged and leaks electricity), the action characteristics of ground fault protection should meet the requirements of the following formula:

the company has attached great importance to the independent research and development ability since its establishment. RA • IA ≤ 50V

in the formula: ra - the grounding resistance and PE wire resistance of the exposed conductive part( ω)

ia - ensure the action current of the protective appliance to cut off the fault circuit (a)

because the size of the grounding fault current is limited by the sum of the grounding resistance at the power end and the grounding resistance of the equipment shell, its current is generally small, and it cannot start the low-voltage circuit breaker to trip or fuse to fuse, which will cause the shell to ground voltage of the leakage equipment to be higher than the safety voltage, so the leakage protector should be used for protection

2.3 ttscope of application of grounding type

it is applicable to buildings with low-voltage power supply away from substations, precision electronic equipment with high grounding requirements, and places requiring fire and explosion prevention

-c system

tn-c system uses neutral line (n) as grounding protection line (PE), which is called protective neutral line, generally known as pen line

3.1 the action characteristics of the ground fault protection of TN system should meet the requirements of the following formula:

ZS • IA ≤ U0

where: ZS - ground fault loop impedance( ω)

ia -- ensure that the protective appliance automatically cuts off the current of the fault circuit within the specified time (a)

u0 -- the nominal voltage of the phase line to the ground (V)

zs includes the impedance of the transformer and the impedance of the phase line and PE (pen) line from the transformer to the ground fault. Due to the large grounding fault current of TN system, the protection device of the fault line acts quickly and cuts off the power supply of the fault circuit to achieve the purpose of protection

3.2 due to the unbalanced three-phase load, there is unbalanced current on the pen line and voltage to the ground, so the metal shell of the electrical equipment connected to the pen line has a certain voltage

3.3 if pen is disconnected, the equipment shell is charged

3.4 if the phase line of the power supply touches the ground, the shell potential of the equipment will rise, spreading the dangerous potential on the pen line

3.5 leakage protector cannot be used on TN-C system trunk line

3.6 although the TN-C system has high sensitivity to ground faults and simple and economical lines, there are a large number of lighting, computer, fire fighting and other equipment in the intelligent building, of which the single-phase load accounts for a large proportion, and it is difficult to achieve three-phase load balance. The unbalanced current of pen line plus the high-order harmonic current caused by fluorescent lamps, thyristors (thyristors) and other equipment in the line, under non fault conditions, It will be superimposed on the medium pen line, causing the voltage fluctuation of the pen line, which will not only electrify the equipment shell (connected with the pen line), causing insecurity to people, but also fail to obtain a suitable potential reference point, and the precision electronic equipment cannot operate accurately and reliably. Therefore, TN-C grounding system can not be used as the grounding system of intelligent buildings

-c-s system

tn-c-s system is composed of two parts, the first part is TN-C system, the second part is TN-S system, and the interface is at the connection point of pen line and PE line. This system is generally used in places where the power distribution of buildings is led from public substations. Before entering the house, it is TN-C system. The pen line is repeatedly grounded at the incoming distribution box. The feeder line of the distribution box separates the N line and PE line to the equipment, and there is no electrical connection. When the three-phase power transformer is well grounded and the three-phase load is relatively balanced, as long as we adopt equipotential connection to make the electronic equipment jointly obtain an equipotential reference point, TN-C-S system can be used as a grounding type of low-voltage distribution system in intelligent buildings

-s system

tn-s system is a low-voltage distribution system that strictly separates neutral line N and protective grounding wire PE. Usually, the system is used when there is an independent substation in the building for two or three cents, and one substation is not expensive

5.1 see 3.1 for the ground fault protection characteristics of TN-S system

5.2 the neutral line N and the protective grounding wire PE are grounded together at the neutral point of the transformer, and there is no longer any electrical connection between the two lines. When the system operates normally, there is no current on PE line, but there is unbalanced current on N line

5.3pe wire shall not be broken, and there is no voltage to the ground, so the metal shell of electrical equipment is connected to PE wire safely and reliably

5.4tn-s system scope of application

tn-s system is safe and reliable, suitable for low-voltage distribution systems such as industrial and civil buildings. This kind of grounding system is generally used in intelligent buildings except for computers and other major electronic equipment with special requirements

6. electrical grounding measures for intelligent buildings

6.1 lightning protection grounding

in order to quickly introduce lightning current to the earth, the grounding for the purpose of preventing lightning damage is called lightning protection grounding. There are building electrical equipment and a large number of electronic equipment and wiring systems in the intelligent building, such as communication automation system, office automation system, fire alarm and fire linkage control system, building automation system, safety protection system, integrated wiring system, closed-circuit television system, garage management system, etc. Seen from the completed building, the roof, floor, side wall and ceiling of the building are almost covered with various wiring. Among them, electronic equipment and wiring system generally belong to the part with low voltage withstand level and high anti-interference requirements, which is most afraid of being struck by lightning. Whether it is direct strike, series strike, counterattack lightning, lightning induction and lightning wave intrusion, electronic equipment will be damaged or seriously disturbed to varying degrees. Therefore, the lightning protection and grounding design of intelligent buildings must comply with the relevant provisions of the code for design of lightning protection of buildings GB (2000 EDITION)

6.2 working grounding

the neutral point of the transformer is directly connected with the earth as a metal, which is called working grounding

the neutral wire (n wire) for grounding must be copper core insulated wire, which cannot be mixed with other grounding wires or connected with PE wire

6.3 safety protection grounding

safety protection grounding is to make a good metal connection between the uncharged metal part of electrical equipment and the grounding body. Connect the electrical equipment in the building and the metal components and metal pipes near the equipment with PE lines, but it is strictly forbidden to connect PE lines with N lines. These measures are not only measures to ensure the safe and effective operation of the electrical system of intelligent buildings, but also necessary means to ensure the safety of equipment and personnel in non intelligent buildings

6.4 shielding grounding and anti-static grounding

electromagnetic shielding and its correct grounding are the best protection methods for electronic equipment against electromagnetic interference. Connect the equipment shell with PE cable; One or both ends of the metal tube of the wire or cable, the metal sheath of the cable and the shielding layer are reliably connected with the PE wire; The wall, top plate, floor reinforcement and metal doors and windows of important electronic equipment room should also be reliably connected with PE wire at multiple points

anti static interference is also important. Anti static grounding requires that in a clean and dry environment, all equipment shells, metal pipes and indoor (including floor) facilities must be reliably connected with PE lines at multiple points

6.5 common grounding system

a general common grounding device shall be used for lightning protection grounding of buildings, grounding of electrical equipment (including electronic equipment), shielding grounding and anti-static grounding of intelligent buildings. The natural grounding bodies such as steel bars, metal objects and pipes in the reinforced concrete of the building are preferred for the common grounding devices. The grounding resistance shall be ≤ 1 ω。 If the requirements cannot be met, artificial grounding body can be added or chemical resistance reduction method can be adopted to make the grounding resistance ≤ 1 ω。

6.6 equipotential bonding

equipotential bonding is the main measure to prevent people from electric shock, electrical fire and anti electromagnetic interference of electronic equipment. Connect the metal shell, metal pipe, cable support, metal trunking, cable metal sheath, building reinforcement and other metal bodies of various equipment of the building to the nearest common grounding device reliably

6.6.1 strong and weak current systems are respectively equipped with their own equipotential grounding terminal boards, which are respectively connected with the common grounding device through the grounding trunk line or grounding busbar

6.6.2 each electrical equipment shall be connected to the equipotential terminal board with a separate PE wire, and several equipment shall not be grounded in series with grounding wires

6.6.3 there should be at least two points at the connection point between the equipotential grounding terminal board and the grounding trunk line or the common grounding device, and they should be in different positions

6.6.4 each equipotential grounding terminal board should be set at a position convenient for installation and inspection and close to various incoming lines, and should not be installed at a place where it is wet or corrosive gas and prone to mechanical damage. The connection point of equipotential grounding terminal board shall have firm mechanical strength and good electrical continuity

6.6.5 all kinds of metal pipes, metal trunking, cable metal sheath, etc. introduced into the building from the outside of the building should share the grounding device at the introduction

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