GB 14821.1-1993 Electric shock protection of electrical installations in buildings
Some standard content:
National Standard of the People's Republic of China
Electric shock protection
Electrical installations of buildingsProtection against electric shockGB14821.1-93
This standard is equivalent to IEC364 -4-41(1992) "Safety Protection of Electrical Installations in Buildings and Protection against Electric Shock". 1 Subject content and scope of application
This standard specifies the requirements for electric shock protection of electrical devices with nominal voltage power frequency AC 1000V and below, DC 1500V and below. and application requirements for protective measures. This standard applies to electrical installations in housing, industry, agriculture, commercial buildings, public buildings, movable buildings, tourist vehicles and similar places, as well as temporary installations in construction sites and exhibition halls. This standard does not apply to electric traction equipment, automotive electrical equipment, marine electrical equipment, aircraft electrical equipment, public road lighting equipment, mining equipment, anti-radio interference equipment (unless the equipment affects the safety of the equipment), buildings Lightning protection. 2 Reference Standards
GB2900.1 Basic terminology of electrical engineering terminology GB4208 Classification of enclosure protection levels
GB4776 Electrical safety terminology
GB6829 Leakage current action protector (residual current action protector ) Low-voltage complete switchgear
GB7251
GB8898 Safety requirements for household and similar general-purpose electronic and related equipment powered by grid power supply 3 Isolation transformers and technical requirements for safety isolation transformers GB13028
3 Terminology
Except for the following terms, the terms used in this standard are quoted from GB4776. 3.1 Electric shock (electric shock) Pathophysiological effects caused by electric shock current passing through the human body or sexual animals. 3.2 Electrical equipment electrical equipment refers to any items or products that generate, transform, transmit, distribute or consume electricity, such as motors, transformers, electrical appliances, measuring instruments, protective appliances, wiring system equipment and electrical appliances.
3.3 Electrical installation is a combination of electrical equipment that achieves one or several specific purposes and has mutually coordinated characteristics. 3.4 Direct contact directcontact
Contact between humans or livestock and live parts.
3.5 indirect contact indirectcontact
Contact between a person or a sexual animal and an exposed conductive part that becomes live under a fault condition. 3.6 The external conductive part extraneousconductivepart was approved by the State Bureau of Technical Supervision on 1993-12-29 and implemented on 1994-10-01
GB14821.1-93
potential) conductive part. 4 General requirements for electric shock protection
The purpose of electric shock protection is achieved by adopting the corresponding measures described in chapters 5, 6 and 7. Protection against electric shock may be applied to the entire installation. It can also be applied to a part of a device or a piece of equipment. If certain conditions for protective measures cannot be met, supplementary measures must be taken to ensure that their safety level is not reduced. The order of the various protective measures described in this standard does not indicate their relative importance. 5 Protection against both direct contact and indirect contact 5.1 Protection against extra low voltage (ELV): SELV and PELV 5.1.1 can be considered to provide electric shock protection when the following conditions are fully met. a. The nominal voltage does not exceed AC 50V, DC 120V Note: ① The AC values ??involved in this standard are root mean square values, and the DC values ??are ripple-free values. ②Ripple-free DC is DC whose effective value of fingerprint wave content is no more than 10%. For example, for a DC ripple-free system with a nominal voltage of 120V, its maximum peak value does not exceed 137V.
b. Powered by the power supply specified in Article 5.1.2; c. Meet all conditions of 5.1.3 and SELV shall also meet the provisions of Article 5.1.4, and PELV shall also meet the provisions of Article 5.1.5. Note: ① If the system is powered by other equipment, such as autotransformers, voltage dividers, semiconductor equipment, etc., with a higher voltage than it, the output loop is considered to be an extension of the input loop, and protective measures applicable to the input loop must be adopted for protection. ② Under certain external influences, lower voltage limits can be specified. 5.1.2 Power supplies for SELV and PELV
5.1.2.1 Safety isolation transformers that comply with the requirements of GB13028. 5.1.2.2 Power supply with a safety level equivalent to a safety isolation transformer (such as a motor generator with equivalent isolation windings). 5.1.2.3 Electrochemical power source (such as battery) or other power source unrelated to the higher voltage circuit (such as diesel generator). 5.1.2.4 Certain electronic equipment that meets the corresponding standards, these electronic equipment have taken measures to ensure that even if an internal fault occurs, the voltage of the lead terminal does not exceed the value specified in Article 5.1.1. In the case of direct contact or indirect contact, if the voltage on the lead-out terminal immediately drops to no more than the value in 5.1.1, a voltage greater than that specified in 5.1.1 is allowed to appear on the lead-out terminal. Note: ① This type of equipment includes insulation testing equipment. ② If the output voltage of the equipment is higher than the provisions of Article 5.1.1, but when measured with a voltmeter with an internal resistance of at least 30,000, the measured voltage is within the limit of Article 5.1.1, it is still considered that the device The equipment complies with the requirements of this article. 5.1.2.5 The selection and installation of mobile safety power sources such as safety isolation transformers or motor generators must achieve Class 1 equipment or insulation equivalent to Class 1 equipment (see Article 7.2).
5.1.3 Circuit configuration
5.1.3.1 The live parts of SELV and PELV circuits must be electrically isolated from each other and from other circuits, and their electrical isolation level shall not be lower than the safety isolation transformer The level of electrical isolation between input and output circuits. Note: ① This provision does not exclude PELV circuit grounding (see 5.1.5). ② In particular, electrical equipment such as relays, contactors, and auxiliary switches must strictly comply with the provisions of this article. 5.1.3.2 Return conductors of SELV and PELV systems must be physically isolated from any other return conductors. When this requirement cannot be met, one of the following measures is required: a: In addition to basic insulation, the SELV and PELV circuit conductors must also be installed in a closed non-metallic sheath; b. The conductors of circuits with different voltages must be Separated by a grounded metal shield or grounded metal sheath; Note: When doing the above treatment, the basic insulation of any conductor only needs to meet the voltage of the circuit where the conductor is located. c. Circuits with different voltages can be included in a multi-core cable or other groups of conductors, but the conductors of SELV and PELV circuits should be insulated individually or collectively, and their insulation level should be considered according to the highest voltage among them. GB14821.1—93
5.1.3.3 Plugs and sockets of SELV and PELV systems must meet the following requirements: a. The plug must not be inserted into the socket of other voltage systems; b. The socket must not be inserted into sockets of other voltage systems Insert the plug, c. The socket must not be equipped with protective wire contacts.
5.1.4 Requirements for SELV circuits
5.1.4.1 The live parts of the SELV circuit are strictly prohibited from being connected to the earth or to the live parts and protective conductors of other circuits. 5.1.4.2 Exposed conductive parts are not allowed to be intentionally connected to one of the following parts: a. Earth;
protective conductors and exposed conductive parts of other circuits; b.
c. External conductive part. Unless the function of the electrical equipment requires connection to external conductive parts, and such connection will not introduce a voltage higher than that specified in 5.1.1.
If the exposed conductive parts of the SELV are likely to come into contact intentionally or unintentionally with the exposed conductive parts of other circuits, protection against electric shock must no longer rely solely on the SELV but also on the exposed conductive parts of other circuits that are easily accessible Take protective measures against electric shock.
5.1.4.3 If the nominal voltage exceeds AC 25V or DC 60V, the following measures should be taken to achieve direct contact protection. a. Use barriers or outer protection with a protection level of at least IPXXB. b. The insulation can withstand AC 500V test voltage for 1 minute. If the nominal voltage does not exceed 25V AC or 60V DC, direct contact protection is generally not required. However, under certain external influences this protection may be required.
5.1.5 Requirements for PELV loopswww.bzxz.net
When the loop is grounded, or the provisions of Article 5.1.4 are not required, the requirements of Articles 5.1.5.1 and 5.1.5.2 must be met. 5.1.5.1 Direct contact protection must be achieved by one of the following measures: a. Use barriers or outer protection with a protection level of at least IPXXB. b. The insulation can withstand AC voltage of 500V. Lasted 1min. 5.1.5.2 If the equipment is within the effective area of ??equipotential bonding and the nominal voltage does not exceed the following values, it is not necessary to install the direct contact protection specified in 5.1.5.1.
a. The equipment is usually only used in dry conditions, and when the live parts are not in large contact with the human body, AC 25V or DC 60V, b. Under any other circumstances, AC 6V or DC 15V. Note: Appropriate connections to the ground can be made within the power supply to achieve grounding of the loop 5.2 Protection to limit discharge energy (under consideration) 5.3 Protection of FELV systems
5.3.1 General requirements
Due to functions For the above reasons, the nominal voltage does not exceed AC 50V and DC 120V, but all the requirements for SELV or PELV in Article 5.1 cannot be fully met, and there is no need to use SELV or PELV, then 5.3.2 and 5.3.3 must be used Supplementary measures specified in Article 1 to ensure protection against direct and indirect contact. This combination of protective measures is called FELV. Note: For example, when there is insufficient insulation between the equipment contained in the circuit (such as transformers, relays, remote switches, contactors) and the circuit with a higher voltage than it, a functional extra-low voltage system should be used.
5.3.2 Direct contact protection
Direct contact protection must be achieved by one of the following measures: a. Use the barrier or outer protection specified in Article 6.2; b. Use insulation equivalent to the minimum test voltage required for the primary circuit. If the insulation of the equipment in the FELV circuit cannot withstand the test voltage required for the primary circuit, the insulation level of the accessible non-conductive parts of the equipment must be strengthened during installation so that it can withstand a test of 1500V AC for 1 minute. Voltage. 5.3.3 Indirect contact protection
GB14821.1-93
Indirect contact protection must be achieved by one of the following measures: a. If the primary circuit adopts a protective method of automatically cutting off the power supply specified in Article 7.1, the exposed conductive part of the FELV circuit can be connected to the protective conductor of the primary circuit. At this time, the live conductor in the FELV circuit is not excluded from contact with the protective conductor of the primary circuit. The protection wire of the primary circuit is connected.
b. When the primary circuit adopts the electrical isolation protection specified in Article 7.5, the exposed conductive part of the FELV circuit equipment may be connected to the ungrounded equipotential bonding wire of the primary circuit. 5.3.4 Plugs and sockets
Plugs and sockets used in FELV systems shall meet the following requirements: the plug cannot be inserted into a socket of other voltage systems, and the socket must not be inserted into a plug of other voltage systems.
6 Direct contact protection
Direct contact protection is also called electric shock protection or basic protection during normal operation. 6.1 Protection with insulation
Insulation is used to prevent any contact with live parts. Live parts must be completely covered with insulation, and the insulation covering must be removed only by destructive means. The insulation of electrical equipment must comply with the relevant standards for the equipment. There are no standards stipulating that the insulation of equipment must be able to withstand the mechanical, chemical, electrical and thermal effects that may be suffered during operation over a long period of time. Paints, varnishes, spray paints and other similar materials generally cannot be used alone as direct contact protection.
Note: The quality of the insulation used during facility installation should be able to pass relevant tests. These tests should be equivalent to the insulation tests performed on similar equipment manufactured.
6.2 Protection with shields and outer shields
Shields and outer shields are used to prevent any contact with live parts. 6.2.1 The live parts must be installed behind the barrier or inside the outer protection with a protection level of at least IPXXB. When holes larger than 12mm appear during the replacement of components such as lamp holders, sockets or fuses, and when holes larger than 12mm are set according to the relevant requirements of the equipment to operate correctly, appropriate measures must be taken to prevent people and livestock from unintentionally touching live parts. Make sure people are aware of the risk of electric shock when reaching into holes.
6.2.2 The level of protection of the horizontal top surface of easily accessible barriers or exterior shields must be at least IPXXD. 6.2.3 Barriers and outer protection must be fixed at the specified position and have sufficient stability and durability to ensure the required protection level and work well with the equipment under normal working conditions (taking into account relevant external influences). Keep an appropriate distance from live parts. 6.2.4 Only when one of the following conditions is met, the barrier can be moved and the outer protection can be opened or the parts of the outer protection can be disassembled. 6.2.4.1 Use keys or tools;
6.2.4.2 After cutting off the power supply to the live parts protected by the barrier or outer shield, the power supply can only be restored after the barrier or outer shield is reset: || tt||6.2.4.3 Have an intermediate barrier to prevent access to live parts. This barrier has a protection level of at least IPXXB and can only be removed with a key or tool.
6.3 Protection with barriers
Barriers are used to prevent unintentional contact with live parts, but they cannot prevent intentional bypassing of barriers and intentional contact with live parts. 6.3.1 The barrier must be able to prevent the following two situations from occurring: a. Unintentionally approaching live parts with the body, b. Unintentional contact with live parts during normal operation of the equipment. 6.3.2 Barriers may be removed without keys or tools, but must be secured so that they cannot be moved unintentionally. 6.4 Protection placed beyond the reach of the arm
GB14821.1-93
Protection placed beyond the reach of the arm is only used to avoid unintentional contact with live parts. 6.4.1 It is strictly prohibited to have parts with different potentials that can be touched at the same time within the reach of the arm. If the distance between the two live parts does not exceed 2.5m, they can be considered to be accessible at the same time (see Figure 1), 2.50m
The limit of the extension range
S·Predetermined human surface|| tt||Figure 1 Outreach Range
6.4.2 If a barrier with a protection level lower than IPXXB (such as railings, mesh screens) is used to restrict a normally occupied position in the horizontal direction, the outreach range must Count from the blocker. Above the head, regardless of intermediate obstructions with a protection level lower than IPXXB, the arm reach of 2.5m should be calculated from s.
Note: The range of reach refers to the range of activities that can be directly touched by bare hands (without tools or ladders). 6.4.3 Where large or long conductive objects need to be held, the size of the conductive object must be taken into account when calculating the reach range involved in Articles 6.4.1 and 6.4.2.
6.5 Use residual current action protector as additional protection. The use of residual current action protector is an additional measure taken to strengthen direct contact protection. 6.5.1 In normal operation, the use of a residual current action protector with a rated residual action current not exceeding 30mA can be used as additional protection when other protection measures fail or the user is negligent. 6.5.2 The residual current action protector cannot be used as a separate means of direct contact protection. 7 Indirect contact protection
7.1 Protection against automatic power supply cutoff
In case of fault, when the contact voltage and its duration cause dangerous pathophysiological reactions to the human body, the power supply should be automatically cut off.
This protective measure needs to coordinate the system grounding type (see GB4776), protective conductor and protective electrical performance. 7.1.1 Basic measures
Note: The system grounding type given in clauses 7.1.3 to 7.1.5 complies with the provisions of clauses 7.1.1 and 7.1.2. 7.1.1.1 Cutoff of power supply
GB14821.1-93
When a fault occurs between a circuit or equipment and a live conductor and an exposed conductive part or a protective conductor, the protective appliance against indirect contact must automatically Cutting off the power supply to the circuit or equipment to a value of expected contact voltage between conductive parts that prevent simultaneous human contact. When AC exceeds 50V and ripple-free DC exceeds 120V, it cannot last long enough to cause harmful and dangerous pathophysiological reactions to the human body. Note: The relationship between the expected AC contact voltage and the maximum cut-off time is shown in Appendix A. In some cases, according to the system grounding type (see 7.1.3.5), the cut-off time can be relaxed to no more than 5 seconds regardless of the contact voltage.
Note: ① In the power generation and distribution system, the allowable cutting time and voltage are greater than the values ??required by this article. ② For some special places or facilities, lower expected contact voltage or shorter cut-off time may be required. ③For IT systems, there is usually no requirement to automatically cut off the power supply when the first ground fault occurs (see 7.1.5). This provision applies to AC and ripple-free DC power supplies of 15 to 1000 Hz. ③ For the meaning of the term “ripple-free”, see Note 5.1.1. 7.1.1.2 Grounding
The exposed conductive parts should be connected to the protective conductor according to the system grounding type. Exposed conductive parts that are accessible at the same time shall be connected individually, in groups or jointly to the same earthing system. For requirements on grounding devices and protective conductors, see IEC364-5-54 "Selection and Installation of Grounding Devices and Protective Conductors for Electrical Equipment in Building Electrical Installations".
7.1.2 Equipotential bonding
7.1.2.1 Main equipotential bonding
The following conductive parts of every building must be connected to the main equipotential bonding conductor: a. Main protective conductor (protected trunk line);
b. Main grounding conductor or main grounding terminal;
Public pipes in the building, such as gas pipes and water pipes; c.
d. Yes Utilized metal structural parts of buildings and metal components of central heating and air conditioning systems. Conductive parts coming from outside the building shall be connected indoors close to the entrance. Equipotential bonding with telecommunications cables must be connected to their metal sheaths, subject to the approval of the competent authority. The main equipotential bonding conductor must comply with the requirements of IEC364-5-54. 7.1.2.2 Auxiliary equipotential bonding
If the automatic cut-off conditions specified in Article 7.1.1.1 cannot be met within a device or a part of the device, auxiliary equipotential bonding shall be implemented in accordance with Article 7.1.6 . Note: ① Adopting auxiliary equipotential bonding does not eliminate the necessity of automatically cutting off the power supply for reasons such as fire prevention and equipment overheating. ② Auxiliary equipotential bonding can be implemented in the entire device, a part of the device, a set of equipment or a location. ③For some special places, some additional conditions may be required. 7.1.3 Protection of TN systems
7.1.3.1 All exposed conductive parts must be connected to the power system grounding point through protective conductors. The protective conductor must be connected to earth in the vicinity of each transformer or generator of the installation.
The grounding point of the power system is usually the neutral point. If there is no neutral point or it is impossible to draw out the neutral point, one phase wire can be grounded at the substation. However, under no circumstances is it allowed to use this phase line as a PEN line (see 7.1.3.2). Note: ① If there are other effective grounding bodies, the protective conductor should be connected to them. In order to ensure that the protective conductor is as close to the ground potential as possible under fault conditions, additional grounding points should be added and evenly distributed. In large buildings such as high-rise buildings, additional grounding of protective wires may not be easily achieved. At this time, the equipotential bonding between the protective conductor and the external conductive part has a similar effect to adding an additional grounding point. ② For the same reason, the protective conductor should be grounded at the point of entry into the building or house. 7.1.3.2 In fixed installations, a single conductor that meets the requirements of the following "Note" may be used as the main protective neutral conductor (PEN conductor). Note: The requirements for PEN conductors are as follows:
GB14821.1-93
① For power circuits that supply power to fixed devices, the cross-sectional area of ??the copper core of the PEN conductor is not less than 10mm, and the cross-sectional area of ??the aluminum core is not less than 16mm . If a concentric neutral cable is used, the outsourced neutral wire is used as a PEN wire, and the PEN wire uses double connectors within the full length of the cable, the minimum cross-section of the PEN wire can be 4mm\,
② shall not be used Residual current operated protectors protect circuits with PEN conductors. When overcurrent operates the protector, it must be ensured that the PEN conductor is cut off at the same time as the phase conductor. ③PEN conductors must be insulated according to the highest voltage they may be subjected to. PEN conductors within switchgear and controlgear packages do not need to be insulated. ④ If from any point of the installation, the PEN conductors are separated into neutral conductors and protective conductors, these conductors are not allowed to be connected to each other from that point on. At the point of separation, separate terminals or busbars must be provided for the connection of the protective conductor and the neutral conductor. 7.1.3.3 The durability and loop impedance of the protective appliance (see 7.1.3.8) must be selected such that when a fault with negligible impedance occurs between a phase conductor and a protective conductor or an exposed conductive part anywhere in the installation, the specified Automatically cut off its power supply within a certain period of time. The following conditions satisfy this requirement:
z, . I,≤U.
(1)
In the formula: Z, the fault loop impedance including the internal resistance of the power supply, the live conductor between the power supply and the fault point, and the protective conductor between the fault point and the power supply, 2;
I. 1 - The operating current to ensure that the protective appliance automatically cuts off the power supply within the time specified in Table 1 or Article 7.1.3.5, A; U. ——Nominal voltage to ground, V.
7.1.3.4 The maximum cut-off time specified in Table 1 may be considered to meet the requirements for cut-off of end circuits that directly supply power to Class I handheld or portable equipment through sockets or not through sockets (see 7.1.1.1 ). Table 1 The maximum cut-off time of TN system
nominal voltage U to ground. , V
110(120)
220(230)
(277)
380(400)
>380(>400)|| tt||Note: ①The voltage value in brackets is the value specified by IEC38. Maximum cut-off time, 8
0.8
0. 4
0. 4
0.2
0.1
②Voltage deviation stated in IEC38 For voltages within the range, the cut-off time should be considered and selected based on the nominal voltage. ③For the voltage between the two levels, use the cut-off time corresponding to the corresponding higher level voltage in the table. 7.1.3.5 The cut-off time of the following circuits is allowed to exceed the provisions of Table 1, but shall not exceed 5s: a. Distribution circuit;
b. The terminal circuit that only supplies power to fixed equipment, on the switchboard that supplies power to the circuit There is no end circuit mentioned in Article 7.1.3.4;
An end circuit that only supplies power to fixed equipment, and the switchboard that supplies power to the circuit is connected to a section 7.1.3.4 that cuts off according to the cut-off time specified in Table 1. The terminal loop described in Article 1, but one of the following conditions has been met: Z.) ohm; or
50
The impedance of the protective conductor between the contact point of the distribution board and the main equipotential bonding does not exceed (U| |tt|| Make equipotential bonding at the switchboard. The bonding range includes the same external conductive parts as the main equipotential bonding. It should comply with the requirements for main equipotential bonding specified in 7.1.2.1. Note: See 7.1.3.9. Article Note.
7.1.3.6 If the overcurrent protector cannot satisfy Articles 7.1.3.3, 7.1.3.4 and 7.1.3.5, Article 7.1.2.2 and GB14821.1-93||tt| Article 7.1.6 implements auxiliary equipotential bonding, and residual current protectors can also be used for protection. 7.1.3.7 When a direct short circuit between the phase line and the ground may occur (such as an overhead line system), in order to protect the conductor and the phase. The voltage to ground of the connected exposed conductive parts does not exceed the agreed voltage limit of 50V, and the following conditions must be met: R
50
≤. Where: Rs—all grounding. Parallel grounding resistance of poles, a,...(2)
Rε - the minimum contact resistance value to ground of an external conductive part not connected to a protective conductor (through which a phase-to-ground fault may occur), Q| |tt||U. Nominal voltage to ground V.
7.1.3.8TN system can use the following protective appliances: a. Overcurrent action protector;
b. Residual current action protector.
In the TN-C system, the residual current operating protector shall not be used in the TN-CS system. When the residual current operating protector is used in the TN-CS system, the PEN conductor shall not be used at its load end, and the connection between the protective conductor and the PEN conductor shall be at the remaining It is carried out on the power supply side of the current-operated protector. 7.1.3.9 Outside the affected area of ??the main equipotential bonding, when the residual current-operated protector is used to automatically cut off the power supply, the exposed conductive part shall not be connected to the protective conductor of the TN system, but must Connect them to a ground electrode with a resistance value that is compatible with the operating current of the protector. The circuit protected in this way is regarded as a TT system and should comply with Article 7.1.4. Note: Outside the main equipotential bonding area. The following protective measures can also be taken in the local area: power supply by isolation transformer
a.
b. Use of additional insulation (see 7.2)
7.1.4 Protection of TT system||tt. ||7.1.4.1 All exposed conductive parts of devices and equipment protected by the same protective electrical appliance must be connected together with protective conductors and connected to a common ground electrode. When several protective electrical appliances are used in series, each protected electrical appliance must be connected to a common earth electrode. The exposed conductive parts protected by electrical appliances should be connected together with protective conductors and connected to their respective ground electrodes. The neutral point of the power system must be grounded. If there is no neutral point, each generator or transformer must have a One phase wire is grounded. 7.1.4.2 The system should meet the following conditions:
R·I.≤50V
In the formula: Ra - the resistance sum of the protective conductor of the exposed conductive part and its grounding electrode,; I. The current that causes the protective appliance to automatically cut off, A. When a residual current operating protector is used, I. is the rated residual operating current I. .(3)
S-type residual current action protector can be used in conjunction with ordinary type residual current action protector. In the distribution circuit, in order to ensure the selectivity of protective appliances, the S-type residual current action protector is allowed to operate within 1 second. When an overcurrent operating protector with inverse time characteristics is used, I. is the operating current that ensures automatic cutoff within 5s. When using an overcurrent action protector with instantaneous tripping characteristics, 1. The minimum current to ensure instantaneous tripping. 7.1.4.3 When Article 7.1.4.2 cannot be met, auxiliary equipotential bonding specified in Articles 7.1.2.2 and 7.1.6 shall be adopted. 7.1.4.4TT system can use the following protective appliances: a. Residual current action protector:
b. Overcurrent action protector (only applied to TT systems with very low RA values). 7.1.5 Protection of TT system
GB14821.1-93
7.1.5.1 The power supply system is insulated from the ground or grounded with a sufficiently high impedance. The ground point is usually a neutral point or an artificial neutral point. If the zero sequence impedance is high enough, the artificial neutral can be connected directly to ground. When there is no neutral point, one phase wire can be grounded through impedance. When a single fault occurs to a relatively exposed conductive part or to the ground, the fault current is very small. When Article 7.1.5.3 is met, it is not necessary to cut off the power supply. At this time, measures must be taken to avoid simultaneous human contact when double faults occur at the same time. Different conductive parts lead to dangerous pathophysiological reactions in the human body. 7.1.5.2 The live conductors of the device shall not be directly grounded. Note: In order to suppress overvoltage or attenuate voltage oscillation, grounding can be done through impedance or artificial neutral point when necessary. When taking this measure, its technical characteristics should meet the device requirements.
7.1.5.3 Exposed conductive parts shall be grounded individually, in groups or collectively. Note: In large buildings, such as high-rise buildings, it is practically impossible to connect the protective conductor directly to the earth electrode. In this case, the grounding of the exposed conductive part can be achieved by the connection between the protective conductor, the exposed conductive part and the external conductive part. The following conditions must be met:
R·I≤50V
where: Ra—the grounding resistance of the exposed conductive part, Q; (4)
Ia—one phase line and The fault current at which a first fault occurs between exposed conductive parts with negligible impedance, A. The Ia value takes into account the influence of the leakage current and the total ground resistance of the electrical installation. 7.1.5.4 If an insulation monitor is installed to monitor the first fault between live parts and exposed conductive parts or the earth, the equipment must be able to emit acoustic and/or optical signals.
Note, ①The first fault should be eliminated in the shortest possible time. ② In addition to indirect contact protection, insulation monitors may also be installed for other reasons. 7.1.5.5 The protection conditions for cutting off the power supply when a second fault occurs after the first fault depends on the connection between the exposed conductive part and the protective conductor as follows:
a. When the exposed conductive part When grounding individually or in groups, it should be determined according to the TT system in Article 7.1.4, except for the second paragraph of 7.1.4.1.
b. When the exposed conductive parts are connected to each other with protective conductors and centrally grounded, the protection conditions shall be determined according to the TN system, and the provisions of Article 7.1.5.6 shall be implemented.
7.1.5.6 When the IT system is not equipped with a neutral conductor, the following conditions must be met: z
21.
When a neutral conductor is equipped, the following conditions must be met zI
U.
In formulas (5) and (6): U. — Nominal voltage AC value between phase line and neutral line, VZ, — Fault loop impedance including phase conductor and protective conductor, Z, — Fault loop impedance including neutral conductor and protective conductor Impedance,; (5)
(6)
I. 1 - The operating current of the protective appliance required when the cutting time complies with the provisions of Table 2. For other circuits, it is allowed to be cut off within 5s (see 7.1.3.5).
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