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Indirect contact with electric shock protection
Jan 17, 2018

Indirect contact with electric shock protection

First, the indirect contact electric shock protection


(1) Under normal circumstances, the electrical equipment is not exposed parts of the metal, such as metal, metal shield and metal framework, in the event of leakage, shell and other metal short-circuit fault occurs when the dangerous contact voltage, this time Human exposure to these exposed metal parts, known as indirect contact with electric shock.


(2) In the case of electrical equipment, lines and other failures, in order to avoid personal injury caused by electric shock accidents, known as indirect contact with electric shock protection, or called the protection of indirect contact with the charged body.


(3) indirect contact with electrical protection measures are the following:


Automatic power supply cut-off (ground fault protection).


② The use of double insulation or reinforced insulation of electrical equipment (ie, Ⅱ grade electrical products).


③ will be dangerous places of electric shock insulation, constitute a non-conductive environment.


④ use of non-grounded local equipotential bonding protection, or to take equipotential pressure measures.


⑤ using special ultra-low voltage.


⑥ implementation of electrical isolation.


Second, the neutral point and zero, neutral line and the difference between the zero line


When the power supply side (transformer or generator) or the load side of the star-shaped wiring, the first end of the three-phase coil (or tail) connected together as a common joint contact points, referred to as the midpoint. Neutral point Neutral point and load neutral point. By the neutral point leads to the neutral line, referred to as the center line.


If the neutral point and the grounding device is connected directly to obtain the earth's reference zero potential, the neutral point is called the zero point, the wire from the zero lead is called the zero line.


Usually 220 volt single-phase circuit two lines in one called "phase line" or "FireWire", while the other line is called "zero line" or "ground." "FireWire" and "ground" in the name, but a common practice in the name, in particular, "ground" is not accurate. Strictly speaking, it should be, if the circuit power side (three-phase distribution transformer neutral point) is grounded, called "zero line"; if not grounded, it should be called "center line" in order to avoid grounding device Line ".


When the three-phase line, in addition to three phase lines, but also leads from the neutral point of a wire, the neutral line, which constitutes three-phase four-wire circuit. This line between the phase line voltage, known as the line voltage, phase line and the voltage between the zero line is called the phase voltage.


Neutral point is grounded, also known as the neutral point system. Neutral point system can be broadly divided into two categories, namely, neutral point grounding system and neutral point insulation system. In accordance with the provisions of the International Electrotechnical Commission (IEC), the low-voltage distribution system is divided into IT, TT, TN three kinds, of which TN system is divided into TN-C, TN-S, TN-C-S three categories.


Third, to protect the ground


The so-called protective earth, that is, the electrical equipment in case of failure may be dangerous to the ground voltage of the metal parts (such as the shell, etc.) with the earth and the electrical connection.


If the electrical equipment is not earthed, the enclosure will be energized when a part of its insulation is damaged and the human body will be exposed to an electrical shock hazard if the enclosure of the electrical equipment damaged by this insulation is present due to the capacitance between the line and the earth. After the protection of electrical equipment grounding, the ground short-circuit current will be along the grounding body and the body through the two channels. Grounding resistance is generally 4 Europe the following, and the human resistance is about 1000 Europe, so the role of the shunt through the grounding body flows through the body's current is almost equal to zero, thus avoiding the short-circuit fault current electric shock hazard.


Fourth, protection to zero


Protection access zero referred to as zero, is the electrical equipment in the normal case of non-charged metal parts (shell), with wires and low-voltage distribution network of zero line (neutral line) directly connected to protect personal safety, to prevent Electric shock accident.


Protection against zero generally with the fuse, release, etc., as a low-voltage direct-neutral grounding system and 380/220-volt three-phase four-wire system (in the IEC standard called TN-C system) .


With the protection of access to zero, when the shell hit short-circuit, the short-circuit current from the phase line through the shell to the zero line, and then return to the transformer neutral point. As the fault circuit resistance, reactance is very small, so the fault current is great, it is enough to make the line protection devices (fuses or automatic switch) rapid action, which will disconnect the power supply equipment, eliminate the danger, play protection effect.


Although the protection of the ground and the protection of access to zero can guarantee personal safety, but the protection of zero access to more than the protection of the ground, because the zero line impedance, short circuit current, thus overcoming the protective grounding requirements of its resistance is very small limitations .


Fifth, the protection of grounding and protection of the difference between zero


The main difference between protective earth and protection is:


(1) the protection of different protection grounding principle is to limit the equipment after the earth-leakage voltage, so that does not exceed the safety range. In the high-voltage system, the protection of grounding in addition to limiting the voltage on the ground, in some cases, but also to promote the action of the power grid protection device; protection is connected with the zero line through the equipment to form a single-phase short-circuit leakage to promote the line Protection devices, as well as the power to cut off the faulty device. In addition, the protection of zero-line network, the protection of zero and repeat the ground can also be prepared to limit leakage when the ground voltage.


(2) The scope of application of different protection Grounding both for general non-grounded high and low voltage power grid, also applies to take other safety measures (such as the installation of leakage protection) low-voltage power network; protection is only applicable to the neutral zero direct Grounded low-voltage power grid.


(3) different line structure If the protective earthing measures, the power grid can be zero line of work, only set protection grounding; If the protection of access to zero measures, you must set the work of the zero line, the use of zero for zero line protection work. Protection should not be connected to the zero line switches, fuses, when working in the zero line on the installation of fuses and other breaking appliances, it must also be installed to protect the ground wire or a zero line.


Sixth, which parts of the electrical equipment should be protective grounding or zero


Where under normal circumstances without electricity, and when the insulation damage, hitting the shell short-circuit or other failures, there may be charged parts of the electrical equipment and its accessories should be grounded or access to zero. These metal parts or accessories include:


(1) Metal casing or base for motors, transformers, circuit breakers and other electrical equipment.


(2) the distribution screen (plate) and the control panel (Taiwan) frame, change, distribution of the metal frame and close to the live part of the metal barrier and metal doors, reinforced concrete frame reinforcement.


(3) metal wire and cable protection and metal skin, AC and DC power cable junction box and the terminal box of the metal shell, bus protective cover and protection network.


(4) lighting fixtures, electric fans and electric equipment, metal base and shell, crane track.


(5) overhead lines and overhead lines of the metal tower, and installed in the tower on the switch, capacitors and other shell and bracket.


(6) the current transformer and voltage transformer secondary winding.


(7) The use of a portable power tool or portable electrical equipment which does not use an isolating transformer, which exceeds the safe voltage.


(8) electrical equipment, transmission.


(9) arrester, protection gap, lightning rod and coupling capacitor base.


7, which parts of electrical equipment, metal parts may not be protective grounding or zero


The following metal parts of electrical equipment, unless otherwise specified, generally do not protect the ground or access to zero:


(1) the safety of the dry place or below the safe voltage voltage (AC 50 volts or less, 110 volts DC) electrical equipment shell.


(2) Enclosures for electrical measuring instruments, relays and other low-voltage electrical equipment mounted on a panel (screen), control panel (s) and electrical distribution frame.


(3) overhead lines on the wood and substation outdoor wooden frame on the insulation porcelain fittings, and insulation damage will not endanger the safety of the insulator metal base.


(4) Grounding support for insulators and bushings on the grounded metal frame, electrical machinery and electrical enclosures on grounded machines.


(5) non-conductive place of low-voltage electrical equipment (including the connection with the metal mechanical equipment, etc.) shell.


(6) is equipped with a high sensitivity to ensure personal safety electrical leakage protection device.


8, to protect the color of the ground wire


China's past production of electrical products, the ground wire are marked with black, this sign has been eliminated. At present, China has implemented international standards, the use of yellow, green color insulated wire as a protective grounding wire. Yellow and green color is the International Electrotechnical Commission to protect the grounding line for the special color standard, has been internationally accepted, our country also in the corresponding standard specifies the use of this color standard. However, Japan, Western Europe, a number of countries using a single green line as the protection of ground wire, so some of our exports of electrical products in these countries also use a single green line as a protective grounding. If you use this export to the domestic electrical products, we must pay attention to, do not take the wrong color of the grounding wire connected to the wrong line, resulting in electric shock. If the protective grounding wire color code is difficult to judge, you should view the instructions, or open identification, or judge with a multimeter.


Nine, in the low-voltage distribution system, the use of protective access to zero should pay attention to matters


In the 1 kV power distribution system, the use of protective access to zero should note the following:


(1) three-phase four-wire low-voltage power supply neutral point must be well grounded, work grounding resistance should meet the requirements to ensure the zero line from work and protection of dual role.


(2) must be in place to repeat the ground zero to prevent zero line break caused danger.


(3) Zero-line circuit shall not be fitted with switches and fuses to prevent the zero line disconnect when the zero-voltage equipment to produce dangerous ground voltage; but not for the protection of the zero line, or another protection connected to the zero line , Can be in the circuit phase and zero line switch and fuse.


(4) on the zero line laying requirements and the same phase line to prevent the zero line disconnection failure.


(5) the protection of all electrical equipment, then the zero line should be connected in parallel to the zero trunk.


(6) in the access system does not allow any device to implement protective grounding.


(7) the zero line of the current carrying capacity should be greater than the phase current carrying capacity of at least 1/2.


(8) the minimum cross-section of the zero line shall not be less than the minimum cross-section specified in the order to ensure that the zero line can withstand short-circuit fault current and automatically remove the faulty equipment power.


10, automatic cut off the basic principles of power supply


Automatic cut off the power supply of the protection measures, refers to the use of appropriate switching devices, when the equipment insulation damage, within a specified time automatically cut off the power to damage the equipment to prevent danger of exposure to dangerous human exposure to electric shock caused by too long . Therefore, the basic principle is: any failure of electrical equipment generated by fault current, to ensure personal safety in the time required to be cut off in time.


This protective measure is premised on the following two interrelated conditions:


(1) the current flow path, or "fault loop", that fault power flow in the loop. The composition of this loop, and distribution system type (or neutral point system). Typically, the use of TN, TT, IT various different systems fault loop is different.


(2) cut off the fault current time, that is, fault occurs, within a certain time cut off the fault current to protect personal safety. The time to cut off the fault current is a function of many factors. For example, the probability of failure occurs, the failure of the human body touched the probability of the device shell. But the key is the human body touches the charged shell, the size of the contact voltage may suffer


11, in accordance with the provisions of the IEC standard work system of the neutral point


According to IEC standards, low-voltage distribution system, there are three neutral point work system:


(1) The neutral point of the TN system power supply is directly grounded, and the exposed conductive part (metal case) of the load device is connected with the grounding point through the protective conductor. TN system is divided into three types: TN-S system, in the whole system, the neutral conductor and the protective conductor is strictly separated from the conductor , That is, the so-called single-phase three-wire and three-phase five-wire system; TN-CS system, the system has part of the neutral conductor and the protection of the conductor function in a conductor, the other part of the neutral conductor and protection body is TN-C system, in the whole system, the neutral conductor and the protection of the conductor function together in a conductor, that is, commonly used in China access zero protection system.


The second letter "N" indicates that the exposed conductive part of the device is directly connected to the ground of the power supply system. The second letter "N" indicates that the power supply system is directly connected to a point (or neutral point) The letter "S" indicates that the neutral conductor and the protective conductor are separated; the letter "C" indicates that the neutral conductor and the protective conductor function together on one conductor.


(2) TT system power system has a point directly to the ground, the equipment exposed conductive part of the grounding and power system grounding is not associated with electricity, China is called the protective grounding system.


The first letter "T" indicates that the point in the power system is directly grounded; the second letter "T" indicates that the grounding of the exposed conductive part of the device is not electrically related to the ground of the power supply system.


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