Prevention of "zero breaking" and safe grounding
the main reason for a large number of burnout of equipment in a phase is the disconnection of the neutral line of three-phase four wire circuit (including the neutral line of TT system and the pen line or neutral line of TN System). In China, it is often called "breaking zero"
"zero breaking" prevention
a large number of burnout of one or two phase equipment sometimes occurs in the three-phase four wire distribution circuit. Sometimes peers think that this is caused by the imbalance of three-phase voltage caused by the imbalance of three-phase load. The voltage of one phase with light load is the highest, which causes a large number of equipment in this phase to burn out. This explanation only explains part of the reasons, but it also has deeper reasons. The main reason for a large number of burned out equipment in a phase is the disconnection of the neutral line of the three-phase four wire circuit (including the neutral line of TT system and the pen line or neutral line of TN System). In China, it is often called "breaking zero"
I. the harm of "zero breaking"
the harm of "zero breaking" can be simply analyzed in Figure 1. In the figure, phase line L1 has no load, L2 has -150 incandescent bulbs, and L3 has -15w incandescent bulbs. The three-phase load is very unbalanced. If the three-phase voltage is measured with a voltmeter, if the neutral line is not disconnected, it will be found that the three voltages are not much different. This is because these three phases are powered by the same 220V winding voltage. Their voltage difference is expected to replace metal and solid plastic components. The difference is that different load currents on the three phase lines produce different voltage drops. According to the specification, the total voltage drop of phase line and neutral line is not more than 5%, so only the three-phase load imbalance will not burn out the equipment in a certain phase
first assume that the neutral line in front of the incandescent bulb is interrupted for some reason, as shown in Figure 1, then 150W and 15W bulbs are connected in series and then connected to a 380V single-phase circuit. We know that the incandescent bulb is basically a resistive load, and its resistance R is inversely proportional to the power P, that is, R ∝ 1/p
therefore, if the resistance of 150W bulb is r, the resistance of 15W bulb is 10R, so the 380V voltage is distributed between the two bulbs in the proportion of 1 and 10. The voltage on the 150W bulb is only 35V, while the voltage on the 15W bulb is as high as 345v, which was burned out soon. For further analysis, the voltage phasor diagram of Figure 1 can be drawn, as shown in Figure 2. It can be seen from the figure that the phase to phase voltage of the three-phase circuit is still 380V, the center point of the load side is transferred from point O to point o ', the neutral line to ground voltage is 190V (in TN system, it can cause electric shock accident), while the no-load L1 phase voltage is as high as 364v, and the three-phase voltage is extremely unbalanced
the life t of incandescent lamp is inversely proportional to the 14th power of applied voltage u, that is, the higher the applied voltage is, the shorter the bulb life is. It should be noted that the service life of incandescent lamp refers to the service time when the luminous flux is reduced to 70% of the rated luminous flux. If the normal life of 15W bulb in Figure 1 is 1000h, its life will be reduced to 1.9h according to the above formula. In this case, the service life of TV picture tube filament will inevitably be reduced to a great extent. Of course, many adult fans of TV are also obsessed with Lego building blocks. When the voltage is too high, it will be heated due to increased iron loss, and when the voltage is too low, it will be heated due to increased copper loss, which will accelerate the aging of engine insulation and shorten the service life. Therefore, the insulation life of the engine will inevitably be shortened no matter the voltage is high or low, but its sensitivity to the voltage is not as serious as that of the incandescent bulb
the danger of "zero breaking" burning the equipment is still hidden, because although the service life of the equipment is shortened after "zero breaking", the bulb is still on and the motor is still running for a period of time at the beginning. It is difficult for people to eliminate the fault even if it is found. It is not until a large number of equipment are burned that it is caused by "zero breaking". At this time, it is too late
II. Prevention of "zero breaking" burning out equipment accident
many peers believe that after the neutral line is repeatedly grounded, using the earth path instead of the interrupted neutral line as the path to return to the power supply can avoid burning equipment accidents. Through phasor analysis and calculation, it is impossible. Because the neutral line impedance is measured in several milliohms, while the earth path impedance is measured in several ohms, there is a great difference. After "breaking zero", the three-phase voltage is still seriously unbalanced, but to a lesser extent
according to foreign experience, the accident of "breaking zero" burning equipment can not be prevented by cutting off the power supply of switch type protective appliances. Various measures can only be taken in the selection and laying of roads to minimize the occurrence of "breaking zero" to prevent "breaking zero" burning equipment. For example, IEC standard stipulates that pen wire can only be used in fixed electrical devices. No matter how small the section of phase wire is, the section of pen wire shall not be less than 10 mm2 copper wire or 16 mm2 aluminum wire, so as to ensure its mechanical strength and prevent "zero breaking". For example, the wire of a three-phase four wire circuit can be used as 3 × 4mm2+1 × 10mm2 copper wire. The section of pen line is not 1/3 or 1/2 of that of phase line, but 2.5 times. This is because if the pen line is broken in the TN system, not only the electrical equipment will lose grounding, but also a large number of single-phase equipment will be burned out due to "zero breaking", with very serious consequences
the United States pays the most attention to the prevention of residential "zero breaking" accidents, because residential electrical accidents are most likely to occur. No matter how much residential power consumption, they all use single-phase distribution transformers to innovate. The State encourages the development of major environmental protection technology and equipment, and the implementation and utilization methods and channels use high-voltage single-phase circuit power supply instead of three-phase circuit power supply. In this way, the equipment burning accident caused by "zero breaking" of three-phase four wire circuit is fundamentally eliminated
under the condition that low-voltage three-phase four wire power supply is widely used in China, in order to prevent the accident of "zero breaking" burning equipment, the following points should be paid attention to in the design, installation and management of electrical lines:
in the three-phase four wire circuit, the cross section of neutral line and pen line should be appropriately enlarged to ensure their mechanical strength, especially the overhead lead-in line from the pole to the power inlet of the building should be no less than 10mm2 copper wire according to the specification requirements, Aluminum wire is not less than 16mm2
take effective measures to prevent the neutral wire from bearing excessive tension
pay attention to the connection quality of neutral wire connector to ensure good conductivity of neutral wire connector. Special attention should be paid to improving the connection quality of aluminum wire, because the surface of aluminum wire is very easy to be non-conductive due to oxidation or corrosion
try to reduce the connection of line terminals and connectors on the neutral line, and try not to connect switches and contacts in series, so as to prevent the risk of "zero breaking" due to poor contact
it is strictly forbidden to connect the fuse in series on the neutral line of the three-phase four wire circuit to prevent the fuse from fusing for various reasons and forming "zero breaking"
ground wire: guarantee of safe life
since taking part in the work, through continuous learning and practice in practical work, as well as continuous learning with experienced engineers in the project, we have realized the importance of safety in the project, not only in structural construction, but also in power distribution construction. If something happens, it will be a major safety accident, which may touch on personal injury and death accidents, which will give the country, Enterprises and individuals have brought immeasurable losses. Therefore, in terms of technology, we should first make the design reasonable and try our best to avoid accidents
now let's talk about some understanding of the ground wire in the electrical system. If we connect multiple electronic devices to the electrical system, the quality of the power supply becomes very important. "Quality" can have many meanings. Stable voltage and no abnormal waveform are the two most expected indicators of the electrical system. Grounding will affect voltage stability, which is very important for personal safety
ground wires connect all live parts of the electrical system or all exposed metal parts near the electrical system. Including metal pipes, shells, supports and other metal objects. This grounding system has two purposes:
1 The safety
grounding system provides a low impedance path for fault current. This allows overcurrent protection devices (fuses and circuit breakers) to detect all current and quickly and safely clear faults
2. Power quality
the grounding system allows all equipment to have the same reference voltage. This can help electronic equipment operate normally and prevent harmful current from flowing on communication lines, insulation or other connections
now let's further study security. Consider that the following systems include a power system with a voltage source (transformer or generator) connected to an disconnector or switchboard. A consumer is powered from this switchboard. Once the circuit is formed, current flows through the consumer to make it operate. The ground wire connects the frame of the consumer to the enclosure of the switchboard. This kind of shell is connected to the ground wire (usually the neutral line), and then connected to the grounding terminal of the conductor transformer. In case of fault (or short circuit), the wire allows current to pass. This current is much larger than the usual load, which can make the circuit breaker jump immediately. In this way, the fault is safely cleared, and the safety infringement on human body is minimized
suppose the ground wire is broken. If a fault occurs, no current passes through the ground wire due to the disconnection of the grounding circuit. The disconnection of ground wire may be caused by illegally cutting a plug, loose connection, incorrect connection of metal pipe or many other reasons. This kind of fault uses the shell of electrical equipment to be electrified. If you touch the electrical equipment and the steel structure of the building, or other pipeline systems, or even the wet concrete floor at the same time, a circuit will be formed, and the current will flow through the human body, causing injury or electrocution
many specifications allow the use of metal tubes as ground wires. Many current designers no longer believe that metal tubes are suitable for this application. Metal pipes have one connection every 10 feet, and low-grade cast iron joints or connectors are often used. For long branch circuits, the impedance of metal tubes may limit the fault current, so that the overcurrent protection device cannot work correctly. For this reason, we believe that copper grounding wire should be used in each power circuit
the second advantage of this copper ground wire is that it can provide equipotential connection for all connected equipment. This often makes the so-called isolated ground wire required by computers or other manufacturers unnecessary