First, protect the ground.
In a three-phase power supply system with ungrounded neutral point, when an electrical device connected to the system is electrified due to insulation damage, if a person stands on the ground and touches the shell with his hand, because there is a distributed capacitor between the power line and the ground, the current will flow back to the power supply through the human body and the distributed capacitor, which will make people get an electric shock, as shown in Figure 6-7- 13. Under normal circumstances, this current is not large. However, if the power grid is widely distributed or the insulation strength of the power grid is obviously reduced, this current may reach a dangerous level, so safety measures must be taken.
Fig. 6-7- 13 The motor without protective grounding touches the shell.
Protective grounding is to reliably connect the metal shell of electrical equipment with the earth with a thick enough metal wire. After protective grounding measures are taken for electrical equipment, the equipment shell has been in good contact with the earth through wires, so when the human body touches the live shell, the human body is equivalent to a parallel branch of grounding resistance, as shown in Figure 6-7- 14. Because the human body resistance is much greater than the grounding resistance, the current passing through the human body is very small, thus avoiding electric shock accidents.
Fig. 6-7- 14 motor collision case with protective grounding.
Protective grounding is suitable for distribution systems with ungrounded neutral points.
Second, protect the zero point
(A) the concept of protection by zero
The so-called zero protection (also known as zero protection) is to connect the uncharged metal parts of electrical equipment with the zero line in the neutral grounding system. Fig. 6-7- 15 is a schematic diagram of fault current when the protection is connected to zero. When a phase insulation damage causes the phase wire to touch the shell and the shell is charged, the phase wire and the neutral wire form a loop due to the protective measures taken by the shell, and the single-phase short-circuit current is large enough to make the protective device (such as fuse) on the line fuse quickly, thus disconnecting the leakage equipment from the power supply and avoiding the possibility of personal electric shock.
Figure 6-7- 15 Protection Zero Connection
380/220V, three-phase four-wire distribution system adopts protective neutral wiring, and the neutral point of power supply is directly grounded.
In the power distribution system with neutral grounding, only protective grounding can be used. If protective grounding is used, personal electric shock accidents cannot be effectively prevented. As shown in figure 6-7- 16, if protective grounding is adopted, the neutral grounding resistance of power supply and the grounding resistance of electrical equipment are considered as 4 Ω, and the power supply voltage is 220V, then when the electrical equipment shell is electrified due to insulation damage, the current between the two grounding resistances is:
Figure 6-7- 16 Consequences of Protective Grounding in Neutral Grounding System
The rated current of fuse melt is selected according to the requirements of protected equipment. If the equipment is easy, in order to ensure the equipment to work under normal conditions, the rated current of the selected melt will be larger. Under the action of 27.5A grounding short-circuit current, it will continue to fuse, and the electrical equipment with charged shell cannot be separated from the power supply immediately, so there is a long-term ground voltage Ud on the equipment shell, and its value is:
Ud=27.5×4= 1 10V
Obviously, this is very dangerous. If the protective grounding resistance is greater than the neutral grounding resistance of the power supply, the grounding voltage of the equipment shell is higher and more dangerous.
(2) Problems that should be paid attention to when the system adopts protection to connect to zero.
1. Zero line plays a very important role in protecting neutral point system. Once the zero line is disconnected, the electrical equipment connected to a section of the line behind the disconnection is equivalent to zero connection or protective grounding, and there is no protection. If there is leakage in the shell of some electrical equipment after the neutral line is broken, which does not constitute a short circuit, then the fuse will blow. Not only will the shell of this equipment be charged for a long time, but also the shells of all protective neutral devices connected behind the circuit breaker will have a grounding voltage close to the phase voltage of the power supply, and the risk of electric shock will increase, as shown in Figure 6-7- 17(a).
For single-phase electrical equipment, even if the shell has no leakage, when the neutral line is disconnected, the phase voltage will appear on the shell of the electrical equipment through a section of zero line behind the load and the disconnection point, as shown in Figure 6-7- 17(b).
Fig. 6-7- 17 consequences of zero line disconnection when protection is connected to zero.
The neutral connection shall be firm and reliable with good contact. The connecting line between the zero line and the equipment shall be bolted. All electrical and gas equipment should be connected in parallel on the zero line, and series connection is not allowed. It is forbidden to install a fuse or a separate cut-off switch on the zero line. In the environment with corrosive substances, in order to prevent the corrosion of neutral wire, the surface should be coated with necessary anti-corrosion coatings.
2. In the three-phase four-wire distribution system where the electrical point of the power supply is not grounded, protective grounding is not allowed, only protective grounding can be used.
In the distribution system with neutral grounding, when a phase line is in contact with the earth, the fuse can be blown by the short-circuit current between the grounded phase line and the power neutral grounding device, and the faulty line can be cut off immediately. However, in the distribution system where the neutral point is not grounded, the system can still operate as usual despite any grounding. But at this time, the earth and the grounding phase pin are equipotential, and the voltage of the shell of the electrical equipment connected to the zero line will be equal to the voltage of the grounding phase line from the grounding point to the neutral point of the power supply, which is very dangerous, as shown in Figure 6-7- 18.
Fig. 6-7- 18 consequences of zero connection of protection in neutral ungrounded system
3. When taking protective measures, it must be noted that it is not allowed to connect one part of the equipment to zero and the other part of the electrical equipment to ground on the same system.
In Figure 6-7- 19, the equipment whose shell is grounded contacts the shell and leaks. When the fuse is blown by accident current, the equipment shell is charged 1 10V, and the potential of the whole neutral line to the ground is raised to 1 10V, so the shells of other equipment with zero connection are/kloc-0. Therefore, it is not allowed to use the mixed method of connecting some devices to zero and grounding some devices in the same system. Even if the fuse meets the fuse requirements, it is not allowed to be mixed. Because the fuse is often replaced in use, it is difficult to ensure that there is no mistake.
Figure 6-7- 19 Error Zero Protection
4. In the system with zero protection, it is necessary to conduct working grounding at the neutral point of the power supply, and repeat grounding at certain intervals and terminals of the zero line.
In the three-phase four-wire distribution system, the secondary neutral point of the distribution transformer is directly connected to the earth through the grounding device, which is called working grounding. Grounding the neutral point of power supply can reduce the grounding voltage of each phase power supply. When people touch the phase power supply, the human body is exposed to the phase voltage. In an ungrounded neutral system, when one phase line is grounded and the human body touches another phase line, it is the line voltage of the power supply that acts on the human body, which is very dangerous. At the same time, the neutral point of distribution transformer is grounded, which provides the necessary conditions for adopting the protection zero connection method. The grounding resistance of working grounding should not be greater than 4Ω, as shown in Figure 6-7-20.
Figure 6-7-20 Schematic Diagram of working grounding
(a) ungrounded power neutral system (b) ungrounded power neutral system
In the neutral grounding system, in addition to the distribution transformer neutral working grounding, one or more places along the neutral line will be grounded again, which is called repeated grounding.
The function of repeated grounding is to reduce the voltage of neutral line to ground when the shell of electrical equipment leaks electricity; If the neutral line is broken, the risk of electric shock can also be reduced.
When the equipment shell leaks electricity, as mentioned above, a short circuit loop is formed through the phase line and the zero line. Short-circuit current can quickly blow the fuse, cut off the circuit, and the metal shell is dead, thus avoiding the danger of electric shock. However, it takes a short time from the leakage of the equipment shell to the fuse blowing. In this short time, there is a voltage to ground in the equipment shell, which is the voltage drop of short-circuit current on the zero line. In this short time, it is still very dangerous if someone touches the equipment shell. If a grounding device is added near the equipment, that is, repeated grounding is implemented, as shown in Figure 6-7-2 1, the grounding voltage of the equipment shell can be reduced.
Figure 6-7-2 1 repeated grounding
In addition, if there is no repeated grounding, when the zero line is disconnected somewhere, all electrical equipment after disconnection is in a state of neither protective grounding nor protective grounding. Once a phase power source collides with the shell, the neutral line behind the disconnection and the shell of the electrical equipment connected with it will have a grounding voltage equal to the phase voltage, which is very dangerous, as shown in Figure 6-7-22.
Figure 6-7-22 Zero line break without repeated grounding
In the case of repeated grounding, when the neutral line is accidentally disconnected and the electrical equipment shell is charged, the phase voltage passes through the leaky equipment shell, forming a loop of repeated grounding resistance and working grounding resistance, and current flows, as shown in Figure 6-7-23. The grounding voltage of leakage equipment shell is the voltage drop of phase voltage on repeated grounding resistance, which reduces the risk of accidents, but it is still dangerous to people. Therefore, neutral wire disconnection accidents should be avoided as far as possible.
Figure 6-7-23 Zero line is broken when grounding is repeated.
In the line with neutral protection, the terminals of the trunk and branch lines of overhead lines and the zero line along each kilometer should be grounded again. When cables and overhead lines are introduced into buildings, neutral wires should also be grounded repeatedly. However, if there are no special requirements, buildings not more than 50m away from the grounding point may not be repeatedly grounded.
Third, the scope of application of protective zero connection and protective grounding
Protective grounding or protective grounding measures should be taken for the metal parts of the following electrical equipment.
(1) Bases and housings of motors, transformers, electrical appliances, lighting appliances, portable and mobile appliances, etc.
(2) the transmission device of electrical equipment;
(3) Secondary winding of voltage and current transformer;
(4) the framework of power distribution panel and control panel;
(5) Metal frames, reinforced concrete main bars and metal fences of indoor and outdoor power distribution devices;
(6) Bushing of screw pipe, metal junction box, cable head and box;
(seven) the power line tower equipped with lightning conductor and the shell of switchgear and capacitor installed on the distribution line tower.