Electrical safety knowledge

Electrical safety knowledge

Safe Voltage When the human body is in contact with electricity, the voltage that does not cause any damage to tissues of the human body (such as the skin, heart, respiratory organs, and nervous system) is called a safe voltage.
The regulations for safe voltage values ​​vary from country to country. For example, the Netherlands and Sweden are 24 volts; the United States is 40 volts; the French exchange is 24 volts; the DC is 50 volts; and Poland and Czechoslovakia are 50 volts.
According to the specific environmental conditions, the safety voltage value is: 65 volts in buildings without high electric shock hazard; 36 volts in buildings with high electric shock hazard; in buildings with special electric shock hazard 12 volts.

Stepping voltage electric shock When a phase of grounding of a live device occurs, the grounding current flows into the earth, and different potentials appear at different points on the ground surface different from the grounding point. The height of the potential is related to the distance from the grounding point. The farther the distance is, the lower the potential is. .
When a person's foot and foot are simultaneously stepped on two points on the ground surface with different potentials, the stride voltage is electrically shocked. If you encounter such a dangerous situation, you should fold your feet 20 meters away from the ground to ensure personal safety.

Phase-to-phase electric shock The so-called phase-to-phase electric shock is when the human body is insulated from the earth, and when two different phase lines are touched at the same time or the human body simultaneously contacts two charged parts of different phases of the electrical equipment, the current flows from the body line to the other body. A phase line forms a closed loop. This situation is called phase-to-phase electric shock. At this time, the human body is directly under the action of the line voltage, which is more dangerous than single-phase electric shock.

Fatal current The minimum current that is life-threatening in a short period of time is called lethal current. In the case where the current does not exceed 100 mA, the main cause of electric shock is the current caused by ventricular fibrillation or suffocation. Therefore, it can be considered that the current causing ventricular fibrillation is a lethal current.

Dangers and related factors when the human body is exposed to electricity The dangers of human body electric shock are related to the following factors:
(1) When the human body is exposed to electricity, the fatal factor is the current through the human body, not the voltage, but when the resistance is constant, the higher the voltage, the greater the current through the conductor. Therefore, the higher the voltage that the human body touches the charged body, the greater the risk. But whether it is high pressure or low voltage, electric shock is dangerous.
(2) The duration of current flow through the human body is another important factor affecting the degree of electric shock damage. The longer the body passes the current, the lower the body resistance, the greater the current that flows, and the more serious the consequences. On the other hand, the human heart contracts and expands once, with an interval of about 0.1 second in the middle, which is most sensitive to current. If the current passes through the heart at this instant, even if the current is small, (zero mA) will cause heart tremor; if the current is not passed at this moment, even if the current is large, it will not cause heart paralysis. It can be seen that if the current duration exceeds 0.1 second, it will inevitably coincide with the most sensitive gap of the heart, posing a great danger.
(3) The way the current passes through the human body is also directly related to the degree of electrical injury. The current passes through the head of the human body, causing people to immediately stun. If the current passes through the spinal cord, the human limb will paralyze. The current through the heart, the respiratory system and the central nervous system will cause nerve malfunction or cause the heart to stop beating, interrupting the blood circulation of the whole body and causing death. . Therefore, the current path from hand to foot is the most dangerous. Second, it is the hand-to-hand current path, again the foot-to-foot current path.
(4) The current frequency has a great influence on the degree of electric shock damage. The power frequency AC of 50H Z is reasonable for designing electrical equipment, but the current of this frequency is also the most serious damage to human body.
(5) The health condition of the person, the dryness and humidity of the skin of the human body, etc. also have a certain influence on the degree of electric shock damage. Patients with heart disease, nervous system disease or tuberculosis have more severe electric shock damage than healthy people. In addition, the dryness of the skin is large, the current passing through is small, the skin's moisture resistance is small, the current passing through is large, and the hazard is also large.

Grounding Protection Grounding protection, often referred to as protective earthing, is to connect the metal casing of the electrical equipment to the grounding body to prevent the operator from touching the equipment casing and getting an electric shock when the casing is energized due to insulation damage of the electrical equipment. In a low-voltage system where the neutral point is not grounded, under normal circumstances, the exposed parts of the uncharged metal of various power devices shall be grounded unless otherwise specified. Such as:
(1) The outer casing of motors, transformers, electrical appliances, portable and mobile electrical appliances.
(2) Transmission of electrical equipment.
(3) The frame of the power distribution panel and control panel.
(4) Cable sheath and power cable junction box, and the outer casing of the terminal box.
(5) Metal protection pipes for power lines, laid steel cables and crane tracks.
(6) A tower equipped with a lightning arrester power line.
(7) The casing and bracket of an electric device such as a switch or capacitor mounted on a power line tower.
The requirements for the grounding resistance of the power device of the low-voltage power network are as follows:
(1) In the low-voltage power network, the grounding resistance of the power device should not exceed 4 ohms.
(2) In a low-voltage power network powered by a transformer with a capacity of 100 kV·A, the grounding resistance of the power device should not be greater than 10 ohms.
(3) In the low-voltage power network with the same grounding device operating in parallel and the total capacity not exceeding 100 kV·A, the grounding resistance of the power device should not exceed 10 ohms.
(4) In areas with high soil resistivity, the grounding resistance of low-voltage power equipment is allowed to increase to 30 ohms when it is difficult to achieve the above grounding resistance.

Contact potential, contact voltage, step potential and step voltage When the ground short-circuit current flows through the grounding device, the ground surface forms a distributed potential, which is at a horizontal distance of 0.8 m from the device on the ground surface and is perpendicular to the device casing, frame or wall. The potential difference between two points at a distance of 1.8 meters is called the contact potential. The voltage that the human body is exposed to when touching the two points is called the contact voltage; the maximum potential difference between the grounding grid center and the grounding grid grounding body is called the maximum contact potential, and the voltage that the human body is exposed to when contacting the two points is called Maximum contact voltage. The potential difference between two points on the ground with a horizontal distance of 0.8 m is called the step potential. The voltage that the human body is exposed to when touching the two points is called the step voltage; the potential difference of the grounding body at the grounding distance of 0.8 m from the ground outside the grounding net is called the maximum step potential, and the human body is two feet. The voltage that is applied to the two points is called the maximum step voltage.

General human body resistance When an electric shock occurs, the current flowing through the human body is determined by the ratio of the electric shock voltage to the human body resistance. Body resistance is not a fixed value. The electrical resistance of each part of the human body is removed from the stratum corneum, and the electrical resistance of the skin is the largest. When the human body is dry and undamaged, the body's resistance can be as high as 4-40 million ohms. If the skin is removed, the body resistance can be reduced to 600-800 ohms. However, the skin resistance of the human body is not fixed. When the skin is sweaty or damaged, the resistance will drop to about 1000 ohms.

Perceptual current
When holding the power supply with your hand, the DC current that feels hot in the palm of your hand, or the alternating current that feels a slight tingling due to nerve stimulation, is called the perceptual current. The subjects placed their hands on a small copper wire. The average current of the DC current was 5.2 mA for men and 3.5 mA for women.

Get rid of current
The current that can be rid of after the electric shock is called the current. According to the measurement results, the male power frequency is 9 mA and the female is 6 mA. When the power frequency current of 18-22 mA (the upper limit of the current) passes through the chest of the human body, the muscle reaction caused by the electric shock will stop the electric current during the energization time, the breathing can be resumed, and there will be no short breathing. Stop and cause adverse consequences.

Basic requirements for safety equipment in electrical equipment
It can be seen from the analysis of the causes of various electric shock accidents that the proportion of accidents caused by the failure of the electrical equipment in the structure and the equipment to meet the safety requirements is very large. Therefore, in order to prevent electric shock accidents in electrical work and ensure the safety of workers, electrical equipment should meet the following requirements in safety technology during design, manufacture, and installation:
(1) The equipment should be protected from grounding.
(2) The live part of the equipment maintains a certain safe distance from each other to the ground and other live parts.
(3) The protective neutral line should be installed in the low-voltage power system.
(4) Reliable protective measures should be taken for exposed equipment on the ground.
(5) Install safety signs in electrical equipment systems and related workplaces.
(6) Take special safety measures according to the characteristics and requirements of certain electrical equipment.

What is repeated grounding?
One or more points on the zero line are reconnected to the earth through the grounding device. The safety function is: in addition to the low-leakage equipment to ground voltage; to reduce the risk of electric shock when the neutral line is broken; shorten the duration of the shell or ground short circuit, improve Lightning protection performance of overhead lines.

Zero protection
In order to prevent the electrical equipment from being damaged by electrical insulation, the connection between the metal casing of the electrical equipment and the neutral point of the power supply transformer is called zero-connection protection. In a low-voltage power network where the neutral point is not directly grounded, the power device should be protected by low-voltage zero-connection. In a low-voltage power network where the neutral point is not directly grounded, the power device should be protected by low-voltage grounding. In the low-voltage power network powered by the same generator, the same transformer or the same busbar, grounding protection and zero-connection protection should not be used at the same time.

What is the difference between grounding and zeroing?
Protective grounding and protective zeroing are two technical measures to maintain personal safety. The differences are:
First, the protection principle is different. The basic principle of the protection grounding of the low-voltage system is to limit the grounding voltage of the leakage equipment so as not to exceed a certain safety range; the protection grounding of the high-voltage system, in addition to limiting the voltage to the ground, in some cases, the protection device in the enabling system The role of the action. The main function of the protection zero is to make the single-phase short circuit of the equipment by the zero line, and the protection device on the line can move quickly.
Second, the scope of application is different. The protective earthing is suitable for general low-voltage ungrounded power grids and low-voltage grounding grids with other safety measures; protective earthing can also be used for high-voltage ungrounded power grids. Ungrounded grids do not have to be protected from zero.
Third, the line structure is different. In addition to the phase line, the protective grounding system only has a protective ground wire. In addition to the phase line, the protection zero system must have a neutral line; if necessary, the protection zero line should be separated from the working zero line; its important device should also have a ground line.