1. High pressure and low pressure
High voltage and low voltage refer to the level of voltage. According to the degree of injury and danger caused by different voltage levels, considering the humidity of the air and the resistance of the human body in different working environments, in electric cars, the voltage is divided into two levels, A and B, for safety, see Table 1 .
|Voltage level||Working voltage/V||Working voltage/V|
|DC (Direct Current)||50~150Hz AC (Alternating Current)|
Class A is a relatively safe voltage level, which is less than or equal to 60V in DC, and lower than AC 30V at the specified frequency of 50~150Hz. Maintenance personnel under A-level voltage do not need to take special protection against electricity.
Class B can cause harm to the human body and is considered to be high pressure. Necessary protective measures must be taken to protect maintenance personnel under B-level voltage.
The high voltage of electric cars has the following characteristics.
①High voltage is generally designed to be above 300V. For example, the total voltage of the power battery of the Tesla Model S is 400V.
②The form of high voltage exists in both direct current and alternating current. This includes the DC power of the power battery, as well as the 220V (or 380V) grid AC power when charging, and the three-phase AC power when the motor is working.
③High voltage has higher requirements on insulation. The insulating material designed on most conventional cars may become a conductor when the voltage exceeds 200, so the insulating material on electric cars needs to have higher insulating properties.
④The high voltage requires a large distance between the positive and negative electrodes. In the case of 12V voltage, the distance between the positive and negative poles needs to be very close to have the possibility of penetrating the air; but if the voltage is higher than 200V, when the distance between the positive and negative poles is far, it will pierce the air and conduct electricity. For example, at a voltage of 30V, breakdown conduction occurs when the distance between the two wires is 10cm.
2. The nature of high-voltage electricity damage to the human body
Usually, when the human body is exposed to alternating current above 30V or direct current above 60V, an accident may occur. The electric shock of the human body does not mean that the human body is exposed to a very high voltage, but that after the excessive voltage passes through the resistance of the human body, an electric current will be formed in the human body, which will cause harm to the human body. Therefore, the essence of high voltage damage to the human body is current.
In the power grid, 36V has always been considered a human-safe voltage. In fact, in high-voltage electric cars, this voltage value is not absolutely safe. There are two main reasons: on the one hand, the resistance of the human body will have individual differences, such as fat and thin, male and female, The resistance value will be different; on the other hand, the different working environment of people will also cause the resistance value of the human body to change. For example, in the humid summer and dry winter, the resistance of the human body will be different. The resistance of the human body will be lower. Also, everyone reacts differently to the flow of electricity through their body, and some people may be able to withstand higher currents. Therefore, the internationally recognized safe voltage is below 60V DC and below 30V AC.
When the voltage reaches a certain value, a corresponding current will flow through the human body. As shown in Figure 1, when there is about 5mA of electricity passing through the human body, it can be regarded as an “electrical accident”, which will make people feel numb. When the current passing through the human body reaches about 0mA, it reaches the limit of exporting the current, and the human body begins to feel pain and can no longer conduct an increase. Long-term retention of 30~50mA alternating current can cause respiratory arrest and ventricular fibrillation. Currents through the body of approximately 80 mA are considered lethal.
In addition, it should be noted that the main reason why the human body conducts electricity is that the blood contains electrolyte components, and the electrolyte components cause the conductivity. The human skin and muscles also have certain electrical conductivity. For most people, the total resistance value of the entire body is low, especially where the aorta is present (thoracic area and trunk), and the greatest danger is the abnormal flutter that stimulates the heart as the current passes through the body.
3. Injury forms of high voltage to human body
What can ultimately harm the human body is the electric current. There are three forms of electric current damage to the human body: electric shock, electric injury and electric field injury.
①Electrical shock means that electric current passes through the human body and destroys the normal function of the human heart, lungs and nervous system.
②Electrical injury refers to the damage to the human body caused by the thermal effect, chemical effect and mechanical effect of the current, mainly referring to arc burns, molten metal splash burns, etc.
③Electromagnetic field injury means that under the action of high-frequency magnetic field, people will experience neurological symptoms such as dizziness, fatigue, memory loss, insomnia, and more dreams.
It is generally believed that the current is more dangerous when it passes through the human heart, lungs and central nervous system, especially when the current passes through the heart. More current passes through vital organs such as the heart and lungs; the second is the current path from one hand to the other.
In addition, when the human body is electrocuted, it is easy to cause people to fall due to severe convulsions, and cause secondary accidents such as falls and falls.
Generally, electric shock accidents cause the most injuries, and there are mainly the following types of electric shock accidents.
①Electrical shock effect, when the current is lower than the conduction limit, there will be a corresponding electric shock response, which will easily lead to injury due to the uncontrolled loss of balance of the limbs.
② thermal effect. Burns and coking can occur at the point of introduction or exit of the current, as well as internal burns. This can lead to overloaded kidneys and even fatal injuries.
③ chemical effect. Blood and cellular fluids become electrolytes and are electrolyzed. This can cause severe poisoning, which is not noticed for several days, and can be extremely damaging.
④ muscle stimulation effect. All bodily functions and human muscle movements are controlled by the brain through electrical stimulation of the nervous system. If the current through the body is too high, the muscles start to twitch, and the brain can no longer control the muscle tissue, which can cause breathing to stop, the heart’s rhythm to be interrupted, and so on.
⑤ The thermal effect of static short circuit occurs. Excessive heat from the tool can cause the material to melt and possibly burn.
⑥ Sparks caused by short circuit. The metal melts quickly, producing flying sparks, and the temperature of the flying metal particles exceeds 5000 ℃, which may cause skin burns and serious eye damage.
⑦Arc light generated when live high-voltage lines are connected and disconnected. Optical radiation may cause electro-optic ophthalmia.
Both DC and AC voltage will cause harm to the human body, but the threshold value of AC voltage to the human body is only 50% of that of DC. The alternating voltage creates an alternating current in the body, which triggers muscle tissue contractions and a fibrillation of the heart. The lower the frequency of the AC voltage, the higher the danger. Alternating current can trigger ventricular fibrillation, which can quickly kill people without first aid.