? Paper Keywords: human body resistance safety current safe voltage
? By explaining the concepts of "human body resistance", "safe current" and "safe voltage", readers finally have a further understanding of the concept of "safe voltage" and learn to look at the problem dialectically. At the same time, learn to analyze specific problems in order to solve the related problems of safe electricity use.
With the development of national economy and the gradual improvement of people's living standards, there are more and more electrical equipment and electrical equipment in production and life. Because electricity itself is invisible and intangible, when people touch or approach equipment and conductors with a certain voltage, it may cause electric shock accidents. Therefore, it is very important to use electricity safely. So, how high is the voltage for safe voltage? "36 volts", how many people with common sense would say that. It seems that this is an unquestionable value. "safe voltage value" is defined as "safe voltage refers to the voltage that will not cause direct death or disability when people are in continuous contact" in the textbook "Safe Use of Electricity" of the National Technical School. Safe voltage value is based on the product of current passing through human body (not exceeding safe current) and human body resistance. Namely:
Us=IsRb
Among them, safe voltage (Volts)
Is- safe current (ampere)
R B- human body resistance (ohm)
The textbook also gives the specific values of IS = 30mA, Rb= 1700 Euro. Therefore, it is calculated that Us is about 50 volts. In fact, the real answer to this question is not so simple. A few years ago, we saw in TV programs that some people could easily perform programs under the condition of being exposed to 220 volts AC. But at the same time, there are also reports that there have been many cases in Japan in which people were electrocuted by 35-volt power supply. So we can't simply answer the concept of "safe voltage". In the specific teaching process, the author thinks that in order to make this problem clear, students should learn to look at the problem dialectically and truly understand the three concepts of "human body resistance", "safe current" and "safe voltage". Now the problem is divided into the following.
First, human resistance.
Generally, the resistance of human body is divided into skin resistance and internal tissue resistance. Because the cuticle of the skin has certain insulating properties, the human body's resistance mainly depends on the cuticle outside the skin. There are great differences between people. Different people have different cuticles, that is, the same person, the thickness of cuticles in different parts is also different. Coupled with the difference of external environment (such as temperature and humidity), this value will change in a wide range due to the influence of many factors such as people's physique and emotions in different periods. It should also be emphasized to the students that the insulation strength of this stratum corneum is very limited, and it will be destroyed with the increase of contact voltage (generally above 50 volts) and the extension of contact time. Once it is destroyed, its resistance value will be reduced or even broken down.
When this skin insulation layer is broken down, the internal resistance of human tissue becomes the main factor of passing current. The internal tissue resistance of human body is generally unstable, about 500~600 ohms, which has little to do with the applied voltage and has little resistance to current.
Second, the safety current
The harm degree of current to human body is also closely related to many factors such as the intensity and frequency of current. First of all, the harm of current to human body has a great relationship with the strength of current. Because the object is human, it is impossible to have a lot of experimental data. Because the experimental conditions and objects are not exactly the same, the statistical data are not exactly the same. But generally speaking, when the current passes through the human body, due to the different physiological conditions of each person, the response to the current is also different. Some people are more sensitive, and their bodies can't stand the power frequency alternating current of several milliamps; Some people don't even care about more than ten milliamps. So it is difficult to find a safe limit current value suitable for everyone.
Men are usually more resistant to electricity than women. The power frequency current is about 16 mA for men and 10.5 mA for women (average). The DC current is about 76 mA for male and 5 1 mA for female (average). It can be seen that power frequency AC current is the most harmful to human body, while DC current and higher frequency current are slightly harmful to human body. We often come into contact with 220 volts alternating current, which should be paid enough attention to.
Third, safe voltage.
As can be seen from the previous explanation, the values of "human body resistance" and "safe current" are relative concepts, which are a general range of specific problems and need specific analysis. After establishing this concept, the problem of safe voltage will be easy to talk about. Just like the formula Us=IsRb given above, we think that under certain conditions, the maximum safe voltage value is the current value allowed to flow through the human body for a short time multiplied by the resistance value of the human body. This is how the 50 volts mentioned above are obtained. To be on the safe side, China limited it to 36 volts.
It should be noted that the above values are stipulated in China's Guide for Grounding Protection of Low-voltage Circuits, and are obtained from "the place where the human body is in a normal state, the skin of hands and feet is dry, and there is great danger after contact with voltage". The guide also gives safe voltage values of other states.
When explaining the safe voltage problem, we need to emphasize to students that high pressure is very dangerous to people's lives, but most people have an instinctive vigilance against it, and the death rate from electric shock is not too high. Usually, people are not alert to low voltage, but people often die. Statistics show that about 85% of electrocution deaths occur at low voltage, so don't be careless because of low voltage, which we should always emphasize to students.
From what has been discussed above, we can see that the so-called "safe voltage" is actually a general statement, which is influenced by many different factors. Therefore, at present, different countries have different regulations on safe voltage. The formulation and selection of safe voltage must take into account the influence of electricity places and appliances on safety. At present, the commonly used safe voltage 36V and12V regulations in China are as follows:
1. Where the height is below 2.5m, the safe voltage of lighting devices, local lights of machine tools, mobile lights, hand-held power tools (such as hand drills) and electrical equipment in wet places can be 36V.
2. Where the workplace is narrow, it is difficult for workers to move, there is a large area of grounding conductor or metal structure (such as in a metal container), there is an environment with high risk of electric shock and a particularly humid place, and 12 volts should be used as the safe voltage.
The above are just the safe voltage values in two kinds of electricity consumption environments that we may often come into contact with, and there are some safe voltage values in special electricity consumption environments, so I won't discuss them here. These safe voltage values usually match the working voltage and action time of the electric shock protector produced by them. Therefore, safe voltage must have an additional condition, that is, the time allowed to pass the current.
Finally, it should be noted that the safe voltage value also needs its power supply. These problems are more professional, and we generally have less contact with electricity users, so I won't go into details here. A good and happy life is based on security. I hope everyone must regulate the use of electricity and use it safely.
References:
[1] Safe use of electricity, a general textbook for electricians in national technical schools (2nd edition) (3rd edition).
[2] Guidelines for Grounding Protection of Low-voltage Circuits, National Standard for Safe Electricity Use (GB3805-83).
;