Electrical practitioners are aware of such a number — 36, which represents what we know as safe voltage. For so many years in the industry, it is not too much to say that this number is deeply rooted in people’s hearts.
Industry regulations safety voltage of 36V, the safety current of 10mA, for the following reasons.
The degree of harm to the human body from electric shock depends mainly on the size of the current through the human body and the length of the energized time. The greater the intensity of the current, the greater the risk of fatalities. The longer it lasts, the greater the likelihood of death. The smallest current value that can cause a person to feel is called the perception current, AC for 1mA, and DC for 5mA. The maximum current that a person can get rid of after being electrocuted is called the get rid of current, AC for 10mA, DC for 50mA. The current that endangers life in a short period is called the fatal current. Such as 100mA of current through the human body 1s, can be enough to make people fatal, so the fatal current is 100mA.
The human body’s reaction to the current:
8~10mA hand has difficulty getting rid of the electrode, there is severe pain (finger joints).
20~25mA The hand is paralyzed quickly, can’t get rid of the electrode automatically, and have difficulty in breathing.
50~80mA breathing difficulty, atrial tremor starts.
According to Ohm’s law (I = U / R), we can learn that the size of the current flowing through the human body is related to the applied voltage and the resistance of the human body. Human body resistance in addition to the person’s resistance should be attached to the human body outside the clothes, shoes, pants, and other resistance, and affect the human body resistance of many factors. Such as damp and sweaty skin, with conductive dust, increase the contact area and pressure with the charged body, as well as clothing, shoes, socks, and other cases of damp oil can make the human body resistance to reduce.
So usually the size of the current flowing through the human body is not calculated in advance. Therefore, to determine the safety conditions, often do not use the safety current, but the safety voltage to estimate: in general, that is, dry and the risk of electrocution in the environment, scientists have experimentally derived, in this case, the maximum resistance of the human body, after the backpropagation of the human body safety voltage value. This value is just 36 V. This is the source of the human safety voltage of 36 V.
However, the safety voltage is not safe, there are also accidents such as 36V electrocution death under voltage. This is because the human body resistance varies from person to person, and is affected by environmental conditions. If the operation site is narrow, wet, or people working in metal containers, mines, pipelines, electrocution is difficult to get rid of the charged body, even if the use of 36V safety voltage, there is still the possibility of death by electrocution.
Therefore, the use of safety voltage should also be used depending on the environment and the corresponding safety voltage. For the human body, the current is the more critical parameter. High voltage will not necessarily kill you, but the strong current will definitely kill you, so why not just write the safety current? Because only the voltage is constant in the standard of the grid, the greater the resistance, the smaller the current through the rated voltage.
For the wet and the risk of electric shock environment (such as metal containers, pipeline welding maintenance), the safety voltage is 12 V. Thus, the current through the human body when electrocuted, can be limited to a smaller range, to a certain extent to protect personal safety.
The size of the human body resistance is an important physical factor that affects the degree of injury to the human body after electrocution. The human body resistance by (body resistance) and (skin), the body resistance is stable, about 500 Ω. Contact voltage of 220V, the average value of the human body resistance of 1900 Ω. Contact voltage of 380V, the human body resistance down to 1200 Ω. After numerous experimental data analyses and research to determine the average value of the human body resistance is generally about 2000 Ω, and in the calculation and analysis, they usually take the lower limit value of 1700 Ω. Because the human body resistance is large, the low voltage must not produce a strong current in the human body. So the low voltage must be safe, often 12V voltage for the absolute safety voltage.