Safety voltage is a voltage that does not endanger life safety under certain conditions and within a certain period of time. According to Ohm's law, the voltage applied to a person can be limited to a certain range, so that at this voltage, the current passing through the body does not exceed a specific allowable range. This voltage is called the safety voltage, also known as extremely low voltage (ELV).
Safety voltage is a safety technical measure that can prevent both indirect contact with electric shock and direct contact with electric shock. Devices that rely on a safe voltage supply belong to Class II devices.
01.Safe voltage limits and ratings
▼limit
The safe voltage limit is the voltage value that must not be exceeded between any two conductors under any circumstances. The Chinese standard stipulates that the limit of RMS value of power frequency safe voltage is 50V, and the limit of DC safe voltage is 120V.
▼ Rated value
The power frequency RMS values of China are 42V, 36V, 24V, 12V and 6V.
●Hand-held power tools used in particularly dangerous environments should be 42V safe voltage III;
●Where there is a danger of electric shock environment use of hand-held lighting and local lighting should use 36V or 24V safety voltage;
●In metal containers, tunnels, Wells and surrounding areas with a large area of ground conductors, such as narrow working places and inconvenient environments, 12V safety voltage should be used;
●6V safety voltage for special places.
●When the electrical equipment adopts a safety voltage of more than 24V, protective measures must be taken to directly contact the electric shock.
Supplement ：
General lighting power supply adopts 220 voltage. In particularly humid places, high temperature places, places with conductive dust or places with conductive ground, for fixed lamps that are easy to reach and have no anti-shock measures, when the installation height is less than 2.2m, 24V safety voltage should be used.
In order to ensure that the human body electric shock will not cause serious injury and casualties, the crane should use low pressure safety operation, often using 36V safety low pressure.
02.Safe voltage level
Safe voltage refers to the voltage that does not directly cause death or disability, and the "safe ultra-low voltage" that allows continuous contact under general environmental conditions is 36V. According to the national standard document "Safety voltage" (GB3805-83) provisions, China's safety voltage is divided into 5 levels, respectively: 42V, 36V, 24V, 12V, 6V, according to different production environments and workplaces, the corresponding level of safety voltage is used, so as to avoid some accidents.
03.Safety voltage setting basis
There are two influencing factors, respectively: shock current and human resistance.
Electric shock refers to the tissue damage and dysfunction caused by the current passing through the human body when the human body directly touches the charged body, and even the heartbeat and respiratory arrest. The greater the current intensity, the greater the fatal danger; The longer it lasts, the more likely it is to die.
According to Ohm's law (I=U/R), the size of the current flowing through the human body is related to the applied voltage and the human resistance. When the current is constant, the greater the resistance of the human body, the higher the safe voltage it can withstand.
Human resistance is divided into internal resistance and external resistance. Since the human body is mainly composed of water, the resistance in the body is basically the same, about 500Ω. External resistance is related to the skin, such as: skin and charged body contact area, dryness, wet sweating, sticky conductive dust, oil, wounds and so on.
In a dry environment, the resistance of the human body is relatively large, generally about 1000-2000Ω. But when the human body sweats or the skin is wet, the resistance drops a lot; If there is a wound in the skin, the external resistance will even drop to zero. In addition to the human's own resistance, we should also consider the resistance of clothes, shoes, pants and other outside the body, such as clothes, shoes, socks wet oil and so on.
In different cases, the resistance of the human body is inconsistent, and the size of the current flowing through the human body can not be calculated and measured. In order to ensure personal safety in different environments, safety voltages corresponding to different environments are specified.
Hand-held power tools used in particularly dangerous environments should use 42V ultra-low voltage;
Hand-held lighting and local lighting used in environments where there is a risk of electric shock should use 36V or 24V ultra-low voltage;
Hand-held lamps used in particularly dangerous environments such as metal containers and particularly humid places use 12V ultra-low voltage;
Ultra-low voltage of 6V should be used in places such as underwater operations.
Human electrocution is caused by the damage caused by the current on the human body, and the voltage is only a factor affecting the size of the current. Because there are too many factors affecting the electric shock current, the minimum voltage is used to ensure personal safety in different environments.
Safety is no small matter, we should treat it with a more rigorous attitude. Putting aside environmental factors to say that 36V voltage is safe, can be said to be rogue. When we talk about safe voltage, we must mention environmental factors.
