Causes and prevention of electric shock on the sec

2022-08-19
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Causes and prevention of electric shock on the secondary line of electric welding machine

causes

electric shock accidents on the secondary line of electric welding machine occur frequently, because the voltage of the secondary line is low (the no-load voltage is generally 50v-90v). People have insufficient understanding of the causes of electric shock, and often mistakenly believe that the secondary line of electric welding machine is "safe", leading to accidents caused by the secondary line causing death are common: there is a young female electric welder in a shipyard who is conducting electric welding in the cabin, Due to the high temperature and poor ventilation in the cabin, the body sweated a lot, and the canvas overalls and leather gloves were soaked. When the welding rod was replaced, it touched the mouth of the welding tongs, fell back due to spasm, and the welding tongs fell on the neck, causing electric shock, which was ineffective and died after rescue. This accident tells us that although the secondary line voltage of electric welding machine is low, it is fatal

the no-load voltage of the electric welding machine is generally about 50v-90v, and the highest level of safety voltage is 42V. The no-load voltage is higher than the safety voltage, which is the main unsafe factor of the secondary line; In addition, after the arc of the general electric welding machine is ignited, the working voltage required to maintain the arc is 16v-35v. Although it is within the safe voltage range, it is in a bad welding environment, such as on metal structures, metal containers, pipelines or underwater. When welding in wet places, if the welder is in poor physical condition and the human body resistance is very low, it may also cause electric shock. Safe voltage is not absolutely safe

in fact, if the problem of electric shock on the secondary line of electric welding machine is discussed in a wider range, the forms of injury caused to the human body are various. During the process of electric welding, the human body contacts with the secondary line, and the current acts on the human body, which will cause current burns, electric branding or the rupture of the body tissue due to central nervous reflex and strong muscle contraction. Mechanical injury such as fracture; Electric arc is generated during electric welding, which can cause electro-optic eye disease or make metal particles penetrate into the skin and cause skin metallization when the operator is not protected properly; In addition, there are more serious electric shocks

analyze the causes of these electric shock accidents, mainly due to the following unsafe factors:

including PMDI spacer electric welding machine and welding circuit, there is an objective risk of electric shock

1 The insulation of the secondary winding of the electric welding machine is damaged, and the insulation of the secondary terminal is poor, causing the shell of the electric welding machine to be charged. However, the electric welding machine is not grounded or grounded, and the leakage protector has problems. The power supply cannot be disconnected, and the shell of the electric welding machine is charged for a long time

2. The only way for the development of the global automotive market is to lighten the vehicle due to the lack of protective cover for the terminal of the secondary output, the exposure of the secondary cable, and the unqualified insulation of the electric welding tongs

3. Because the metal structure, pipeline, track, crown block hook or other metal objects of the plant are used as the welding circuit

poor welding environment

1 Welding operations are carried out in metal containers, pipelines, metal structures or wet places, underwater,

2 In summer, the temperature is high, the humidity is high, the welders work for a long time, the ventilation is poor, the human body sweating and fatigue, the body is sick or in a bad mood, resulting in a significant reduction in human resistance

welders' protective measures are not in place

1 Welders did not wear labor protection articles as required, especially insulating gloves, insulating shoes and protective masks

2. The insulating articles fail due to humidity, damage and other reasons

3. In dangerous environment, there is no insulating pad under your feet

due to the existence of the above substances and environmental risk factors, when the welder makes mistakes or touches the abnormal electrification in the case of normal operation, the following phenomena will occur: the hand or body of a person contacts the leakage of the welding machine shell, the leakage part of the secondary wire and welding tongs, the normal electrification part of the welding rod and welding tongs, and the other hand, foot or some part of the body contacts the workpiece, metal conductor or wet place, It will form a current circuit and cause electric shock

preventive countermeasures

in order to prevent secondary line electric shock accidents and avoid serious injuries caused by untimely rescue, practical and effective preventive measures must be taken in management, technology and first aid

1. Welder qualification must be strictly managed. Electric welders are one of the special types of work clearly stipulated by the state. Their training, assessment, certification, review and personnel use management must strictly implement the national regulations and put an end to the phenomenon of welding without certificates

2. Welders must wear complete PPE; Welding operators should ensure that the quality of work clothes, insulating shoes, insulating gloves, protective masks and shading lenses is qualified, and the performance meets the requirements of national standards. This is the most basic and effective measure to prevent electric shock, electric burns, skin metallization, electric eyes and electric branding

3. Qualified products must be selected for electric welding machines. The electric welding machine shall be equipped with thermal protection device to prevent overload, and there shall be obvious overload indication on the nameplate for regulating current; There should be good insulation between the conductive parts of the electric welding machine, and the insulation resistance value should not be less than 5m Ω between the primary circuit and the secondary circuit, and not less than 2.5 m Ω between the live part and the shell and the frame; The experimental voltage value of dielectric strength meets the requirements of national specifications; Therefore, the length of the power line of the welding machine can be used as a reference for the plastic mold industry. Generally, the length of the power line should not exceed 2 ~ 3 meters; The working temperature of the electric welding machine is generally between -10 ℃ +40 ℃, and the relative humidity of the air is ≤ 90% at 20 ℃. Under special circumstances, such as low or high temperature, excessive humidity, low air pressure and corrosive or explosive conditions, the electric welding machine suitable for the performance of special conditions should be used, or necessary protective measures should be taken; The electric welding machine should be repaired and maintained every six months. In case of failure, the power supply of the welding machine should be cut off immediately and repaired in time

4. The terminal ports of the primary and secondary lines of the electric welding machine must have good protective covers to prevent human bodies from accidentally touching the live body. If the protective cover is made of metal material, it must be prevented from touching or connecting the protective cover with the terminal posts and metal wires of the wiring port to avoid electrification of the protective cover

5. The secondary cable of the electric welding machine must be well insulated and must not be exposed. The secondary cable shall be selected according to the national standard gb50133-85, and its insulation resistance shall not be less than 1m Ω; It shall also have good conductivity. The wire core shall be multi strand thin copper wire (with a diameter of 0.2mm-0.4mm), which is light, soft and easy to operate; The cross-sectional area shall be determined according to the difference between the welding current used and the length of the cable, so as to prevent the insulation layer from burning due to overheating in use; Generally, the length is not more than 20-30 meters; The secondary cable and electric welding machine can be connected by equipment coupler (commonly known as quick change connector). There can be no more than two connectors, and the joints should be well insulated

6. The quality of welding tongs must meet the safety requirements, and its insulation resistance shall not be less than 1m Ω; The dielectric strength shall reach that the insulation part can withstand the AC experimental voltage with the effective value of 1000V without flashover or breakdown through the test; Welding tongs and resistance to welding spatter meet the specified requirements; The mechanical properties meet the specified requirements through tensile test and impact resistance test; It is forbidden to immerse the welding tongs in water for cooling during use

7. The use of electric welding machines shall adhere to the principle of "one machine, one switch, one leakage and one box". That is, each electric welding machine must be equipped with an independent power control box, which has an iron shell switch (or automatic air switch) and leakage protector with the required capacity. When the welding machine is overloaded, it should be able to automatically cut off the power supply; It is forbidden for multiple welding machines to share one power switch; The metal shell of electric welding machine and power control box must be grounded (or connected to neutral), and the grounding resistance is RD2; Leakage protectors must be products that conform to gb6829-86 leakage current operated protectors. In this way, it can prevent the poor internal insulation of the electric welding machine and the poor insulation of the wiring port of the primary line or secondary line from electrifying the metal shell of the electric welding machine and causing electric shock to the human body

8. Install the no-load automatic power-off device of the electric welding machine. Generally, the welding arc voltage is 16v~35v (lower than the safety voltage), that is, the output voltage of the power supply after the arc striking, that is, the secondary line voltage will automatically drop to the working voltage before the welding can continue stably. At this time, the safety degree of the secondary line voltage is high, but when the welding is stopped, the secondary line voltage will change to the no-load voltage of 50V ~ 90V, which is more dangerous. If the power supply of the welding machine can be cut off at this moment, The hidden danger of the secondary line and even the primary line can be eliminated from the root. The no-load automatic power-off device of the electric welding machine has the function of automatically cutting off the power supply of the electric welding machine within the set time. Its function principle is shown in Figure 1 (execution unit: control the connection and disconnection of the main circuit; timing unit: realize the timing control of secondary output circuit breaking; stabilization unit: use current to control the stability of timing unit in welding; detection unit: realize the detection of secondary circuit voltage)

when the welder stops welding (the welding handle leaves the workpiece), the device will automatically cut off the power supply within the set time (1 ~ several seconds). When the welding is restarted and the welding rod touches the workpiece, the device will automatically send electricity and start arc. In this way, it can prevent the welder from forgetting to turn off the power supply when the welder is out of service, so that the welder is always in the pre start state; In addition, in the case of electric shock caused by human misoperation or poor insulation of the equipment itself, due to the interruption of welding, the time-delay switch will automatically cut off the power supply within the set time, reducing the time of electric shock of human body, so that the person who gets an electric shock is not injured or slightly injured, and avoiding the serious consequences of electric shock accidents. According to national regulations, when welding operations are carried out in special hazards, such as metal containers, pipelines, metal structures, wet places, underwater and high altitude, the electric welding machine must be equipped with no-load automatic power-off protection device. It can be said that the electric welding machine equipped with no-load automatic power-off device is an extremely effective method to prevent secondary line electric shock accidents technically

9. Welding in poor environment. Use "one pad, one set" to prevent electric shock. When welding in metal containers, pipes, metal structures and wet places, there is a great risk of electric shock. In addition to the measures of installing automatic power-off protection device for electric welding machine by using the driving sail prepared based on this kind of graphene material, the method of adding "one pad one set" can also be used to prevent electric shock, that is, adding insulating pads under the feet of welders. When welding is stopped, remove welding rods and put "insulating sleeves" on welding tongs

10. It is strictly forbidden to use plant components, metal structures, tracks, pipes or other metal objects to lap instead of welding cables. Using these metals as welding cables is easy to cause electric shock, and at the same time, sparks will be generated due to poor contact, causing fire

11. Electricians are responsible for the installation, maintenance and inspection of welding equipment. In case of equipment failure during work, the power supply should be cut off immediately, and a full-time certified electrician should be found for maintenance. Welders are not allowed to deal with it without authorization

12. Overload is not allowed during the use of electric welding machine. If overloaded, the electric welding machine will be burned due to overheating, causing danger; If the insulation is burnt out, it may cause electric leakage and electric shock. The heating of the electric welding machine is related to the current and the power on time, so overload refers to two aspects: one is that the welding current exceeds the rated current value, and the other is that the use time exceeds the rated temporary load rate. The temporary load rate refers to the ratio of the specified time to the power on working time, that is:

temporary load rate = (power on working time/specified time) × 100% when the welder carries out manual arc welding, the working cycle shall be fixed according to the national regulations (gb807-74, jb686-74)

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