In order to contribute additional knowledge about electrical safety during the labor process, Galaxy M&E would like to send readers an article about the causes of electrical accidents.

According to statistics of the Ministry of Labor - Invalids and Social Affairs, in 2016, there were 799 occupational accidents leading to death, where the cause of electrical insecurity accounted for 12.5%, equivalent to 107 people died. In order to contribute additional knowledge about electrical safety during the labor process, Galaxy M&E would like to send readers an article about the causes of electrical accidents.

Power supply plays an extremely important role in the construction process. In addition to the benefits that electricity brings, risks are always mentioned when utilizing electrical energy. Fire accidents, 70% of the reasons come from using or operating non-standard power consumption devices. Electrical accidents, more than 30% of the cases are fatal. In order not to cause unintended incidents, it is imperative that we understand the causes of electrical accidents.

The elimination of factors leading to electrical accidents should be informed right from the start when we plan a construction project. All risks of electrical accidents must be listed and solutions to prevent and eliminate them should be prepared.

3 general causes of electrical accidents can be listed, as follows:

- Accidental or intentional direct/indirect contact with electrical wire or equipment. Accidental cases of electrical contact often occur with objects with damaged insulation. Intentional cases occur when human operations do not comply with electrical safety regulations or due to lack of knowledge, experience in construction/repair/operation of electrical equipment. Some indirect cases of accidents are due to the failure to comply with the safe distance from the grid.

- Workers' bodies are exposed to electrical leakage in the ground due to the broken power lines hitting the ground, and the dissipation of current does not meet safety requirements. This phenomenon is called "step voltage”.

- When one does not strictly follow the procedure of electricity switching. Switching off or cutting off the power without checking carefully whether the related circuits will be manipulated when there are parts operating in the network without notice. Sudden power interruptions prevent construction workers from preparing for accident prevention as well as appropriate manufacturing operations.

In addition to the three general causes mentioned above, electrical accidents also depend on many different factors such as the location of the current to the human body, the voltage, the contact time, the current intensity, the working environment. The content below will show in detail each specific cause.

1. The position of the body in contact with electrical current

The position of the body in contact with the electrical current is extremely important because it produces an electrical current that flows through the body. The current flowing through the body must be directed from an energized source to ground or metal objects to continue.

The most dangerous electrical current: Normally, the flow of electricity from the right hand, through the body leading to the foot to connect to the ground is the most dangerous flow to the human body. When the electrical current is in contact with the right hand, and your feet are wet and not protected with shoes, the current will continue to flow into the ground to form a closed circuit causing heart arrhythmia, which creates the heart to fail. In addition, the flow of electricity from the left to the right hand equals that the person will become an intermediate electrical connector device. This is also the flow of the highest risk of death compared to other forms.

Less dangerous electrical current: The flow of electricity from one leg to the other leads to less danger because the shortest current does not pass through vital organs such as the heart, the brain, and the lungs.

Due to the fact that electrical current flows vertically through the body via the heart, it is the most dangerous to humans.

2. The influence of voltage

Voltage changes the electrical resistance of the human body. Low voltage can create burns on the skin where electricity has been in contact with. Electrical burns are caused by heat. The colors of the burn are white or black. At the burn, the skin's electrical resistance reaches 0 making that area becoming an electrical conductor.

With voltages less than 50 Volts there is rarely a burn; whereas at 220 Volts, it usually creates traces even when the time of contact with voltage is less than 1 second. With a voltage higher than 700 Volts, the skin is often punctured.

2.1. The higher the voltage is, the more dangerous it is to humans

For medium/high voltages electricity (> 1 kV, 20 kV, 35 kV, 110 kV...), the main conductor is a bare wire. When humans are close to the medium/high voltage wire, the current will discharge an electrical arc causing the heart to stop beating and paralyze breathing. This phenomenon is similar to lightning. After the victim falls into an uncontrolled status, the electricity will continue to burn the body.

The question is, why do medium and high voltage wires often have no electrical insulation?

The reason is because high voltage requires insulation layer (if any) to be thick, thus increasing the weight of the wiring. When the weight of the wiring system increases the strength of the electrical poles, the distance between the poles must be reduced to ensure the weight is taken care of properly. In addition, when there is an insulating layer, the heat emission of conductors will be affected. The heat generated is proportional to the voltage and duration of use. Increased heat will affect the physical properties of the conductor, causing fire / short-circuiting, even melting, and cutting off wires.

2.2. Safe voltage in factories, factories, offices or civil works

As stated in part 1 of the article, in addition to indirect contact with the current, the danger of direct contact voltage depends on two factors simultaneously: working environment and magnitude of the voltage. .

In humid environments (100% warmth), 6 Volts offer no danger to humans.

In factory environments, factories with dust (heavy metals or types of conductive dust), the maximum safe voltage is 12 Volts.

In a mechanical workshop, a carpentry workshop is dusty but the dust does not have conductive properties, 36 Volt is the maximum for safety.

In the working environment at offices / civil constructions, the maximum safety voltage is 60 Volts.

3. Time of impact of electrical current.

Although the magnitude of the current intensity does not pose a direct danger to humans, with the voltage exceeding the safety level (as stated in section 2), the longer the current exposed, the greater the danger level. With a shorter time of 0.2 seconds, the current flow is not dangerous, from 0.2 seconds or more, human life is at risk. However, there were cases where with only 0.1 second of contact with the current, the human being could have been in a dangerous status.

During the cardiac cycle, the heart has an average of about 0.1 seconds of rest (in the status between contraction and relaxation), in which the heart is very sensitive. When the electrical current does not impact the right time of the heartbeat, a small current (about 10mA) passing through has a level of danger equal to that of a 10A current entering the normal status. The effects of the current time is shown in the table below:

Electrical current and impact time correspond to body's reaction.

4. Dangers of electricity when passing through the human body

The electrical current through the human body causes reactions that change the mental state, change blood components / cause burns. People begin to feel an electric current through their body when the current intensity is about 0.6 ~ 1.5 mA with alternating current and 5 ~ 7 mA with direct current. The harm of electrical current through the body depends on the amount of current that was just exposed to the body, the current flowing through the body and the current being retained in the body.

New current which was just exposed to the body: is a current within the range of 0.6 ~ 5 mA. It is also considered as when people are still capable of separating themselves from the electrical current.

Current through the body: is a current within 15 mA with alternating current and 50 ~ 80 mA with direct current.

Current retained in the body: A current of about 15 ~ 20 mA with alternating current and 80 mA with direct current. The electrical current retained in the body is extremely dangerous because it causes convulsions of the nervous system, muscle twitching and then paralysis, cardiac arrest leading to death.

Many studies of electrical accidents show that only with 20 ~ 25 mA AC which can create danger. Above 50 mA can lead to fatal accident.

Impact of electrical current on the human body.

5. Electrical shock due to step voltage

The step voltage is the voltage that humans suffer when their feet come into contact within the current within the ground/floor. When a person moves within the range of the impact current, a step voltage will be applied to the feet, causing electric shock.

Stepping voltages mainly occur in cases when the grounding system is not up to standard or faulty or the conductor is broken / exposed and in contact with the ground.

The resistance of the soil through which the current flows will decrease with greater distance to the point where the current flows into the soil. At a certain distance, this resistance actually becomes zero. The area where the actual current is suppressed is called the potential area 0. At the point of touching the ground, the voltage which is relative to the ground will be: Uđ = Id.Rd (Id is the ground current intensity, Rd is the resistance point to the current).

Measures to prevent electric shock in the area where the step voltage is: When seeing the conductor break / open / fall to the ground / floor, immediately hop the trigger outside the range of 20 m from the point where the step voltage appears. At a distance of 20 m, the step voltage is approximately zero magnitude.

6. Electrical accident due to the working environment

The level of electrical safety depends very much on the working environment. As stated in section 2.2, in a humid environment the safety voltage limit is lower. Depending on the characteristics of the working environment, ones should adjust the level of safety during operation and the use of electrical equipment.

Low-risk environment: An environment where the air humidity does not exceed 75% when the temperature is between 5 ~ 25^oC, and the working floor is made of insulating material (with large resistance), the gas does not contain suspended conductive dust.

Dangerous environment: Working environment with humidity above 75%, average temperature is above 25oC. Places where there is a lot of conductive dust such as coal grinding workshops, reinforced broaching workshops, mechanical filing processing ... With room temperature above 30 degrees, workers are prone to sweating, reducing the resistance of the body, increasing the risk of shock electricity.

Extremely dangerous environment: is a working environment with a humidity above 100%. In this environment, walls, partitions, and floors often experience condensation. In addition to humidity, the working area is an explosive processing workshop, the explosive gas with a lot of dust / gases forming explosive mixtures are also extremely dangerous in these working environments. In the environment containing flammable substances, when the temperature of electrical equipment is high, if there is no good ventilation or heat dissipation system will be an indirect cause of fire. The initial fire may be in small scale, but if it is not detected in time, it will create fires on the wires, seriously threatening human life and surrounding assets.

Above are 6 causes of accidents caused by electricity. In addition to the general causes mentioned above, there are many other factors affecting the safety during construction, repair, operation ... of electrical equipment. In order to ensure absolute safety during the labor process, minimize the causes of electrical accidents, factories/companies must necessarily be equipped with professional and experienced units, consulting, designing, constructing and installing electromechanical systems, instructions for use as well as providing mechanical and electrical maintenance services. Having so, the business has not only limited the number of work accidents caused by electricity, but also contributed to promoting the productivity of using equipment, reducing costs and time to manage the apparatus to help operate electrical safety. For workers working in an environment with many electrical equipment, before starting the work, it is necessary to understand and master the basic elements that help prevent electrical accidents.

Galaxy M&E