Contact hazard conditions in three-phase networks. Occupational Safety and Health

OCCUPATIONAL SAFETY AND HEALTH
Free library / Occupational Safety and Health

Contact hazard conditions in three-phase networks. Occupational Safety and Health

Occupational Safety and Health

Occupational Safety and Health / Legislative basis for labor protection

The analysis of the hazard conditions of three-phase electrical networks practically comes down to determining the magnitude of the current flowing through a person and assessing the influence of various factors: the circuit for including a person in the circuit, the network voltage, the circuit of the network itself, its neutral mode, isolation of current-carrying parts from the ground, etc. .

In a three-phase three-wire network with isolated neutral and phase voltage U_Ф with good insulationR_OF = 0,5 MΩ) current flowing through a person I_Ч is of little importance and a single-point (to some phase, and therefore also called single-phase) human touch is not dangerous:

I_Ч = U_Ф / ((R_Ч + R_Software + R_OF/3),

where: R_Ч - resistance of the human body; R_Software - floor and shoe resistance; R_OF- insulation resistance of phase conductors.

In such networks, it is very important to ensure high insulation resistance and monitor its condition in order to timely eliminate faults. However, if such a network has a large capacity relative to earth (branched cable lines, long overhead lines), then a single-phase touch will be dangerous, despite the good insulation of the wires.

In networks with an isolated neutral, it is especially dangerous to touch a healthy phase when any other phase is shorted to ground. In this case, the person is switched on under full line voltage.

In networks with earthed neutral, the earthing resistance of the neutral R₀ very small (2-8 ohms) compared to leakage resistance (insulation) R_FROM. Therefore, the current flowing through a person, when touched by a phase conductor, is determined by the phase voltage of the network U_Ф, floor and shoe resistance R_Software and neutral earthing resistance R₀:

I_Ч = U_Ф / ((R_Ч + R_Software + R₀)

It follows that touching a phase of a three-phase network with a grounded neutral during its normal operation is more dangerous than touching a phase of a normally operating network with an isolated neutral.

In emergency operation, when one of the phases of the network is closed to ground through a relatively low resistance R_PC when a person touches one of the other two phases, he is energized somewhat more than the phase voltage, but less than the linear one. This is one of the advantages of earthed neutral networks from a safety point of view.

Regarding networks with voltages above 1000 V, it should be noted that they are long, have a significant capacitance and a high value of insulation resistance. Therefore, in these networks, current leakage through the active insulation resistance can be neglected and only current leakage through the phase capacitance relative to earth can be taken into account. Therefore, touching these networks is dangerous regardless of the neutral mode.

In accordance with the PUE, networks with a voltage of 6-35 kV are made with an isolated neutral or with grounding of the neutral through a reactive coil in order to reduce the ground fault current.

Networks with a voltage of 110 kV and above are performed with neutral grounding.

The choice of the network scheme, and, consequently, the neutral mode of the current source is made on the basis of technological requirements and safety conditions.

According to technological requirements, at voltages up to 1000 V, a four- or five-wire network is preferred, since such a network allows the use of two operating voltages: linear and phase.

According to the security conditions, the choice of one of the two systems is made taking into account the conclusions obtained when considering these networks.

It is advisable to use networks with an isolated neutral, provided that the insulation is maintained well and the network has a low capacity (networks of electrical laboratories, small enterprises, etc.).

Networks with grounded neutral should be used where it is impossible to provide good insulation of wires (due to high humidity, aggressive environment, high capacitive currents, etc.). An example of such networks are large modern enterprises, housing and communal services networks.

Authors: Fainburg G.Z., Ovsyankin A.D., Potemkin V.I.

We recommend interesting articles Section Occupational Safety and Health:

▪ Basis for the adoption and application of state regulatory requirements for labor protection

▪ Laser radiation

▪ Labor protection of minors

See other articles Section Occupational Safety and Health.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Machine for thinning flowers in gardens 02.05.2024

In modern agriculture, technological progress is developing aimed at increasing the efficiency of plant care processes. The innovative Florix flower thinning machine was presented in Italy, designed to optimize the harvesting stage. This tool is equipped with mobile arms, allowing it to be easily adapted to the needs of the garden. The operator can adjust the speed of the thin wires by controlling them from the tractor cab using a joystick. This approach significantly increases the efficiency of the flower thinning process, providing the possibility of individual adjustment to the specific conditions of the garden, as well as the variety and type of fruit grown in it. After testing the Florix machine for two years on various types of fruit, the results were very encouraging. Farmers such as Filiberto Montanari, who has used a Florix machine for several years, have reported a significant reduction in the time and labor required to thin flowers. ... >>

Advanced Infrared Microscope 02.05.2024

Microscopes play an important role in scientific research, allowing scientists to delve into structures and processes invisible to the eye. However, various microscopy methods have their limitations, and among them was the limitation of resolution when using the infrared range. But the latest achievements of Japanese researchers from the University of Tokyo open up new prospects for studying the microworld. Scientists from the University of Tokyo have unveiled a new microscope that will revolutionize the capabilities of infrared microscopy. This advanced instrument allows you to see the internal structures of living bacteria with amazing clarity on the nanometer scale. Typically, mid-infrared microscopes are limited by low resolution, but the latest development from Japanese researchers overcomes these limitations. According to scientists, the developed microscope allows creating images with a resolution of up to 120 nanometers, which is 30 times higher than the resolution of traditional microscopes. ... >>

Air trap for insects 01.05.2024

Agriculture is one of the key sectors of the economy, and pest control is an integral part of this process. A team of scientists from the Indian Council of Agricultural Research-Central Potato Research Institute (ICAR-CPRI), Shimla, has come up with an innovative solution to this problem - a wind-powered insect air trap. This device addresses the shortcomings of traditional pest control methods by providing real-time insect population data. The trap is powered entirely by wind energy, making it an environmentally friendly solution that requires no power. Its unique design allows monitoring of both harmful and beneficial insects, providing a complete overview of the population in any agricultural area. “By assessing target pests at the right time, we can take necessary measures to control both pests and diseases,” says Kapil ... >>

Random news from the Archive

DNA instead of a hard drive 20.08.2015

Scientists from the ETH Zurich have demonstrated that information encoded by a person in DNA can be stored without any loss for about 2000 years.

With the advent of the digital age, the problem of preserving information for future generations has acquired a new significance. As you know, at the moment an external hard drive can store up to five terabytes of information for about 50 years (under ideal storage conditions). Scientists decided to overcome these boundaries by taking DNA as a memory device a couple of years ago. In theory, even a small part of it can contain up to 300 thousand terabytes of information, and yesterday at the 250th exhibition and conference of the American Chemical Society, scientists demonstrated how long this information can be stored.

They encoded 83 kilobytes of text from the Swiss Federal Charter of 1291 and the 71th century Archimedes Palimpsest into DNA, then sealed the DNA to silicon spheres and kept them for a week at a temperature of about 10 degrees Celsius, which is the equivalent of keeping the same material at a temperature of 2000 degrees Celsius. Celsius is about XNUMX years old. When the information was decoded after all the procedures, no errors were found in it.

Now that the researchers have demonstrated a method for how to store DNA synthetically for a long time, they face a new challenge. According to the head of the research team, Dr. Robert Grass, "in the DNA storage there is a drop of liquid with floating molecules in which information is encoded. Now we can read everything that is in this drop. But I can not point to any particular place and read a separate file". Therefore, now scientists are developing a system of labels for different pieces of information so that they can be searched.

True, there is some bad news. As with many technologies in the early stages of development, storing information in DNA costs a lot of money. Encoding and storing a couple of megabytes of data now costs thousands of dollars, which means DNA-based hard drives will not be available to users anytime soon.

Other interesting news:

▪ Sapper rats are better than metal detectors

▪ Bilingualism improves brain health and mental health

▪ System for detecting and neutralizing asteroids dangerous for the Earth

▪ Home solar inverter LG Micro Inverter LM320KS-A2

▪ Legumes force bacteria into symbiosis

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ section of the site Household electrical appliances. Selection of articles

▪ article Roman law. Lecture notes

▪ How did the formation of centralized states take place in Western Europe? Detailed answer

▪ article Rector. Job description

▪ article Means for manicure. Simple recipes and tips

▪ article Resistors. Philips color coding. Encyclopedia of radio electronics and electrical engineering

Leave your comment on this article:

All languages of this page

Home page | Library | Articles | Website map | Site Reviews