Vitaly
Good afternoon What explains this significant difference in the permissible values ​​of current flowing through a cable of the same cross-section when open and closed types wiring?

Answer:

Wires or cables laid openly ( exposed wiring), cool better than wires and cables ( hidden electrical wiring) laid in pipes or hidden under plaster, suspended ceiling and behind hemmed walls. Wires with rubber insulation allow long-term heating of their cores, not exceeding 65 °C, and wires with plastic insulation - 70 °C. The cross-section of the current-carrying conductors is selected based on the maximum permissible heating of the conductors, at which the insulation of the wires is not damaged.

If you cannot carry out electrical measurements yourself, then use the services of specialists from a mobile electrical laboratory.

Electric current in metals is the ordered movement of electrons under the influence of electric field. The most convincing evidence of the electronic nature of current in metals was obtained in experiments with the inertia of electrons (the experiment of Tolman and Stewart):

Coil with a large number turns of thin wire (Fig. 9.1) was brought into rapid rotation around its axis. The ends of the coil were connected using flexible wires to a sensitive ballistic galvanometer. The untwisted coil was sharply slowed down, and a short-term current arose in the circuit due to the inertia of the charge carriers. The total charge flowing through the circuit was measured with a galvanometer. When braking a rotating coil, each charge carrier e with mass m is acted upon by a braking force, which plays the role of an external force, that is, a force of non-electric origin:

The external force per unit charge is, by definition, the field strength of the external forces:

Consequently, in the circuit when the coil is braking, an electromotive force arises:

During the braking of the coil, a charge q will flow through the circuit equal to:

where is the length of the coil wire, I is the instantaneous value of the current in the coil, R is the total resistance of the circuit, is the initial linear speed wire. The good electrical conductivity of metals is explained by the high concentration of free electrons, equal in order of magnitude to the number of atoms per unit volume. The assumption that electrons are responsible for the electric current in metals arose much earlier than the experiments of Tolman and Stewart. Back in 1900, the German scientist P. Drude, based on the hypothesis of the existence of free electrons in metals, created the electronic theory of metal conductivity. This theory was developed in the works of the Dutch physicist H. Lorentz and is called classical electronic theory. According to this theory, electrons in metals behave like an electron gas, much like an ideal gas. Electron gas fills the space between the ions that form the metal's crystal lattice. Due to interaction with ions, electrons can leave the metal only by overcoming the so-called potential barrier. The height of this barrier is called the work function.
At ordinary (room) temperatures, electrons do not have enough energy to overcome the potential barrier. According to the Drude–Lorentz theory, electrons have the same average energy of thermal motion as the molecules of a monatomic ideal gas. This allows us to estimate the average speed of thermal motion of electrons using the formulas of molecular kinetic theory:

When an external electric field is applied to a metal conductor, in addition to the thermal movement of electrons, their ordered movement (drift), that is, an electric current, occurs. The electron drift velocity is in the range of 0.6 – 6 mm/s. Thus, the average speed of ordered movement of electrons in metal conductors is many orders of magnitude less average speed their thermal movement. The low drift speed does not contradict the experimental fact that the current in the entire circuit DC installs almost instantly. Closing the circuit causes the electric field to propagate at a speed c= 3·10 8 m/s. After time (l is the length of the chain), a stationary distribution of the electric field is established along the chain and the ordered movement of electrons begins in it.
In the classical electronic theory of metals, it is assumed that the movement of electrons obeys Newton's laws of mechanics. In this theory, the interaction of electrons with each other is neglected, and their interaction with positive ions is reduced only to collisions. It is also assumed that with each collision the electron transfers to the lattice all the energy accumulated in it. electric field energy and therefore after the collision it begins to move with zero drift speed. Despite the fact that all these assumptions are very approximate, the classical electronic theory qualitatively explains the laws electric current in metal conductors: Ohm’s law, Joule–Lenz’s law and explains the existence electrical resistance metals
Ohm's Law:

Electrical resistance of the conductor:

Joule-Lenz law:

However, in a number of issues, classical electronic theory leads to conclusions that are in conflict with experiment. This theory cannot, for example, explain why the molar heat capacity of metals, as well as the molar heat capacity of dielectric crystals, is equal to 3R (Dulong and Petit's law). The classical electronic theory also cannot explain the temperature dependence of the resistivity of metals: the theory gives, while from experiment the dependence ρ ~ T is obtained. The most a shining example The discrepancy between theory and experiment is superconductivity. The qualitative difference between metals and semiconductors (dielectrics) lies in the nature of the dependence of specific conductivity on temperature. For metals, the conductivity decreases with increasing temperature, while for semiconductors and dielectrics it increases. At T o K, pure metals have conductivity s o ¥. For semiconductors and dielectrics at T o K, s o 0. There is no qualitative difference between semiconductors and dielectrics in terms of electrical conductivity. Manifestation in some substances metallic properties, and for other semiconductor and dielectric ones can be consistently explained only within the framework of quantum theory.
According to quantum concepts, the energy of electrons in an atom can change in a discrete manner. Moreover, according to the Pauli principle, there can be no more than one electron in one quantum state. As a result, electrons are not collected at any one energy level, but sequentially fill the allowed energy levels in the atom, forming its electron shells. When approaching large number atoms and the formation of a crystal structure chemical bonds between atoms are formed due to electrons located in the outer, valence, electron shells.
According to the Pauli principle, atoms cannot bunch together into a dense mass, since in this case there would be many particles in one quantum state with half-integer spin - the proper angular momentum (L = ħ/2). Such particles are called fermions, and they include, in particular, electrons, protons, and neutrons. They are named after the Italian physicist E. Fermi, who was the first to describe the peculiarities of the behavior of groups of such particles. When a large number of atoms come together within a solid, the initial energy level of the valence electron in the atom splits into N sublevels, where N is the number of atoms forming the crystal. As a result, a zone of allowed energy levels for electrons in a solid is formed (Fig. 9.2).

Fig.9.2
In metals, the outer valence shells are not completely filled, for example, silver atoms have one electron in the outer shell 5s1, while, according to the Pauli principle, there could be two electrons with different spin orientations, but the second electron in the outer shell of the silver atom is simply No. When N Ag atoms approach each other and the outer energy level 5 splits s 1 1 into N sublevels, each of them is filled with two electrons with different spin orientations. As a result, when N silver atoms approach each other, an energy band appears that is half filled with electrons. The energy corresponding to the last filled electronic level at 0 K is called the Fermi energy eF≈kTg. The distance between neighboring energy levels DE is very small, since N is very large, up to.
e F ~ 1¸10 eV, Δ E=eF/ N << kT» 0.025 eV.

The distance between adjacent allowed levels of electrons in metals is much less than the energy of thermal motion of electrons even at the lowest temperatures. If you place a conductor in an electric field, turning it on, for example, in a closed circuit with an EMF source, then electrons will begin to move from a point in the conductor with a lower potential to a point with a higher potential, since their charge is negative. But movement in an electric field means an increase in the energy of the electron, and according to quantum concepts, a transition to a higher energy level of an electron is possible if this neighboring level is free. In metals, there are quite enough such free levels for electrons located near the Fermi level, so metals are good conductors of electric current.
However, this conductivity is provided not by all free electrons of the metal, but only by those located near the Fermi level. The concentration of such electrons is approximately equal to nT /T g, Where T g= 5×10 4 K– degeneration temperature.

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The word “electrical wiring” is familiar to everyone, but not everyone will correctly explain its meaning. For many, the concept is associated exclusively with wires. In fact, this is a whole system that includes:

• insulated wires laid in the building;
• unarmored cables;
• their protective structures;
• connecting and fastening elements.

And most importantly, this whole system called “electrical wiring in the house” has a specific purpose - to deliver electricity from the power source and distribute it to consumers.

It is also important to understand the difference.

Electrical wiring is carried out in networks with voltages up to 1000 V: in buildings and structures (inside or outside), on the territories of institutions and enterprises, in courtyards and neighborhoods, on construction sites and personal plots.

Cables and wires with voltages over 1000 V, laid in special channels, wells, and on supports are already power transmission lines (power lines).

TYPES AND TYPES OF ELECTRICAL WIRINGS

According to placement, electrical wiring is of two types:

• internal (mounted inside the rear);
• external (laid along the external walls of buildings).

According to the method of installation, both external and internal wiring are also classified into hidden and open.

OPEN ELECTRICAL WIRING

With open wiring, cables and wires are laid along wall surfaces, ceilings and other building elements of the building.

There are several installation methods:

• free suspension;
• directly on the wall surface or ceiling;
• in electrical baseboards;
• on strings;
• in platbands;
• on cables;
• in trays;
• on rollers;
• on insulators;
• in boxes;
• in the pipes.

In turn, open wiring is divided into three more subtypes:

Stationary.

This is wiring that is permanently connected and cannot be disconnected without the use of a special tool.

Portable.

The contacts of such wiring are connected to each other using plug connectors (not twisted or soldered), that is, such wiring can be disconnected at any time.

Mobile.

Used to connect mobile machinery to the electrical network.

The advantages of open wiring are as follows:

1. It is easy to install. Not even the most experienced electrician, but an ordinary man who understands electricity, can do such electrical wiring in an apartment with his own hands.

2. No additional equipment is required (such as a power tool for cutting grooves).

3. The integrity of ceilings and walls is minimally damaged during installation.

4. The wiring is available at any time for inspection or repair of damage.

5. It is mobile; if you need to move a switch or socket to another location, this can be done without any problems.

Disadvantages of open wiring:

1. It is unattractive and does not always fit into the interior.

2. During installation, it is necessary to take into account the technical standards and requirements of the premises (it cannot be used everywhere).

3. The open installation method is the most dangerous in terms of fires. If the load exceeds the permissible limit, overheating of the wiring and a fire is possible, which will immediately spread to the wallpaper or decor.

HIDDEN ELECTRICAL WIRING

Hidden electrical wiring is laid inside the structural elements of the building - in floors and foundations, in wall surfaces, under removable floors. There are several ways to lay a cable or wire:

• in pipes;
• in furrows under plaster;
• in a flexible metal sleeve;
• in the voids of a building structure;
• in the channels;
• in boxes;
• in niches of plasterboard structures.

Nowadays it is rare, but in Soviet times such a method as embedding conductors into a building structure during its manufacture was often used.

Advantages of hidden wiring:

1. It is not visible, does not spoil the interior of the room and does not interfere with any finishing work.

2. This installation method is characterized by a high level of electrical safety. All conductors are hidden, which means touching live parts and electric shock are minimized.

3. Being under a layer of plaster, hidden wiring does not have air access, as a result of which it has high fire safety.

4. Hidden wiring elements are not exposed to solar or mechanical influence, thereby increasing their service life.

Disadvantages of the hidden method of laying electrical wiring. postings:

1. It is practically impossible to repair such wiring. If a wire burns out somewhere, finding the location of the damage will be very problematic.

2. Labor-intensive installation.

3. It is necessary to immediately clearly think through the locations of sockets and switches, and the routes for laying wires, since it will be difficult to change anything in the future.

4. It will be necessary to draw up an exact wiring diagram. After all, when you need to drill a hole to attach a picture or shelves, you need to be sure that you won’t hit the wire with the drill. You can, of course, do without a circuit, but then you will have to buy a special device to detect hidden wiring.

EXTERNAL ELECTRICAL WIRING

In another way, external electrical wiring is called street wiring. It is laid outside houses, along the walls of structures and buildings, as well as between them on special structures or supports.

Such wiring is necessary to supply voltage to lighting lamps, alarm systems, CCTV cameras and utility rooms (garage, workshop, barn, sauna, bathhouse, swimming pool). All this must be taken into account at the stage of construction of buildings and structures.

External electrical wiring in a country house is no less important than internal wiring. In addition to the need to provide sufficient lighting in the area, at any time you may need to connect a power tool (drill, grinder, lawn mower, pump) in the yard. Don’t drag the carriers across the entire house and property; when planning the power supply, it is better to mark out places for installing 2-3 sockets.

The most important difference between external electrical wiring is that it is exposed to atmospheric factors - rain, snow, wind, sun rays. Therefore, it must be reliably protected from precipitation, from mechanical influences and accidental human contact with live parts.

General requirements for electrical wiring.

These requirements apply equally to both apartments and private houses:

1. All boxes, sockets and switches must be located in such places that they are always easily accessible.

2. All connections and branches are made only in boxes.

4. Grounding and grounding conductors are connected only by welding.

5. Grounding of stationary plates is carried out by separate conductors.

According to the PUE (Electrical Installation Rules), the following are also regulated:

• standards for the number of sockets per room;
• the distances at which sockets and switches should be placed in relation to the floor and communications;
• norms for laying wires (vertical and horizontal sections).

ELECTRICAL WIRING IN THE HOUSE AND APARTMENT

Apartment wiring is not as expensive in terms of finances, effort and time as wiring in a private home. In multi-storey buildings, each apartment is already connected to an electrical network from the general distribution panel to the input circuit breaker.

Do-it-yourself wiring in apartments is within the capabilities of many and is carried out according to the following approximate plan:

1. You should start by drawing up a diagram.
2. Based on the diagram, you need to make markings on the walls and calculate the required amount of wire, cable, switching devices and fittings.
3. Next comes the installation work - cutting grooves, laying conductors in them, installing boxes, switches and sockets, and connecting all elements.
4. The final stage is checking the operation of the assembled circuit.

Electrical wiring in a private house.

Electrical wiring in a private house is also quite possible to do with your own hands by a person who understands electricity. Just keep in mind that it differs from the apartment option, since it has additional requirements.

Power supply should be provided from a 220 V or 380 V network, with a TN-C-S grounding system. It is prohibited to lay conductors along heated surfaces, as well as near a stove, chimney or fireplace (this rule is especially true for saunas and baths).

Particularly difficult when laying wiring yourself in a private house is the installation of the input. First you will have to go around several institutions to obtain permission and technical conditions.

There is a lot of work to be done on assembling the distribution panel and laying the input line (digging a trench for the cable or installing a tap from an overhead power line). And only then can you proceed with the internal electrical wiring according to the same plan as described above for the apartment option.

If you are not sure that you can install reliable and high-quality electricity. wiring in an apartment or private house yourself in accordance with the requirements of the PUE, it is better to invite a specialist. Remember that 65% of all electrical fires occur in the home electrical system.

* * *

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The materials on the site are for informational purposes only and cannot be used as guidelines or regulatory documents.

First, let's figure out what internal wiring is. This is, simply put, all the wires and cables with fasteners through which current flows in your home.

There are several types of electrical wiring

1. passes along the surface of ceilings, walls or other parts of the building.
2. Hidden electrical wiring is laid inside walls or other elements of buildings.
3. External electrical wiring is laid along the outer parts of buildings. It can also be either open or hidden.

We've sorted out the types of wiring, now let's proceed directly to the installation of internal electrical wiring. And so, depending on its type, it is produced at different stages of construction.

Closed wiring

Closed wiring should be installed only before all finishing work begins, since if the room has already been finished, additional costs will be required for re-finishing. Mounted in parts between distribution boxes. The wires are laid in pre-made grooves (grooves) and then plastered.

Installation of open wiring

Produced after finishing work. The wires are secured with tin plate brackets, porcelain insulators or special plastic fasteners that are installed every half meter. If the walls consist of flammable materials, then passing through them, the wires must be laid in metal or plastic pipes.

Preparation

Before you pick up a tool, you need to carefully calculate everything and draw up a plan. Switches are located near doorways. If the wiring runs in unheated or damp rooms, then you need to use wires in a special sheath, or put them in metal pipes.

When drawing up a plan, the required cable length is calculated. The wire is taken with a margin of 10-15 centimeters at the ends so that switches, lamps, sockets can then be connected and twisted together in junction boxes. It is recommended to save the plan diagram, as you may need to repair the wiring or simply know where the wires go if you need to hammer a nail into the wall.

Electrical wiring installation

The cable must be laid strictly vertically or horizontally. On turns and bends it should pass at 90 degrees. If it intersects with metal parts, it is necessary to strengthen the insulation. You can strengthen the insulation simply by wrapping several turns of electrical tape at the point of contact with the metal.

We punch the grooves (grooves) using a wall chaser, a hammer drill, or, in the absence of either, simply with a chisel and a hammer. If the walls are wooden, then sheet asbestos is placed under the wire.

It is worth paying special attention to the fact that the wire, even if it is flat, cannot be fastened with nails. We insulate even the unpowered ends, and after the walls are plastered, the ends are connected to each other or connected to switches and sockets. Aluminum wires only need to be soldered together, but for copper both soldering and twisting are suitable. In sockets, switches and lamps, we clamp the stripped ends with screws in the terminals.

The most reliable method of hidden wiring is laying wires in the thickness of walls, partitions and ceilings in specially provided or technological voids. In this case, the wires are not visible to the eye and do not “eat up” the space. However, this method is quite expensive both in terms of time and finances, so it is rarely used.

It is a little easier to lay cables in special wall grooves called grooves. They can be made in brick walls or solid blocks using a hammer drill. It is better to lay the cable in the duct in a flexible tube made of non-flammable PVC, which will protect it from damage during the installation process.

You can also lay cables along the surface of the walls, securing them every 25-30 cm with nails or alabaster. Then the wires will fall under a layer of putty and plaster, which is used to level the walls for finishing. This option is also safe, since the wiring is located in the thickness of fireproof material. True, this method also involves opening the walls if repairs are necessary.

Therefore, with any hidden wiring option, it is necessary to provide the required number of outlets for sockets and switches, and install junction boxes at the places where the wires are connected. They are closed with plastic lids, which is both aesthetically pleasing and safe.

Open wiring

Exposed wiring allows you to install additional outlets, lights, switches, as well as additional low-voltage equipment, telephones, speakers and computers without opening walls or ceilings. This method is the only one possible in wooden log houses. Switches and sockets with this installation method are not attached directly to the wall surface, but on special insulating socket pads.

The easiest way to install open wiring is to attach the wires in metal tubes to the walls. This is not aesthetically pleasing, but it is possible in utility rooms, in the attic or in the country house.

Installation of wires in plastic cable channels (boxes) is more aesthetically pleasing. They consist of two parts: a base and a cover. The plastic cable channels themselves are not electrically conductive, so they are not grounded. During installation, turn off the power, secure the base to the wall or baseboard using glue or self-tapping screws, route the wires and snap the lid onto each cable channel box. After that, connect the wires and install sockets.

Plastic cable ducts are made from polyvinyl chloride (PVC), which, unlike polyamides, polyesters and other plastics, does not support combustion. They come in white, cream, gray, dark brown colors, and also with a film coating that imitates the texture of wood. They produce cable channels of various sections, 1.5-2 m long. There are also cable channels in the form of a plinth with an internal cavity for laying cables. Systems of electrical baseboards and boxes usually include special sockets that are mounted close to the baseboard and match it in color and design.

Open wiring kits are also available for sale, in which the sockets are already mounted on the cable channel and connected to the wires.

The terms “open” and “hidden wiring” will not surprise anyone now. There are, however, disagreements regarding their definition. For example, what kind of wiring is it under plaster? Open or hidden? Anyone will say: hidden. And he will be right. And if the wiring in the corrugated pipe is fixed to the wall, is it hidden or exposed? Everyone will say: open, and again they will be right. Here's a tricky question: the wiring is hidden above the suspended ceiling. Is it hidden or open? And the one under the drywall on the walls?

What do the PUE (Electrical Installation Rules) say about this? And the PUE says this:

2.1.4. Electrical wiring is divided into the following types:

1. Open wiring- laid along the surface of walls, ceilings, along trusses and other construction elements of buildings and structures, along supports, etc.
...
2. Hidden wiring- laid inside the structural elements of buildings and structures (in walls, floors, foundations, ceilings), as well as over ceilings in floor preparation, directly under a removable floor, etc.
...

Whether drywall is considered part of the wall or not - opinions vary. And in my opinion, open wiring is one that is easy to access and does not require dismantling structures (walls, ceilings...). And if repairs require breaking plaster or drywall, this is hidden wiring.

What is good about one and bad about the other? Here, too, everything is ambiguous. If I live in a one-room apartment in a common shelving unit, then it would be better not to see it, this wiring. Let it hide in the walls without killing the already dull design of the living cell. I got an outlet for the TV and refrigerator - and I don’t need anything else, either today, or tomorrow, or in the foreseeable future. Because you can’t move the refrigerator to another corner, you can’t plug the TV into another place.

Having your own home is a completely different philosophy. Today I arranged the furniture like this, tomorrow I completely swapped the bedroom and kitchen. Yesterday I didn’t have a microwave, today I got a dishwasher. Previously there was one TV, now there are two and each has its own receiver. A computer appeared, or some other device - you also need power, and other cables...

The bathroom was renovated - an automatic lighting switch with a motion sensor was installed... And so almost every year, all my life. My wiring in the house has probably been changed dozens of times. Everything changes, something new appears, the old is thrown away. New sockets appear and then disappear out of sight. What the hell is this hidden wiring?

But even the so-called open wiring In most cases, it is quite possible to hide it out of sight, leaving fairly quick access to it for control and repair. For example, after the last renovation, the wires are almost invisible in my house. They are hidden somewhere in baseboards, somewhere in cable ducts behind furniture or even inside built-in cabinets.

Metal hoses are laid in wooden partitions and throughout the attic from switches to ceiling chandeliers. And only where there is no way to hide it, it is forced to be in plain sight in cable ducts.

There are no distribution boxes at all. All necessary connections are made in cable ducts and baseboards using soldering. I talked about connection methods in the article Connecting wires. Terminal blocks or soldering? For powerful consumers (electric stove, heating elements) - separate individual wires from the machines without any intermediate connections.

At any moment, if some new synchrophasotron suddenly appears in the house, there are no problems connecting it to the home network. And even if a rearrangement or redevelopment occurs, it is not difficult to change the wiring. Because it is available.

Questions like “what are the pros and cons”, “which is better” are very common... And there are numerous attempts to create such a list of pros and cons. Someone baselessly claims that in one case the aging of insulation occurs faster, someone talks about some kind of protection from accidental damage - believe me, this is all from the evil one. But everyone agrees on one thing: they say, open wiring violates the design.

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