The most scandalous issue is grounding (resetting)

Generally speaking, it can be noted that the great and terrible power of electricity has long been described, calculated, introduced in thick tables. The regulatory frame, which defines the paths of sinusoidal electrical signals with a frequency of 50 Hz, can plunge any neophyte into awe with its volume. And yet, every visitor to technical forums has long known that there is no more scandalous issue than grounding.

The mass of conflicting opinions doesn't really do anything to establish the truth. Moreover, this issue is really serious and requires more careful consideration.

Basic concepts

If you miss the introduction of the "electrical engineer's bible" (PUE), then to understand grounding technology, you should refer (for starters) to chapter 1.7, which is called «Grounding and electrical safety precautions».

In point 1.7.2. PUE says:

Electrical installations in terms of electrical safety measures are divided into:

• electrical installations above 1 kV in networks with an effectively earthed neutral (with large earth fault currents) ,;
• electrical installations above 1 kV in networks with isolated neutral (with low grounding currents);
• electrical installations up to 1 kV with grounded neutral;
• electrical installations up to 1 kV with isolated neutral.

The majority of residential and office buildings in Russia use a solidly grounded neutral... Point 1.7.4. reads:

A dead earthed neutral is the neutral of a transformer or generator connected to an earthing device directly or through a low resistance (eg through current transformers).

The term is not entirely clear at first glance - neutral and grounding device are not found at every turn in the popular science press. Therefore, below all incomprehensible places will be gradually explained.

Let's introduce some terms - so it will be possible to speak at least one language. Perhaps the points will appear "out of context". But PUE not fiction and such separate use must be fully justified — like the use of separate articles of the Penal Code. However, the original PUE is quite readily available both in bookstores and on the web—you can always refer to the original source.

• 1.7.6. Grounding any part of an electrical installation or other installation is the intentional electrical connection of that part to an earthing device.
• 1.7.7. Protective earthing is the earthing of parts of an electrical installation to ensure Electrical safety.
• 1.7.8. Working grounding is the grounding of every point of the current-carrying parts of an electrical installation, which is necessary to ensure the operation of an electrical installation.
• 1.7.9.Zeroing in electrical installations with a voltage of up to 1 kV is the intentional connection of parts of an electrical installation that are not normally supplied with an earthed neutral of a generator or transformer in three-phase current networks, with a dead earthed output of a single-phase current source, with a dead earthed central point of the source in DC networks.
• 1.7.12. A grounding conductor is called a conductor (electrode) or a set of metal-connected conductors (electrodes) in contact with the ground.
• 1.7.16. A ground wire is a wire that connects parts that need to be grounded to a ground wire.
• 1.7.17. The protective conductor (PE) in electrical installations is a conductor used to protect people and animals from electric shock. In electrical installations up to 1 kV, the protective conductor connected to the grounded neutral of the generator or transformer is called the neutral protective conductor.
• 1.7.18. Neutral working wire (N) in electrical installations up to 1 kV is a wire used to supply electrical receivers, connected to the grounded neutral of a generator or transformer in three-phase current networks, with a grounded output from a single-phase current source, with a dead point of the source in three-wire DC networks. Combined zero protective and zero working conductor (PEN) in electrical installations up to 1 kV is a conductor that combines the functions of zero protective and zero working conductors. In electrical installations up to 1 kV with a solidly grounded neutral, the neutral working conductor can perform the functions of a neutral protective conductor.

Rice. 1. The difference between protective grounding and protective «zero»

So, a simple conclusion follows directly from the PUE conditions.The differences between "ground" and "zero" are very small... At first glance (how many copies are broken in this place). At the very least, they should be connected (or even can be made "in one bottle"). The only question is where and how it is done.

Along the way we note paragraph 1.7.33.

Earthing or grounding of electrical installations must be carried out:

• at voltages of 380 V and more alternating current and 440 V and more direct current — in all electrical installations (see also 1.7.44 and 1.7.48);
• at nominal voltage above 42 V but below 380 V AC and above 110 V but below 440 V DC — only in rooms with increased danger, particularly dangerous and in outdoor installations.

In other words, there is no need to ground or neutralize a device connected to 220 volts AC at all. And there is nothing particularly surprising in this - the third wire is not actually observed in ordinary Soviet contacts. We can say that the Eurostandard (or the new edition of PUE, which is close to it), which is manifested in practice, is better, more reliable and safer. But according to the old PUE, they lived in our country for tens of years ... And what is especially important, the houses were built by whole cities.

However, when it comes to grounding, it's not just about the supply voltage. A good illustration of this is VSN 59-88 (Goskomarkhitektura) «Electrical equipment of residential and public buildings. Design Standards» Excerpt from Chapter 15. Grounding (Grounding) and Safety Precautions:

15.4. For grounding (grounding) of metal boxes of household air conditioners, stationary and portable household appliances of class I (without double or reinforced insulation), household electrical appliances with a capacity of St.1.3 kW, casings of three-phase and single-phase electric stoves, boilers and other heating equipment, as well as metal non-conductive parts of technological equipment in rooms with wet processes, a separate wire with a cross section equal to must be used phase one, placed by the circuit board or shield to which this electrical receiver is connected, and in the lines supplying medical equipment—from the ASU or the building's main switchboard. This wire is connected to the neutral wire of the supply network. The use of a working neutral wire for this purpose is prohibited.

It turns out to be a normative paradox. One of the results visible at the household level was the completion of Vyatka-automat washing machines with a coil of single-core aluminum wire with the requirement to perform grounding (by the hands of a certified specialist).

And one more interesting moment: 1.7.39. In electrical installations up to 1 kV with a solidly grounded neutral or solidly grounded output of a single-phase current source, as well as with a solidly grounded midpoint in three-wire DC networks, a reset must be carried out. The use in such electrical installations of grounding the housings of electrical receivers without their grounding is not allowed.

In practice, this means - if you want to "ground" - first "zero". By the way, this is directly related to the famous question of "battery charging" - which, for a completely incomprehensible reason, is mistakenly considered better than grounding (earthing).

Grounding parameters

The next aspect to consider is the numerical parameters of grounding. Since it is physically nothing more than a wire (or set of wires), its main characteristic will be resistance.

1.7.62. Resistance of the earthing device, kTo which neutrals of generators or transformers or outputs of a single-phase current source are connected, at any time of the year should not be more than 2, 4 and 8 ohms respectively at a line voltage of 660, 380 and 220 V at three-phase current source or 380, 220 and 127 V of single-phase current source. This resistance must be ensured by taking into account the use of natural grounded electrodes, as well as grounded electrodes for multiple grounding of the neutral conductor of overhead lines up to 1 kV with the number of outgoing lines at least two. In this case, the resistance of the grounding electrode located in close proximity to the neutral of the generator or transformer or the output of the single-phase current source must be no more than: 15, 30 and 60 Ohm, respectively, on line voltages of 660, 380 and 220 V on the three-phase current source or 380, 220 and 127 V single-phase current source.

For lower voltage, higher resistance is acceptable. This is completely understandable - the first purpose of grounding is to ensure the safety of a person in the classic case of a "phase" hitting the body of an electrical installation. The lower the resistance, the less potential can be "on the body" in the event of an accident. Therefore, the first step is to reduce the danger of higher voltages.

In addition, it should be noted that earthing is also used for the normal operation of fuses. For this, it is necessary that the breakdown line «just in case» has significantly changed properties (first of all, the resistance), otherwise the triggering will not happen.The greater the power of the electrical installation (and the voltage consumed), the lower is its working resistance and, accordingly, the grounding resistance must be lower (otherwise the fuses will not work due to a slight change in the total resistance of the circuit ).

The next standardized parameter is the cross section of the wires.

1.7.76. Grounding and neutral protective conductors in electrical installations up to 1 kV must have dimensions not smaller than those specified in the table. 1.7.1 (see also 1.7.96 and 1.7.104).

It is not advisable to give the entire table, an excerpt is sufficient:

For bare copper the minimum cross-section is 4 sq. mm, for aluminum — 6 sq. mm For insulated 1.5 sq. mm and 2.5 sq. mm respectively If the grounding wires enter the same cable with the power wires, their cross-section can be 1 sq. mm for copper and 2.5 sq. mm for aluminum.

Grounding in a residential building

In a normal "household" situation, users of the power grid (ie, residents) deal only with the group network (7.1.12 PUE. Group network — a network of panels and distribution points to lamps, sockets and other electrical receivers). Although in old houses, where the panels are installed directly in the apartments, they have to deal with part of the distribution network (7.1.11 PUE. Distribution network — network from VU, VRU, main switchboard to distribution points and panels). It is desirable to understand this well, because often "zero" and "ground" differ only in the place of connection to the main communications.

From this, the first grounding rule is formulated in PUE:

7.1.36.In all buildings, the lines of the group network, laid from group, floor and apartment shields to lamps of the general radiation, plugs and stationary electrical receivers must be three-wire (phase — L, zero working — N and zero protective — PE wires). It is not allowed to combine neutral working and neutral protective conductors from different group lines. It is not allowed to connect zero working and zero protective conductors of screens under a common terminal.

These.3 (three) wires must be laid from the floor, apartment or group panel, one of which is a protective zero (not grounded at all). Which, however, does not at all prevent it from being used for grounding a computer, a cable shield or a "tail" of lightning protection. Everything seems simple, and it is not entirely clear why to delve into such difficulties.

You can look at your home contact... And there's about an 80% chance you won't see the third contact there. What is the difference between neutral working and neutral protective conductors? In the control panel, they are connected on the same bus (although not at the same point). What would happen if you used a working zero in this situation as a backstop?

Let's assume that a careless electrician ephase and zero melt in the valve, this is difficult. Although users are constantly afraid of this, it is not possible to make a mistake in any state (although there are unique cases). However, the "working neutral" goes through multiple switches, probably through several distribution boxes (usually small, round, mounted in the wall near the ceiling).

It is much easier to confuse the phase with zero there (he himself did it more than once).As a result, there will be 220 volts in the event of an improperly "grounded" device. Or even simpler — a contact will burn out somewhere in the circuit — and almost the same 220 will go to the box through the load of the electric consumer (if this is an electric stove with a power of 2-3 kW, this will not seem enough) .

For the function of protecting a person — frankly, this situation is useless. But for connecting the grounding lightning protection of the APC type it is not fatal because there is a disconnection of high voltage. It would be unequivocally incorrect to recommend such a method from a security perspective. Although it must be admitted that this rule is broken very often (and usually without any adverse consequences).

It should be noted that the lightning protection capabilities of the working and protective zero are approximately equal. Resistance (to the connecting bus) differs slightly, and this is probably the main factor affecting the flow of atmospheric pickups.

From the further text of the PUE, it can be noted that literally everything in the house must be connected to the zero protective wire:

7.1.68. In all premises, it is necessary to connect open conductive parts of lamps for general lighting and stationary electrical receivers (electric stoves, boilers, domestic air conditioners, electric towels, etc.) to the neutral protective conductor.

In general, it is easier to imagine the following illustration:

Rice. 2. Grounding diagram

The picture is quite unusual (I will do it for everyday life). Literally everything in the house must be grounded to a dedicated bus.Therefore, the question may arise - after all, we live without it for tens of years and everyone is alive and well (and thank God)? Why change everything so seriously? The answer is simple — there are more and more electricity consumers and they are more powerful. Accordingly, the risks of defeat also increase.

But the dependence of safety and cost is a statistical value, and no one cancels the savings. Therefore, blindly laying around the perimeter of the apartment with a copper strip with a decent section (instead of a plinth), placing everything on it, up to the metal legs of the chair, is not worth it. Because you should not walk in a fur coat in the summer, but constantly wear a motorcycle helmet. This is already a question of adequacy.

It is also worth ascribing independent digging of trenches under the protective contour in the area of ​​an unscientific approach (in a city house, this will certainly bring nothing but trouble). And for those who still want to experience all the pleasures of life — in the first chapter of PUE there are standards for the production of this fundamental structure (in the absolutely literal sense of the word).

Summarizing the above, the following practical conclusions can be drawn:

• If the group network is made with three wires, a protective neutral can be used for grounding / neutralization. He was actually invented for this.
• If the group network is made with two wires, it is recommended to run a protected neutral wire from the nearest shield. The cross-section of the wire must be more than the phase (more precisely, you can consult the PUE).