Electricity is man's best friend and worst enemy. Of course, now it is almost impossible to imagine life without it. Unfortunately, there were some bad moments, such as electric shock. You can be shocked if you touch not only a bare current-carrying part, but also a harmless-looking body of an electrical appliance. In this article we will try to explain in simple terms what grounding is and what it is intended for. In addition, we will consider what a difavtomat and RCD are and what they are used for.
Concept definition
In short and simple terms:
Grounding is a device that protects a person from electric shock if all electrical equipment is connected to the ground. In an emergency, dangerous voltage "flows" to the ground.
Protection is the main purpose of grounding. It consists in connecting an additional, third ground conductor to the wiring, which is connected to a device such as a ground electrode. He, in turn, has good contact with the ground.
Grounding is working and protective for its intended purpose. The worker is necessary for the normal functioning of the electrical installation, the protective one is necessary to ensure electrical safety (prevention of electric shock).
Usually, grounding (ground electrode) looks like three electric rods driven into the ground, at the same distance from each other, located at the corners of an equilateral triangle. These rods are interconnected by a metal strip. You could see such rods near houses and structures.
You may also have noticed that metal strips are fixed on the walls of many buildings inside or outside, sometimes painted with yellow and green alternating stripes - this, it is also connected to the ground electrode. A ground bus is needed in order not to pull a ground wire from each electrical installation.
The third conductor is usually connected to the housing of electrical appliances, providing protection against the appearance of dangerous voltage on it. In cables, it usually has a smaller cross section than adjacent "working" cores and a different color of insulation - yellow-green.
Grounding Requirements
The requirements for a protective earth loop are as follows:
- All electrical installations must be grounded, including the metal doors of electrical cabinets and shields.
- The resistance of the grounding device must not exceed 4 ohms in electrical installations with a grounding neutral.
- Nessesary to use .
We figured out what grounding is, now let's talk about what it is for.
Why does a person get electrocuted
Consider two typical situations when you are shocked:
- The washing machine did its job properly, and when you wanted to turn it off, you felt that its body was “pinching” you. Or even worse, when you touched it, you were seriously “twitched”.
- You decided to take a bath, turned on the water, holding the faucet, you felt the same effect of electricity - a tingling or a strong blow.
Both situations are solved by connecting grounding to the instrument cases and all metal parts in the bathroom and installing an RCD or differential machine at the input of electricity to a house or a group of consumers.
How grounding works
To begin with, let's figure out why a dangerous voltage appeared on the body of a washing machine or other electrical equipment. Everything is quite simple - the insulation of the conductors for some reason deteriorated or was damaged and the damaged area touches the metal case of one of the equipment parts.
If there is no grounding or zeroing of electrical equipment, then when a person touches a damaged device, it may occur (potential difference on the surface between the points of contact). If you are near damaged equipment, you may experience (potential difference between feet in contact with the ground). The touch voltage and step voltage can be dangerous to humans. To reduce their value to a safe value, protective grounding is used.
Even such small values as 50 mA are dangerous for a person - such a current can lead to ventricular fibrillation and death.
So the principle of operation of grounding is as follows: the cases of all electrical appliances are connected to the ground electrode, and an RCD is additionally installed. In the event of a dangerous voltage on the case, grounding always attracts a dangerous potential to a safe ground potential and the voltage "drains" to ground.
What are RCDs and difavtomatov used for?
Simply grounding devices is fine, but it's even better to provide additional protection. For this, they came up with (RCD) and.
A difavtomat is a device that combines an RCD and a conventional circuit breaker in its case, so you save space in the electrical panel.
RCD - responds only to. The principle of its operation is as follows: it compares the amount of current through the phase and through the neutral wire, if part of the current has flowed to the ground, then it instantly reacts, turning off the circuit. They are distinguished by sensitivity from 10 to 500 mA. The more sensitive the RCD, the more often it will work, even with minor leaks, but you should not install too rough an RCD for your home.
The principle of operation of a secure circuit in simple terms:
When a phase enters the body of grounded electrical equipment, current begins to flow between the phase wire and the body. Then the RCD notices that a current has passed through the phase wire, part of the current has to be done somewhere and a smaller current has returned through the neutral wire, after which this circuit is de-energized. So you are protected from electric shock.
If you install an RCD in a two-wire electrical circuit without a ground conductor and there is a possibility of current leakage somewhere, it will work only after you touch this place and the current will flow to the ground through you. In this case, you will be safe too.
That's all we wanted to say about this issue. Now you know what grounding is, when and how it is installed and what it serves. We hope that the information was presented for you in a clear and understandable way!
The electrical connection of an object made of conductive material to earth. Grounding consists of a grounding conductor (a conductive part or a set of interconnected conductive parts that are in electrical contact with the ground directly or through an intermediate conductive medium) and a grounding conductor connecting the grounded device to the grounding conductor. The grounding conductor can be a simple metal rod (most often steel, less often copper) or a complex set of special-shaped elements.
The quality of grounding is determined by the value of the electrical resistance of the grounding circuit, which can be reduced by increasing the contact area or the conductivity of the medium - using many rods, increasing the salt content in the ground, etc. in Russia, the requirements for grounding and its device are regulated.
Protective grounding conductors in all electrical installations, as well as zero protective conductors in electrical installations with voltage up to 1 kV with a solidly grounded neutral, including tires, must have the letter designation PE and color designation with alternating longitudinal or transverse stripes of the same width (for tires from 15 to 100 mm ) yellow and green.
Zero working (neutral) conductors are indicated by the letter N and blue. Combined zero protective and zero working conductors must have the letter designation PEN and color designation: blue along the entire length and yellow-green stripes at the ends.
Errors in the grounding device
Wrong PE conductors
Sometimes water pipes or heating pipes are used as a ground conductor, but they cannot be used as a ground conductor. There may be non-conductive inserts in the plumbing (such as plastic pipes), the electrical contact between the pipes may be broken due to corrosion, and finally, a part of the pipeline may be dismantled for repair.
Combining a working zero and a PE conductor
Another common violation is the union of the working zero and the PE conductor beyond the point of their separation (if any) along the distribution of energy. Such a violation can lead to the appearance of quite significant currents in the PE conductor (which should not be current-carrying in the normal state), as well as to false trips of the residual current device (if installed). Incorrect separation of the PEN conductor
The following way of “creating” a PE conductor is extremely dangerous: a working neutral conductor is determined directly in the socket and a jumper is placed between it and the PE contact of the socket. Thus, the PE conductor of the load connected to this outlet is connected to the working zero.
The danger of this circuit is that a phase potential will appear on the grounding contact of the socket, and therefore on the case of the connected device, if any of the following conditions are met:
- Rupture (disconnection, burnout, etc.) of the neutral conductor in the area between the socket and the shield (and further, up to the grounding point of the PEN conductor);
- Swap the phase and zero (phase instead of zero and vice versa) conductors going to this outlet.
Protective function of grounding
The protective effect of grounding is based on two principles:
Reduction to a safe value of the potential difference between a grounded conductive object and other conductive objects that have a natural ground.
Removal of leakage current when a grounded conductive object contacts a phase conductor. In a properly designed system, the appearance of a leakage current leads to the immediate operation of the protective devices ().
Thus, grounding is most effective only in combination with the use of residual current devices. In this case, for most insulation failures, the potential on grounded objects will not exceed dangerous values. Moreover, the faulty section of the network will be disconnected within a very short time (tenths of hundredths of a second - the RCD trip time).
Grounding operation in case of electrical equipment malfunctions A typical case of electrical equipment malfunction is the phase voltage entering the metal case of the device due to insulation failure. Depending on what protective measures are implemented, the following options are possible:
The case is not grounded, there is no RCD (the most dangerous option). The case of the device will be under phase potential and this will not be detected in any way. Touching such a malfunctioning device can be fatal.
The case is grounded, there is no RCD. If the leakage current along the phase-housing-grounding circuit is large enough (exceeds the trip threshold of the fuse that protects this circuit), then the fuse will trip and turn off the circuit. The highest operating voltage (relative to earth) on a grounded case will be Umax=RGIF, where RG ? ground electrode resistance, IF ? the current at which the fuse that protects this circuit operates. This option is not safe enough, since with a high resistance of the ground electrode and large fuse ratings, the potential on the grounded conductor can reach quite significant values. For example, with a grounding resistance of 4 ohms and a 25 A fuse, the potential can reach 100 volts.
The case is not grounded, the RCD is installed. The case of the device will be at phase potential and this will not be detected until there is a path for the leakage current to pass. In the worst case, leakage will occur through the body of a person who has touched both a faulty device and an object that has a natural ground. The RCD disconnects the section of the network with a malfunction as soon as a leak occurs. A person will receive only a short-term electric shock (0.010.3 seconds - the RCD operation time), which, as a rule, does not cause harm to health.
The case is grounded, the RCD is installed. This is the safest option since the two protective measures complement each other. When a phase voltage hits a grounded conductor, current flows from the phase conductor through an insulation fault into the ground conductor and further into the ground. The RCD immediately detects this leak, even if it is very small (usually the RCD sensitivity threshold is 10 mA or 30 mA), and quickly (0.010.3 seconds) disconnects the section of the network with a malfunction. In addition, if the leakage current is high enough (greater than the threshold of the fuse protecting that circuit), then the fuse may also blow. Which protective device (RCD or fuse) will turn off the circuit depends on their speed and leakage current. It is also possible for both devices to operate.
Ground types
TN-C
The TN-C system (fr. Terre-Neutre-Combine) was proposed by the German concern AEG (AEG, Allgemeine Elektricitats-Gesellschaft) in 1913. Working zero and PE-conductor (Protection Earth) in this system are combined into one wire. The biggest drawback was the formation of a linear voltage (1.732 times higher than the phase voltage) on the housings of electrical installations during an emergency zero break.
Despite this, today you can find this in the buildings of the countries of the former USSR.
TN-S
To replace the conditionally dangerous TN-C system in the 1930s, the TN-S system (fr. Terre-Neutre-Separe) was developed, in which the working and protective zero were separated directly at the substation, and the ground electrode was a rather complex design of metal fittings.
Thus, when the working zero was interrupted in the middle of the line, the electrical installations did not receive line voltage. Later, such a grounding system made it possible to develop differential automata and automatons that are triggered by current leakage, capable of sensing a small current. Their work to this day is based on Kirghof's laws, according to which the current flowing through the phase wire must be numerically equal to the current flowing through the working zero current.
You can also observe the TN-C-S system, where the separation of zeros occurs in the middle of the line, however, in the event of a break in the neutral wire, up to the point of separation of the case, they will be under line voltage, which will pose a threat to life when touched.
The operation of modern electrical equipment is unacceptable without well-organized protection against accidental electric shock. For these purposes, special devices are used, which are called grounding. Thus, grounding is a deliberately organized system that provides normal operating conditions for electrical equipment.
About grounding in simple words
The very concept of "grounding" comes from the word "earth", that is, soil or soil, the purpose of which is to serve as a drain for dangerous currents flowing through a specially organized circuit. For its formation, it is necessary to have an inseparable connection of all parts of the protective system, which starts from the point of contact of the grounding element body and ends with the element of the grounding device (GD) immersed in the ground.
External ground loop of a private house (left). Indoor grounding (right), the grounding conductor is indicated by a dotted line.
According to the definitions given in the technical documentation, grounding is a deliberate electrical connection of the metal casings of the units with a special grounding loop. Based on the facts considered, it can be concluded that grounding is the intentional electrical contact of the protected equipment with the ground.
Grounding Requirements
After you have figured out what is the definition of the very concept of grounding, you can move on to those categories and norms that are introduced by the current standards. According to the PUE, the following requirements are primarily imposed on the grounding device:
- the purpose of the charger is to effectively divert dangerous currents to the ground, for which their design provides for a whole set of conductors and metal rods;
- all parts of the electrical installation are subject to grounding, including the metal doors of the shields;
- the total contact resistance of the contacts in the grounding system should not exceed 4-30 ohms;
- when it is arranged in distributed loads, it is necessary to use a potential equalization system (its purpose is to eliminate the uneven distribution of voltages).
Additional Information: Since the main purpose of grounding is to ensure the safety of personnel working with the equipment, special attention is paid to the reliability of operation during its operation.
The quality of its work is ensured by a whole range of preventive measures and periodically organized tests.
In order to answer this question, you will need to familiarize yourself with the malfunctions that periodically occur in existing electrical equipment. The fact is that during its long-term operation, the destruction of the insulation and the appearance of contact between the bare wire of the power supply and the body of the electrical installation may occur.
Parts of steel blanks protruding from the ground by 10-15 cm are welded together with metal plates 40 mm wide (at least 4 mm thick). In the upper part of one of the vertical electrodes, a contact zone is arranged in the form of a threaded bolt welded onto it. On it, by means of a nut, the end of a copper bus running from the body of the grounded device is fastened, the cross section of which should not be less than 6 sq. mm.
Additional Information: To reduce the resistance of the emergency current drain circuit, this connection is sometimes made welded.
Upon completion of the main work, the trench with the structure placed in it is covered with previously thrown earth, from which stones and unnecessary debris are removed.
According to the requirements of the PUE, any grounding system must comply with technical standards in terms of the maximum permissible resistance to leakage current. Its value should be:
- less than 8 ohms in industrial networks with a phase voltage of 220/127 Volts;
- less than 4 ohms for line voltages of 380 volts;
- no more than 30 ohms in household networks (this figure is considered the maximum allowable).
The copper core laid from the structure of the charger is fixed with its second end on a special bar mounted on the switchboard of the object (at home, in particular). It is called the main ground bus (GZSH), and it is intended to assemble all protective conductors in one place. Copper conductors diverge from it directly to consumers (through sockets to the instrument cases).
Natural and artificial grounding
Natural grounding is an object or structure that has reliable contact with the ground due to its functions. This category includes:
- water and heating pipes laid directly in the ground;
- any metal structures and their elements that have good contact with the soil;
- sheaths of welding and similar cables;
- metal mortgages and tongues, etc.
Worth noticing! In this case, the arrangement of functional grounding will not require special efforts, since the elements of the natural grounding conductor are already ready for connecting grounding conductors.
In a situation where such systems cannot be found, you have to deal with the installation of home-made memory.
Artificial grounding is considered to be a deliberately organized electrical contact of two bodies, one of which is the protected device, and the second is the so-called "ground loop". This component is a special distributed (sometimes point) structure based on metal rods placed deep in the ground.
As a rule, steel bars with a diameter of up to 12 mm and a length of at least 2.5 meters are used as vertically hammered electrodes. To equip horizontal jumpers that provide electrical contact between two bodies, metal corners 50x50x6 mm and 2.5-3 meters long are taken (they can be replaced with pipes with a diameter of about 6 mm or more).
What is grounding for? Video
To understand why you need grounding in the house, you will have to familiarize yourself with its main purpose. As noted in the previously presented section, grounding serves to protect a person from a dangerous potential that accidentally appeared on the body of the operating equipment. The easiest way to get acquainted with the order of its work and purpose is on the numerous examples presented in the videos.
In conclusion, we note that understanding the purpose of grounding will help preserve the health of people working with electrical equipment.
Protective earth is an intentional electrical connection to earth or its equivalent of metallic non-current-carrying parts that may be energized.
Purpose of protective earthing- reduce to a safe value the voltage relative to the ground on the metal parts of the equipment that are not energized, but may become energized due to a violation of the insulation of electrical installations. As a result of a short circuit to the case of grounded equipment, the contact voltage decreases and, as a result, the current passing through the human body when it touches the cases.
Grounding of electrical equipment, buildings and structures is also used to protect against the action of atmospheric electricity.
Protective grounding is used in three-phase three-wire networks with voltage up to 1000 V with an isolated neutral, and in networks with a voltage of 1000 V and above - with any neutral mode.
Grounding device
Grounding device- this is a combination of a grounding conductor and grounding conductors connecting the grounded parts of an electrical installation with a grounding conductor.
Distinguish between natural and artificial ground electrodes.
For grounding devices, natural grounding conductors should be used first of all:
- water pipes laid in the ground;
- metal structures of buildings and structures having
- reliable connection to the ground;
- metal sheaths of cables (except for aluminum ones);
- casing pipes of artesian wells.
It is forbidden to use pipelines with flammable liquids and gases, pipes of heating mains as ground electrodes.
Natural grounding conductors must be connected to the grounding network at least in two different places.
The following are used as artificial grounding conductors:
- steel pipes with a diameter of 3-5 cm, wall thickness of 3.5 mm,
- 2-3 m long;
- strip steel with a thickness of at least 4 mm;
- corner steel with a thickness of at least 4 mm;
- bar steel with a diameter of at least 10 mm, a length of up to 10 m or more.
For artificial ground electrodes in aggressive soils (alkaline, acidic, etc.), where they are subject to increased corrosion, copper, copper-plated or galvanized metal is used.
Aluminum sheaths of cables, as well as bare aluminum conductors, cannot be used as artificial grounding conductors, since they oxidize in the soil, and aluminum oxide is an insulator.
Each individual conductor in contact with earth is called single grounding, or electrode. If the grounding conductor consists of several electrodes connected to each other in parallel, it is called group grounding.
To immerse vertical electrodes in the ground, they first dig a trench with a depth of 0.7-0.8 m, after which they clog pipes or corners using mechanisms. Steel rods with a diameter of 10-12 mm are buried in the ground using a special device, and longer ones using a vibrator. The upper ends of the vertical electrodes immersed in the ground are connected by a steel strip by welding.
The protective earthing device can be implemented in two ways: contour location of grounding conductors and remote.
With contour placement of grounding switches, potential equalization is ensured in case of a single-phase ground fault. In addition, due to the mutual influence of the ground electrodes, the contact voltage and the step voltage in the protected area are reduced. External groundings do not have these properties. But with a remote placement method, there is a choice of a place for deepening the ground electrodes.
In rooms, grounding conductors should be located in such a way that they are accessible for inspection and reliably protected from mechanical damage. On the floor of the premises, grounding conductors are laid in special grooves. In rooms where the release of caustic vapors and gases is possible, as well as with high humidity, grounding conductors are laid along the walls on brackets 10 mm from the wall.
Each body of the electrical installation must be connected to the grounding conductor or to the grounding line using a separate branch. Sequential connection of several grounded housings of electrical installations into a grounding conductor is prohibited.
The resistance of the grounding device is the sum of the resistances of the grounding conductor relative to the ground and the grounding conductors.
The resistance of the earth electrode relative to the earth is the ratio of the voltage on the earth electrode to the current passing through the earth electrode to the earth.
The value of the ground electrode resistance depends on the resistivity of the soil in which the ground conductor is located; type of dimensions and arrangement of elements from which the earthing conductor is made; the number and relative position of the electrodes.
The value of the resistance of ground electrodes can vary several times depending on the time of year. Grounding conductors have the greatest resistance in winter when the soil freezes and in dry times.
The highest permissible value of grounding resistance in installations up to 1000 V: 10 Ohm - with a total power of generators and transformers of 100 kVA or less, 4 Ohm - in all other cases.
These standards are justified by the permissible value of the contact voltage, which in networks up to 1000 V should not exceed 40 V.
In installations above 1000 V, ground resistance R 3 is allowed<= 125/I 3 Ом, но не более 4 Ом или 10 Ом.
In installations over 1000 V with high earth fault currents, the resistance of the grounding device should not exceed 0.5 Ohm to ensure automatic disconnection of the network section in the event of an accident.
Zeroing and protective shutdown
Zeroing- this is a deliberate electrical connection with a zero protective conductor of metal non-current-carrying parts that may be energized.
Zero protective conductor - a conductor connecting the parts to be nulled to the neutral point of the current source winding or its equivalent.
Zeroing is used in networks with voltage up to 1000 V with a grounded neutral. In the event of a phase breakdown, a single-phase short circuit occurs on the metal case of electrical equipment, which leads to a quick operation of the protection and, thereby, automatic disconnection of the damaged installation from the mains. Such protection are fuses or maximum circuit breakers installed for protection against short-circuit currents; magnetic starters with built-in thermal protection; contactors with thermal relays and other devices.
In the event of a phase breakdown on the case, the current goes along the path "case - neutral wire - transformer windings - phase wire - fuses". Due to the fact that the resistance during a short circuit is small, the current strength reaches large values and the fuses work.
The purpose of the neutral wire in the electrical network is to provide the amount of short-circuit current necessary to turn off the electrical installation by creating a low-resistance circuit for this current.
The neutral wire must be laid in such a way as to exclude the possibility of a break; it is forbidden to install fuses, switches and other devices in the neutral wire that can violate its integrity. The conductivity of the neutral wire must be at least 50% of the conductivity of the phase wire. Bare or insulated conductors, steel strips, aluminum cable sheaths, various metal structures of buildings, etc. are used as zero protective conductors.
Zeroing control of electrical equipment is carried out upon its acceptance into operation, as well as periodically during operation. Once every five years, the impedance of the "phase-zero" loop should be measured for the most remote, as well as the most powerful electrical receivers, but not less than 10% of their total number.
Safety shutdown is a special case of protective zeroing. Unlike zeroing, protective shutdown can be used in any networks, regardless of the accepted neutral mode, voltage value and the presence of a neutral wire in them.
Protective shutdown is a protection system that automatically turns off the electrical installation in the event of a danger of electric shock to a person (in the event of a ground fault, a decrease in insulation resistance, a grounding or zeroing fault). Protective disconnection is used when it is difficult to ground or neutralize, and also in addition to them in some cases.
Depending on what is the input value, to which the protective shutdown reacts, the following protective shutdown circuits are distinguished: on the case voltage relative to earth; for earth fault current; for zero sequence voltage or current; to the phase voltage relative to earth; for direct and alternating operating currents; combined.
Protective shutdown is carried out using circuit breakers equipped with a special protective shutdown relay. The protective shutdown response time is no more than 0.2 s.
In order to provide reliable protection when working under voltage, electrical installations are grounded. Protective earth is the intentional electrical connection of the machine housing to a grounding device. According to the principle of operation, all grounding is divided into two types. It can be performed in the form of protective grounding and grounding, which have exactly the same function, which consists in protecting people from the effects of electric current, in case of touching the case or other parts with broken insulation.
The essence of protective grounding
With a protective clamping device, a deliberate connection of parts of electrical installations and a grounding device is carried out. Thus, electrical safety is ensured in case of accidental contact with certain parts that are energized. This situation, as a rule, occurs during an insulation breakdown, when voltage appears between the case and the phase. In the presence of grounding, the current will not pose a danger, since the protective grounding, which has a very low resistance, will act as a conductor.
The main components of grounding are the grounding conductor itself and the grounding conductors. Grounding conductors can be natural and artificial. In the first case, these are metal structures that have a reliable connection to the ground. Earthing electrodes of artificial origin are steel rods, pipes or angles, the length of which must be at least 2.5 m. They are hammered into the ground and interconnected using welded wire or steel strips. To make grounding more efficient, it is necessary to reduce its resistance by increasing the number of artificial grounding conductors.
Protective earthing device
The essence of the protective is the deliberate electrical connection of certain parts of electrical installations with a neutral wire.
As a rule, such electrical installations are not under normal voltage. In these cases, any phase shorted to the case leads to its short circuit with the neutral wire. A very high current occurs, therefore, the equipment must be quickly and completely disconnected. This is precisely the main function of zeroing. The entire design of the protective grounding consists of a zero working and zero protective conductor.
