The power grid is the foundation modern world. Almost all modern Appliances powered by electricity, it is a convenient source of energy. But there is also back side medals - high risk of electric shock. Without the right approach to the design of equipment and the design of electrical networks, electricity will do more harm than good. Grounding is one way to ensure safety.
In simple words about grounding
Grounding is a set of solutions and devices for protection against electric shock and ensuring the operation of protective equipment.
Domestic power grids have. What does it mean? If we consider this issue in a simplified way, then three-phase generators are installed at power plants. Their windings are connected according to the star scheme. The connection point of the windings is neutral.
If you ground the star connection point, as shown in the figure above, you will get a power line with a solidly grounded neutral. The potential of this point and the neutral wire will be equal to the ground potential.
The grounding device is called. Usually these are three metal pins killed into the ground at the same distance from each other, being, as it were, at the vertices of a triangle, while they are connected to each other by a steel strip by welding. The length of the pins and their cross section is calculated for specific conditions and requirements for this object.
The ground conductor is inserted into the electrical panel of the house or apartment and connected to the ground bus. It is a metal strip with terminal blocks. Earth conductors from each grounded appliance or outlet are connected to it. If the device is not connected through a socket, then its own grounding conductor is laid to it, and it is connected to a special terminal connected to the housing.
All ground conductors and busbars are insulated or colored with alternating stripes of green and yellow.
By type, grounding is protective and working. As you might guess, protective grounding performs the functions of protection against electric shock, and the working one is necessary for the normal functioning of electrical equipment.
Thus, grounding is called the electrical connection of the housing of electrical appliances with a ground electrode.
To understand what grounding is for, first we will understand in what cases and why we are shocked. The main thing that is needed for the flow of electric current is the potential difference.
This means that if you stand on the floor and grab a bare wire or other current-carrying part with your hands, then the current through your body and the floor will drain into the ground.
Attention:
An alternating current with a power of only 50 mA is already dangerous for humans.
And if you take hold of the current-carrying part with both hands and hang on it without touching the ground, then most likely nothing will happen, of course you shouldn’t check this. Therefore, birds are not electrocuted on wires. But back to talking about grounding. As we have already said, the housings of electrical appliances are grounded. What is it for?
Wiring and other equipment components, such as electric motors, heating elements, etc., in the normal state, do not have phase contacts with the device case, metal hose or cable armor. But in case of a malfunction, the phase may end up on the case. This can happen when the insulation of the windings of motors and transformers is damaged, the dielectric layer of heating elements is broken, the insulation of the connecting wires inside the device and cable lines is damaged.
As a result, a dangerous potential will appear on the case, plain language: the case will be "under phase". When you touch it while standing barefoot on tiles, concrete and even wooden floor- You will be electrocuted. In the worst case, it can lead to death.
Most often, this situation occurs as a result of water heating tanks, flow heaters. And this is especially brightly felt when touched at the same time. washing machine and plumbing and heating pipes, or in the case of a water tank, when you take a shower or bath, you are shocked.
The last problem is solved by the organization (grounding the bath and other metal parts of the water supply).
If the body of the damaged device is grounded, a dangerous voltage will drain to the ground and (or) a protective device will operate - a residual current device (RCD) or a differential current circuit breaker (difavtomat). We have already considered what these devices are and how they work in articles earlier:
If the body is zeroed, it will work, as this will be a short circuit to the body (zero in this case). Difautomats and RCDs determine current leakage by comparing the currents of the phase and neutral wires - if the current in the phase is greater than zero, then the current flows into the ground, through the ground wire or through the human body. Such devices operate at a differential current (current difference), usually 10 mA or more.
Therefore, it is a complex device with a large set of switching protective devices, and the presence of grounding is mandatory in all buildings built or renovated after 2003. That is, 3-wire single-phase or 5-wire three-phase electrical wiring must be laid in them. If you want to express your opinion on grounding issues - write in the comments about it.
The presence of a grounding contact in modern electrical outlets has become commonplace. It corresponds to the contact on the plug of any electrical appliance. Let's try to figure out why grounding is needed.
What is grounding
Grounding is the connection of conductive elements that are not normally energized to a ground electrode - a metal structure buried in the ground with low electrical resistance. As the mentioned conductive elements, the metal case of the electrical installation, the working bodies of machines or household appliances etc.
The shielding braids of electrical cables are also grounded.
What is grounding for?
Depending on the purpose, there are several types of grounding:- functional;
- for lightning protection.
Protective ensures the safe operation of electrical installations.
The functional is used to operate the device or circuit - it plays the same role as the neutral conductor in the mains.
In lightning protection systems, the ground electrode is connected to the lightning rod.
Principle of operation
The ground loop functions due to the ability of the soil to absorb an electrical charge. If the equipment case is energized as a result of insulation breakdown, then the charge will drain to the ground. When the user touches the case, the current will still follow the path of least resistance, i.e. through ground, and not through the human body. Without grounding, in such a situation, the user would receive an electrical injury.
The condition for the normal functioning of the grounding is a low resistance of the grounding conductor. This value depends on the soil parameters:
- density;
- humidity;
- salinity;
- ground contact area.
The ability of the soil to absorb the charge drops sharply when it freezes. Therefore, the grounding pins are driven in to a depth below the freezing mark, which depends on the latitude of the area. Data on the depth of soil freezing for different regions Russian Federation are given in SNiP "Construction climatology".
Visual demonstration of grounding
On rocky, sandy and permafrost soils, which are difficult to penetrate, electrolytic ground electrodes from an L-shaped perforated pipe are used. Inside contains a reagent that forms a salty environment. The latter is characterized by high conductivity and low freezing point. The long part of the ground electrode is buried in a shallow trench, the short part is brought to the surface. It is used in three ways:
- for backfilling a new reagent;
- for pouring water (provokes chemical reaction during the dry season).
Another modern version ground electrode - . It consists of many sections connected by threaded or otherwise. As they are driven into the ground, more and more sections are screwed on. So such a ground electrode, unlike the classic one of several pins, can be installed at any depth. The sections are connected according to special rules and using conductive paste. When clogging, a special nozzle is used that protects the thread from damage. The modules are made of steel and coated with copper or zinc, which reduces their resistance and increases their service life.
Electrolytic and modular grounding are expensive, because their traditional counterparts remain in demand. The pins in this design are arranged differently:
- at the vertices of an equilateral triangle near the object;
- at the corners of the object;
- around the perimeter of the object.
The number of rods and the distance between them are determined by calculation.
The resistance of the earth electrode is checked periodically. The maximum allowable value is 30 ohms.
Combined protection of earthing devices and fuses
Grounding not only removes dangerous current, but in the presence of a protection device causes the emergency equipment to turn off. When a phase conductor contacts a grounded case, the network operates in a mode close to a short circuit (short circuit), accompanied by a sharp increase in the current strength in the circuit. An automatic switch (VA) reacts to this, which must be installed at the input of the electrical line to the object.
True, this is possible only with a very low resistance of the ground electrode, which is extremely rare. In most cases, the probability of a VA tripping is quite low. For example, with a grounding resistance of 10 ohms, the current in the circuit will be I \u003d 220 / 10 \u003d 22 A. Automatic machines, according to the requirements of GOST, can withstand a current that is 1.42 times higher than the nominal value for an hour. That is, a 16 A machine with a current of 22 A will not turn off for almost 60 minutes (16 * 1.42 = 22.72 A).
Grounding scheme
More reliable automatic protection - or. This device compares the currents in the phase and neutral conductors and, if a difference is detected, indicating leakage, disconnects the circuit. By sensitivity, that is, the minimum amount of current leakage that causes operation, RCDs are divided into several categories:
- Protecting against electric shock: 10 mA - installed in rooms with high humidity and 30 mA - in dry ones.
- Fire-fighting - for 100, 300 and 500 mA.
Fire protection RCDs are used at facilities where a short circuit can cause a fire. They protect sections of the network where electric shock is practically excluded, for example, lighting circuits.
They are not interchangeable. VA protects against short circuits and overloads, RCD - against electric shock. Ideally, the input and each consumer group should be protected by both VA and RCD.
Grounded non-electrical equipment
Constructions that are not connected with electricity in any way are also connected to the ground electrode system:
- Fences and other structures on overpasses and galleries, in which a dangerous potential difference is induced at a lightning discharge at close range. The same can happen with a pipeline or container containing a combustible substance. Due to the induced voltage, sparking is possible, followed by an explosion, therefore such structures are also grounded.
- Products in which a static charge accumulates during operation. Basically, these are pipelines and containers: static electricity is formed due to the friction of the particles of the transported medium. For this reason, the rate of fuel supply to airliners is limited.
- Pipelines of considerable length. In accordance with the law of electromagnetic induction, in such pipelines, when the Earth's magnetic field changes, and it is always unstable under the influence of the solar wind, so-called stray currents are formed. Therefore, they are connected with a certain step to the ground electrodes.
Difference from zeroing
Zeroing is the connection of the conductive parts of an electrical installation to a dead-earthed neutral of a current source (to a neutral conductor). Its resistance is much less than the resistance of the ground electrode. Therefore, when the phase is closed to the zeroed case of the device, a short-circuit current is guaranteed to occur, leading to the operation of the circuit breaker.
In the most common grounding system of the TN type, both grounding and grounding are carried out simultaneously.
Connection to the neutral core is carried out above the RCD. Otherwise, the currents in the phase and neutral conductors after the phase is closed to the case will remain equal and the protection device will not work.
About grounding systems
Several grounding systems are used, indicated by a combination of letters. The letters have the following meaning:
- I: insulated conductor;
- N: there is a connection to a solidly earthed neutral;
- T: there is a connection to the ground wire.
There are three main types of grounding systems:
- IT type- system with insulated neutral wire. In this system, it is isolated from or in contact with the neutral through a high value resistor or an air gap. Does not apply to residential buildings. Designed for connecting devices with special requirements for safety and stability. It is mainly used in laboratories and medical institutions.
- Type TT- system with independent grounding. The best option. It provides for the use of two grounding conductors - for the source of electric current and metal elements unprotected systems. The earth wire (PE) in this system is independent, and its performance in the area between the equipment and the transformer is improved. There may be difficulties in selecting the diameter for your own ground electrode. This disadvantage is compensated by the installation of a protective shutdown system.
- TN type. The ground wire in such a system is combined with the neutral, therefore, when a phase breaks down on the case, a short circuit occurs and the machine disconnects the circuit. This ensures a high level of security.
Various grounding systems
TN systems are the most widely used. There are three subspecies:
- TN-S: option with zero and divided working conductor. In order to increase safety, instead of one neutral wire, two are used: one is used as a protective one, the second one is used as a neutral one with a connection to a solidly grounded neutral. Such a system provides the best protection against electric shock.
- TN and TN-C-S: option with PEN-wire and a pair of zeros. A neutral wire is connected to the equipment, split into PE and N conductors.
- In TN-C-S after separation, a second grounding conductor is installed, which ensures uninterrupted operation of the system.
Advantages of the TN system:
- the device is quite simple;
- protection against lightning discharges;
- to protect the wiring, it is enough to install circuit breakers.
Flaws:
- there is a possibility of zero burnout from the outside with subsequent breakdown of the metal cases of the equipment;
- potential equalization equipment is required.
The TN system is not well suited for rural areas.
The lives of people sometimes depend on the correct organization of grounding. Organization means not only the device, but also the timely control of the resistance of the ground electrode. Due to oxidation or changes in soil parameters, it may be overestimated, as a result of which the protective effect of grounding will be lost.
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 conditions functioning of 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, an inseparable connection of all parts of the protective system is necessary, 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 point is that in the process long-term operation possible destruction of the insulation and the appearance of contact of the bare wire of the power supply with the body of the electrical installation.
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 constructions and their elements having 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, take metal corners 50x50x6 mm and 2.5-3 meters long (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 a system designed to prevent the effects of electric current on a person by deliberately connecting to the ground the housing and non-current-carrying parts of the equipment that may be energized. Grounding systems can be natural or artificial.
What is grounding and why is it needed?
Grounding devices are intentional connection by conductors electric type various electrical outlets.
The purpose of grounding is to prevent the effects of electric current on a person. Another purpose of protective grounding is to divert voltage from the body of the electrical installation through a grounding device to ground.
The main purpose of grounding is to reduce the potential level between the point that is grounded and the ground. This reduces the current strength to the lowest level and reduces the number of damaging factors in contact with parts. electrical appliances and installations in which there was a breakdown on the hull.
What is neutral?
Neutral is a zero protective conductor that connects the neutrals of electrical installations in three-phase electric current networks. Scope of use - zeroing of electrical installations.
The step-down substation, where the transformer installation is located, is equipped with its own ground loop. This circuit consists of a steel tire and rods buried in the ground in a special way. A cable with 4 cores is laid to the consumption sources in the electrical panel from the substation. When the consumer of electricity needs power from a three-phase type circuit, then all 4 cores must be connected. When a different load is connected to the conductors, a neutral displacement occurs in the system, in order to prevent this displacement, a neutral conductor is used. It helps to symmetrically distribute the load on all phases.
What are PE and PEN conductors?
A PEN conductor is a conductor that combines the functions of a zero protective and zero working conductor. It comes from the substation and is divided into PE and N conductors, directly at the consumer.
PE conductor is a protective earth that we use, for example, in an apartment in a socket with earth. PE-conductor is used for grounding devices, installations and devices where the voltage level does not exceed 1 kV.
This type of grounding is used for safety purposes only. This grounding ensures a continuous connection of all exposed and external parts. The mechanism ensures that the current drains to the ground, which appeared as a result of the ingress of electric current on the body of a device.
PEN-conductor (combination of zero protective and zero working conductor) is used when using a TN-C type grounding system.
Types of artificial grounding systems
In the classification of grounding systems, there are natural and artificial types of grounding.
Grounding systems of artificial type:
- TN-S;
- TN-C;
- TNC-S;
Types of grounding - decoding of the name:
- T -- grounding;
- N - connection of the conductor to the neutral;
- I - isolation;
- C - combining the options of a functional and neutral wire of a protective type;
- S -- separate use of wires.
Many people are interested in the question of what is called working grounding. In another way, it is called functional. The answer to this question is given by paragraph 1.7.30 of the PUE. This is the grounding of the points of current-carrying parts electrical installation. It is used to ensure the functioning of electrical appliances or installations, and not for protective purposes.
Also, many are concerned about the question of what is protective grounding. This is the process of grounding devices to ensure electrical safety.
Systems with solidly earthed neutral of the TN earthing system
These systems include:
- TN-C;
- TN-S;
- TNC-S;
According to clause 1.7.3 of the PUE, a TN system is a system in which the neutral of the power source is deafly grounded, and the open conductive parts of the electrical installation are connected to the deafly grounded neutral of the source by means of zero protective conductors.
TN includes elements such as:
- midpoint grounding, which is related to the power supply;
- external conductive parts of the device;
- conductor of neutral type;
- combined conductors.
The source neutral is solidly grounded, and the external conductors of the installation are connected to the solidly grounded midpoint of the source using protective type conductors.
It is possible to make a ground loop only in electrical installations, the power of which does not exceed 1 kV.
TN-C system
In this system, the zero protective and zero working conductors are combined into one PEN conductor. They are combined throughout the system. The full name is Terre-Neutre-Combine.
Among the advantages of TN-C, only easy installation of the system can be distinguished, which does not require much effort and money. Installation does not require improvement of already installed cable and overhead power lines, which have only 4 conductive devices.
Flaws:
- increases the likelihood of receiving an electric shock;
- line voltage may appear on the body of the electrical installation during an open circuit;
- high probability of loss of the grounding circuit in case of damage to the conductive device;
- such a system only protects against short circuits.
TN-S system
The peculiarity of the system is that electricity is supplied to consumers through 5 conductors in three-phase network and through 3 conductors in a single-phase network.
In total, 5 conductive sources depart from the network, 3 of which perform the function of the power phase, and the remaining 2 are neutral conductors connected to the zero point.
Design:
- PN is a neutral mechanism that is involved in the circuit of electrical equipment.
- PE is a solidly grounded conductor that performs a protective function.
Advantages:
- ease of installation;
- low cost of purchase and maintenance of the system;
- high degree of electrical safety;
- no contour creation required;
- the ability to use the system as a current leakage protection device.
TN-C-S system
The TN-C-S system involves the division of the PEN conductor into PE and N in some section of the circuit. Usually the separation takes place in the shield in the house, and before that they are combined.
Advantages:
- a simple device of a protective mechanism against lightning;
- protection against short circuit.
Cons of using:
- low level of protection against combustion of the neutral conductor;
- the possibility of phase voltage;
- high cost of installation and maintenance;
- voltage cannot be switched off automatically;
- there is no outdoor current protection.
TT system
TT is designed to provide a high level of security. Installed in power plants low level technical condition, for example, where bare wires are used, electrical installations that are located in the open air or fixed on supports.
TT is mounted according to the scheme of four conductors:
- 3 phases supplying voltage are displaced at an angle of 120 ° between themselves;
- 1 common zero performs the combined functions of a working and protective conductor.
TT Benefits:
- high level of resistance to deformation of the wire leading to the consumer;
- short circuit protection;
- Can be used in high voltage electrical installations.
Flaws:
- sophisticated lightning protection device;
- impossibility to track the phases of the short circuit of the electrical circuit.
Systems with isolated neutral
During the transmission and distribution of electric current to consumers, a three-phase system is used. This makes it possible to ensure symmetry and uniform distribution of the current load.
Such a device creates a regime that involves the use of a transformer box and generators. Their neutral points are not equipped with an earth loop.
The isolated type of neutral is used in the power circuit when connecting the secondary windings of transformer installations according to the triangle circuit and in the absence of power during emergencies. Such a network is a replacement chain.
An insulated neutral contributes to the penetration of the insulating coating during a short circuit and the occurrence of a short circuit in other phases.
IT system
The IT system up to 1000 V provides grounding through a high resistance level and is equipped with a power supply neutral.
All external elements of the electrical installation, which are made of conductive materials, are grounded. Among the advantages, low current leakage rates during a single-phase short circuit can be distinguished electrical network. An installation with such a mechanism can function for a long time even in emergency situations. There is no difference between the potentials.
Disadvantage: current protection does not work in the event of a ground fault. During operation in single-phase short circuit mode, the probability of electric shock increases when touching the second phase of the installation.
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 used as a grounding conductor water pipes or heating pipes, but they cannot be used as a ground conductor. There may be non-conductive inserts in the plumbing (for example, 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 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 one 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
TN-C (fr. Terre-Neutre-Combine) system proposed 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 (French Terre-Neutre-Separe) was developed, in which the working and protective zero were separated directly at the substation, and the ground electrode was a fairly complex structure 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.
