Electricity is not joking, so the protection of the apartment from short circuit is the task of every man. Along with building a house, planting a tree and the birth of a son. That in the family there was no accident. According to the Soviet Union standards, there were no multi-apartment buildings for protective earthing. Even if it was required, the case of the device was nullified. The standards that have come to us with European technology have changed. Devices are now designed so that they need grounding. If you leave everything as is, an accident can happen. Let's see how the protection of the apartment is performed against a short circuit.
Organization of a grounding loop in the private sector
It's no secret that today many people live in their own homes. Not always projecting organizations provide for everything. In the electrical project at home there may be no verification of the ground loop. It is possible to assemble this structure sufficiently independently. To do this, you need a little reinforcement, a good shovel and skillful hands. It is necessary to dig in the courtyard of ditch of any shape with a depth of about one meter and a width of about a third of a meter. The length of the pit must be at least 8 meters. After every half meter, 50 cm rods are driven into the bottom of the pit. The whole structure resembles a belt-pile foundation, so for the people following in the construction, the picture itself will not be new.
Ground loop
The hammered reinforcing bars must be joined together by a steel profile of any shape and a sufficiently large section. As a rule, almost any corners are suitable. It is important that there is a reliable electrical contact at the welding location. Is it possible to connect the corners with wire, as they do with the foundation of the filling foundation? We do not guarantee that after some time such a design will not fail. Surely corners will rust, and electrical contact will be lost.
The assembled structure must be connected to the house ground bus with a thick copper wire. It does not matter whether one vein is available or several, the main thing is that the resistance is sufficiently small. This can be, for example, an ordinary copper wire for external mounting of a relatively large section. Let's say 6 square millimeters.
After assembling the ground loop, it is necessary to check its resistance. The normal value should be the proportion of Ohm. For sure, many do not have a home for special equipment for measuring ground resistance. In this case, radio amateurs suggest using a very original method. To do this, it would be nice to have a transformer at hand, so as not to overload the network. Its output voltage can be quite standard, for example, 9, 12 or 27 volts. Through the resistance of a small value, let's say, 50 ohm, we start to pass current through our ground loop. As a result, a resistive divider is formed, whose shoulder values are proportional to the voltage incident here.
The total value at the point of the output winding can be measured in advance. For example, there may be a deviation of 9, 12 or 27 volts, indicated above. Of course, this need to be fixed, because the accuracy depends on the result. Many people are wondering how to connect the transformer together, the ground bus and the resistance. For this purpose one terminal of the transformer winding is grounded together with one of the resistance sides. Here such here business!
Then we need to measure the voltage drop across our resistance. Let's assume that at a nominal value of 27 volts, we have a value of 26.8. Now we can calculate the resistance of our grounding from a simple proportion. 26.8 / 0.2 = 50 / R, where R and is the desired value. The result is 0.37 ohms. The calculated value is slightly higher than the desired value. Therefore, on the side of connecting the bus of the ground loop, it is possible to excavate the ditch in the other direction and, in addition to the circuit, drive in the reinforcement and weld it with a steel profile. This will increase the contact of the structure with the ground, which will lead to a decrease in the resistance to a predetermined value. We remind you that this is 0.1 Ohm.
Indoors, the petal of each outlet must be connected to a grounded earthing bar. Separately, we need to talk about the kitchen and the bathroom. In these places it is necessary to mount a system of equipotential bonding by the standard. Such menacing words actually mean only that all metal parts in contact with water are joined together by copper veins of a sufficiently large cross section. In turn, both potential equalization loops are combined and together they are connected to the grounding bus of the house.
Short circuit
How does the ground loop protect the house from short circuit
By itself, the ground loop is not the protection of the home from someone's fault. Just as a result of a breakdown of insulation, the leakage current will not do any harm to anyone. In most cases, no one will even have time to touch a dangerous site. Almost instantly, due to the increased leakage current, it will knock out plugs. This is why the resistance of the ground loop should be as small as indicated above. In this case, the voltage of 220 volts forms a current equal to 2200 amperes on the plugs. This is an incredibly high value, which literally burns wiring. In theory, according to the idea, the fuse in the traffic jam should be the first to refuse.
But modern machines in addition to such antediluvian devices contain something else. And they really help to protect the apartment from short circuit. These devices are called differential protection devices. Inside is the anchor of the measuring relay, which is the executive module of the entire device. The bottom line is that through two coils, the input and output currents flowing through the apartment flow. While these quantities are equal, magnetic fields, formed by inductances, counterbalance each other. But, say, somewhere there was a breakdown of insulation on the ground. In this case, due to the small resistance of our ground loop, almost all the current will go to the garden. As a result, the equality of the coil fields will instantly be disturbed. A fixed anchor will cut off the power of the house by electricity.
Consequences of short circuit
Protection of the apartment from short circuit is realized in exactly the same way. The difference is that the earthing loop must be organized by electricians management company. If nothing similar in the vicinity is observed, then you need to call the master and ask them to perform all the necessary operations. Very often the house already has a ground loop, simply, in accordance with Soviet regulations, it is used, for example, for lightning conductors. Is it safe to connect your home network to it? Probably not. But according to the requirements of modern standards, the responsibility of an electrician is to organize an earthing loop. And how to do it, this is already a matter of the management company and lawyers.
A grounding bar is usually present on the access panel.
Protection against short circuit by proper arrangement of household appliances
Protection from short circuit with the help of automata is more a struggle with consequences. And the prerequisites for the occurrence of an abnormal situation may be the wrong location of household appliances in the apartment. Especially it concerns the bathrooms and the kitchen. The standards stipulate that you can not install devices that are powered by 220 volts, for example, in the bathroom. More precisely, under certain conditions such actions are allowed. But! Who among us has ever heard of these very conditions? There are at least three options, when the technology can still be installed no closer than 60 cm from the bath:
The successful arrangement of furniture and appliances in the bathroom
- The supply voltage does not exceed 50 volts alternating current or 120 volts direct current. This refers to the so-called higher class of electrical safety devices. It is believed that 50 volts AC is not capable of striking a man to death. But even in this case, isolation of current-carrying parts is necessary. It must withstand a voltage of 500 volts for one minute.
- It is allowed to connect household appliances in the bathroom through a protective transformer. The bottom line is that the galvanic connection to the ground loop is broken, due to which the possibility of hitting people is blocked electric shock.
- Finally, the third measure to combat the danger is the installation of the differential protection devices mentioned above. In this case, the house must have a ground loop. Otherwise, the differentiated protection device simply will not work. More precisely, fuses inside the modules will burn, but this can be accompanied by fatal consequences for the owners of the device. Never joke with electricity, especially if there is water nearby.
That is, the protection of an apartment from a short circuit is performed not only by installing the equipment listed above on the list items, but also by the correct placement of household appliances. For example, the power of the jacuzzi lighting can not exceed 12 volts by voltage (one of the conditions for placing household appliances in the bathroom zones). It is in this case by definition that a person is not able to get a fatal electric shock. There are a lot of electrical safety rules, and we do not have the opportunity to list everything. Those who wish can read the relevant standards. For example, one of them regulates the performance of electrical installation in the bathroom.
Our review came to an end. We hope that now readers have an idea of how to protect the apartment from short circuits.
The tripping of the protective equipment is due to short circuits in the wiring and current collectors or to overload.
Consider when an emergency power outage occurs.
Short circuit
If any part of the electrical wiring or appliance (light bulbs, iron, etc.) breaks the insulation and the phase wire touches zero, a short circuit will occur.
To protect against short circuits, fuses are used. A simple (in the form of a "plug") fuse is a fusible insert included in the phase wire, which, when the current rises, will burn and open the circuit long before serious troubles occur. Structurally, the fuse is designed in such a way that this micro crash does not lead to spoilage of the safety shoe. The automatic fuses are designed so that in the event of a short circuit the increase in current results in the activation of an instantaneous electromagnetic release that disconnects the electrical circuit without harming itself. In order to turn on the electricity again after eliminating the short circuit, simply press the white button (red turns off) or throw the lever-switch down when the fuse is triggered.
It should be noted that the automatic fuses installed at the beginning of each home line (working group) protect from short circuit not only the apartment network, but also the outdoor one. If they did not exist, an emergency short circuit would result in the failure of a transformer substation, or rather an electrical power shield of a higher level, so that a significant number of users would lose electricity, and without emergency service call it would not be necessary. And if there is an "automatic" it is enough to turn it on after it has been triggered.
It becomes clear and the need for several lines in the apartment: if one line is out, there are others in the reserve. It is convenient, if from each working group the emergency light bulb in the meter area or an emergency socket, into which it is possible to turn on a portable lamp, is supplied.
How to determine the location of the closure
To accurately and quickly determine the location of the closure, you should use the method of sequential load switching. To do this, turn off all electrical appliances that are in the apartment. Then replace the burnt plug, include a PA or a circuit breaker.
In the event that a short circuit occurs again immediately and protection is triggered, the most likely fault location is the wiring or the socket outlet.
If protection does not occur immediately, then it is advisable to turn on the lighting devices one by one, then the other electrical equipment - before a short circuit occurs. In the lamps, the damage is most often in cartridges.
In the event that the protection is triggered some time after the load is switched on, it is necessary to disconnect a part of the electric receivers (to reduce the load), since in this case the load of the network exceeds the current of the protection to be activated.
What is Overload?
If a short circuit is an accident, a force majeure situation, then the overload, that is, the operation of the network and electrical appliances connected to it, at current values significantly exceeding the rated current, does not cause harm immediately and therefore is more insidious.
Overloading can be caused by both external causes and internal ones. External causes are the increased voltage in the network. Internal - the inclusion in the line of devices that consume an unacceptably high power for this line.
To protect against overload, so-called thermal releases are used, which are usually integrated into short-circuit protection devices, forming combined automatic switches.
For sale, in addition to conventional fuses ("plugs"), there are domestic and imported circuit breakers of various denominations: stationary (fixed with screws to the panel of the shield or mounted on the mounting panel) and "plug", designed for screwing into the shoe for ordinary plugs.
Correctly selected rated currents of tripping of protection means both from a short circuit and from an overload are as follows:
16 A - for the line (group) with a total maximum consumption of 10 A (2.2 kW); it can be lighting devices or plug sockets for low-power devices;
25 A - for a line with a total power of up to 4 kW (about 18 A) - for example, for a network of plug sockets, with separate appliances with a capacity of about 1 kW.
Before you buy and plug into the network of any household appliance, you need to make sure that the voltage to which it is calculated corresponds to the power supply voltage. Do not connect devices that do not match the mains voltage.
In modern apartments, a large number of new electrical appliances are used, so you should pay attention to the current or power consumed by them and to calculate whether the fuses and wiring will allow them to be switched on.
Devices protective shutdown (RCD)
Circuit-breakers that protect the mains and electrical appliances from short circuits and overloads do not help when a person touches exposed live wires or live parts of appliances. In these situations, residual current devices (RCDs) are used that react to earth leakage currents.
Such devices are triggered when the current flows between the phase conductor and:
earthing wire (not to be confused with zero);
grounded housing;
land.
These currents are called leakage currents. They can occur in two cases:
when the phase conductor is closed with one of three primary grounding types;
when a person touches a bare phase wire under voltage.
In the second case, the leakage current flows through the human body, creating a threat to his life.
Thus, devices of this type perform two functions: fire protection in case of short-circuit "phase-to-earth" and disconnection of the network when the person touches bare exposed live parts.
The use of residual current devices (RCDs) in combination with circuit breakers minimizes the risk of human injury and virtually eliminates the possibility of ignition of electrical appliances and wiring in cases of overload or short circuit in the network.
And also RCDs are often called differential protection devices, differential relays, differential modules, and so on.
Currently, there are two main categories of RCDs: voltage-dependent - "electronic" and independent of supply voltage - "electromechanical".
When choosing an RCD, the following factors must be considered:
the location of the RCD;
RCD parameters: rated load current, rated differential breaking current, thermal resistance;
solution schemes for various systems of grounding supply networks;
selection of RCD type.
The installation of a residual current device must only be carried out by a qualified specialist who has a license to perform electrical installation work.
Almost everyone in his life encountered a short circuit. But more often it was: flash, cotton and everything. So it was only because it was short circuit protection.
Short-circuit protection device
The device can be an electronic, electromechanical or simple fuse. Electronic devices are mainly used in complex electronic devices, and we will not consider them within the framework of this article. Let's focus on fuses and electromechanical devices. To protect the household electricity supply, fuses were first used. We used to see them in the form of "traffic jams" in the electrical panel. 
There were several types, but the whole defense was that there was a thin copper wire inside this "plug", which burned out when there was a short circuit. It was necessary to run to the store, buy a fuse or store houses, perhaps not soon needed fuse stock. It was uncomfortable. And the light appeared automatic switches, which at first looked like "plugs".
It was the simplest electromechanical circuit breaker. They were produced for different currents, but the maximum value was 16 amps. Soon, higher values were required, and technological progress made it possible to produce machines as we now see them in most of the electrical shields in our homes.
How does the machine protect us?
There are two types of protection in it. One type is based on induction, the second is on heating. A short circuit is characterized by a high current flowing through the short-circuited circuit. The machine is arranged in such a way that the current flows through the bimetallic plate and the inductor. So, when a large current flows through the machine, a strong magnetic flux appears in the coil, which drives the release mechanism of the machine. Well, the bimetallic plate is designed to flow the rated current. When the current flows through the wires, it always causes heat. But we often do not notice it, because heat dissipates and it seems to us that the wires do not heat up. The bimetallic plate consists of two metals with different properties. When heated, these two metals are deformed (expand), but as one metal expands more than the other, the plate begins to bend. The plate is selected in such a way that when the nominal value of the machine is exceeded, due to bending, it activates the release mechanism. Thus, it turns out that one protection (inductive) works for short-circuit currents, and the second for currents, which flow continuously through the cable. Since the short-circuit currents are fast-paced and run in the network for a short time, the bimetallic plate does not have time to heat up to such an extent that it deforms and turns off the machine.
Short circuit protection circuit
In fact, there is nothing complicated in this scheme. The circuit is installed, which disconnects either the phase wire or the entire circuit at once. But there are nuances. Let us dwell on them in more detail.
- You can not put individual automata in the phase chain and the zero chain. For one simple reason. If, suddenly, a zero-point machine trips during a short circuit, the entire electrical network will be under voltage, because the phase machine will remain on.
- Do not install a wire with a smaller cross-section than the automatic device allows. Very often in apartments with old wiring to increase power, put more powerful machines ... Alas, this is the most frequent cause of short circuits. That's what happens in such cases. Suppose, for clarity, there is a wire, copper, section 1.5 mm, which is able to withstand current up to 16 A. It is put on the machine 25A. To this network, we include the load, say 4.5 kW, the current will flow through the wire 20.5 amperes. The wire will begin to heat up strongly, but the machine will not turn off the network. As you remember, the machine has two types of protection. The short-circuit protection does not work yet, because there is no short circuit, and the rated current protection will operate at a value exceeding 25 amperes. So it turns out that the wire heats up a lot, the insulation begins to melt, but the machine does not work. In the end, there is a breakdown of insulation and a short circuit appears and finally an automatic machine is triggered. But what would you get? The line can no longer be used, it must be replaced. This is easy if the wires are laid open. But if they are hidden in the wall? You are provided with a new repair.
- If the aluminum wiring is more than 15, and the copper wire is more than 25 years old, and you are going to do repairs - uniquely change to a new wiring. Despite the investment, this will save you money. Imagine that you have already made repairs, and in some junction box there was a bad contact? This, to speak of copper wire (in which, as a rule, only insulation grows old or the places of joints become oxidized or weaken with time, then they start to get warm, which leads to the destruction of twisting even more quickly). If we talk about aluminum wire, it's still worse. Aluminum is very ductile metal. When the temperature fluctuates, the compression and expansion of the wire are quite significant. And if the wire had a microcrack (factory marriage, technological marriage), then in time it increases, and when it gets big enough, so the wire in this place is thinner, then when the current flows, this area starts to warm up and cool down, which only speeds up the process . Therefore, even if it seems to you that everything is normal with the wiring: "After all, it worked before that!" - it is better to change all the same.
- Raspail boxes. There are articles about this, but I will go through them briefly here. NEVER MAKE A SCROLLER !!! Even if you do well, it's twisting. Metal has the property of shrinking and expanding under the influence of temperature, and twisting weakens. Try not to use screw clamps for the same reason. Screw clamps can be used in open wiring. Then, at least, you can periodically look in the boxes and check the status of the wiring. The screw terminals of the "PPE" type are best suited for this purpose, or terminal connections type "WAGO", for power wiring best suited screw clamps such as "Walnut" (these clamps have two plates that are tightened by four screws, in the middle of another plate, ie with the help of such clamps you can connect copper and aluminum wires). Leave a supply of bare wire at least 15 cm. This has two purposes: if the bad twist contact, the wire manages to dissipate heat, well, you have the opportunity, in case of anything, to twist the twist. Wires should be positioned so that there is no overlap between the phase and ground with the grounding. Wires can cross, but do not lie on top of each other. Try to arrange the twisting so that the phase wire is in one side, and zero and grounding in the other.
- Do not connect copper and aluminum wires directly. Either use "WAGO" terminal blocks, or "Nut" clamps. This is especially true for wires intended for connecting electric cookers. Usually, when doing repairs and transferring the socket for the plates, increase the cable. Very often, these are aluminum wires that build up copper.
- A little special. Do not save on switches, sockets (especially for electric cookers). The fact is that at the present time it is quite difficult to find good outlets for electric stoves (I'm talking about small towns), so it's best to either use the U739M "Walnut" clamps or find a good outlet.
- When tightening the terminals on the sockets, do it more tightly, but do not rip off the thread, but if this happens, better change the outlet immediately, do not expect to "maybe".
- When laying a new electric path, use the regulations: 10-15 cm from corners, ceiling, walls (on the floor), jambs, window frames, floor (on the wall). This you will protect yourself when installing, for example, suspended ceilings or skirting boards, which are fastened with dowels, for which it is necessary to punch a hole. If the wire is in the corner between the floor and the wall, it's very easy to get into the wire. All wires must be placed horizontally or vertically. So it will be easier for you to understand where you can continue a new hole, if you suddenly need to hang a shelf or picture or TV.
- Do not connect more than 4 outlets (from one to the other). In the kitchen, I do not recommend connecting more than two, especially where there is a plan to use an oven, a kettle, a dishwasher and a microwave in one place.
- On the oven it is best to lay a separate line or connect it to the line from which the cooking surface is fed (for very often they consume about 3 kW.) Not every outlet is capable of withstanding such a load, but if another powerful consumer is connected to it for example, a kettle), you risk getting a short circuit because of the strong heating of the connection in the socket by the loop.
- Avoid using extension cords to turn on powerful electrical appliances, such as oil heaters, or use extensions made by well-known manufacturers, rather than Chinese "no name". Carefully read what power is able to power this extension cord, and do not use it if it has less power than you need to power. When using an extension cord, try to avoid twisting the wire. If the wire simply lies, it manages to dissipate heat. If the wire is twisted, the heat does not have time to dissipate and the wire begins to heat up significantly, which can also lead to a short circuit.
- At once in a single socket (through a tee or an extension cord with several outlets) several strong consumers. On a good outlet it is allowed to include a load of 3.5 kW, not very good up to 2 kW. In houses with aluminum wiring in any outlet no more than 2 kW, or even better on a group of outlets that feed from one machine do not include more than 2 kW.
- Before placing in each room on the heater, make sure that the rooms are powered from different machines. As the saying goes: "And a stick sometimes can fire" - as with automatic weapons: "And the machine sometimes can not work," and the consequences of this are pretty cruel. Therefore, protect yourself and loved ones.
- Carefully handle the heating devices, make sure that the wire does not get on the heating elements.
Short circuit circuit breaker
Why did I render this a separate item? It's simple. It is the machine that provides protection against short circuits. If you install, it is necessary, then you need to put the machine, or put it straight away (this device is two in one: a RCD and an automaton). Such a device disconnects the network in the event of a short circuit, when the rated current is exceeded, and at a leakage current when, for example, you are under voltage and an electric current flows through you. I will remind you once again: the RCD does not protect you from a short circuit, the RCD protects you from electric shock. Of course, it may be that the RCD disconnects the network in case of a short circuit, but it is not intended for this purpose. The operation of the RCD in case of a short circuit is absolutely random. And all the wiring can burn, maybe everything is in a flame, and the RCD does not disconnect the network.
Related materials.
Protection devices in internal networks of residential buildings with a voltage of 380/220 V are fuses and automatic air switches. Power receivers, besides them, are protected against overloads (10.3) by means of thermal relays integrated in magnetic starters; Magnetic starters also provide protection against self-starting. Self-launching is used in residential buildings only in smoke removal systems, which should be taken into account when calculating networks and selecting security devices.
It should be borne in mind that the main contacts of magnetic starters are not designed to trip short-circuit currents (10.2). In addition, the thermal relays of most existing designs of magnetic starters themselves need to be protected against short circuits (10.1), because when the short-circuit currents (10.2) pass, the relay heating element can burn out faster than the relay will have time to shut down the motor. Therefore, when using magnetic starters with thermal relays to protect against overloads (10.3), it is necessary to additionally install fuses (10.4) or circuit breakers (10.5) for short circuit protection (10.1) in these circuits.
It is allowed to consider these relays as thermally stable without checking by calculation if the branch to the electric receiver is protected by one of the following devices: a fuse-link with a rated current not exceeding the longest permissible current of the thermal relay by more than 4 times; automatic circuit-breaker (10.5) with a thermal release, the rated current of which does not exceed the maximum long-time permissible current of the thermal relay by more than 2 times. These conditions predetermine the number and unit power of the electrical receivers, which can be connected to a "chain" with one common protection device on the branch, but with individual control devices and overload protection (10.3) for each electric drive monica.
Fuses (10.4) due to a simple device and low cost have become very popular in networks up to 1000 V, including in the electrical networks of residential buildings. Fuses (10.4) and fuse-links are characterized by rated voltage, rated current and the maximum breaking current.
The rated voltage of the fuse (10.4) is the voltage corresponding to the highest rated voltage of the circuits in which this fuse is allowed to be installed. The rated current of the fuse (10.4) is the highest current that the live parts (cartridge, contact racks) are designed for. The fuse-links can be inserted into the same cartridge for different rated currents.
The rated current of the fuse (10.4) is equal to the largest of the rated currents of the fuse-links intended for installation in this fuse. The rated current of the fuse-link is the largest, the current that the fuse-link withstands for an indefinitely long time. The maximum breaking current of the fuse (10.4) is the greatest melting current, at which arc suppression is still ensured without damaging the fuse holder.
At present, fuses with closed cartridges are used almost exclusively as the safest and having a high switching capacity.
Consider the principle of the most common fuses (10.4) of the series PR2 and PN-2.
A collapsible fuse with a fiber tube of the PR2 series has a cartridge made of a thick-walled fiber tube on the ends of which brass bushings are fitted, protecting the tubes from rupture. Fusible inserts are rolled with screws to the knives, which in turn are fixed with brass clips screwed onto the cartridge hubs. The cartridge assembly with a fuse-link is inserted into the pins, to which suitable and bypassing conductors are attached. Thus, the fuse turns on in series in the line cut. At a current exceeding a certain value, the fuse-link becomes very hot, and then melts. A powerful electric hot air is formed, the temperature inside the fiber cartridge sharply increases. With the appearance of an arc from the surface of the fiber, intensive emission of gases occurs, the pressure inside the cartridge greatly increases, which contributes to the deionization of the space and the effective extinction of the arc. However, the high pressure developed in the cartridge requires its special strength, which limits the switching capacity of fuses of this type. In this regard, fuses with fiber cartridges are much inferior to fuses with fillers. In addition, they are quite expensive and require for the manufacture of scarce materials and are more cumbersome than the fuses with fillers, discussed below. However, the PR2 fuses have the advantage of being easy to replace the fuse-link, whereas in fuses with fillers, the entire cartridge has to be replaced.
Currently, the most common are fuses series PN - 2 with protective cartridges filled with quartz sand. Inside the cartridge there is a fuse-link. In such "backfill" fuses, intense arc suppression is facilitated by the branching of the arc in the finest intervals between the grains of sand. Having a large surface, the filler grains absorb heat well and cool off the evolved gases.
As a result, the pressure in the cartridge decreases sharply when the insert material evaporates. Deionization and arc quenching occur so rapidly that, in the event of a short circuit, the current does not have time to reach its amplitude value. Therefore, such fuses are current-limiting. For example, a PN-2 series fuse with 100 and 250A cartridges transmits a current of no more than 5 kA.
Among the common types of fuses, we note the filling fuses of the NNP series, produced for currents up to 60 A. In principle, they are analogous to the PN-2 series fuses. For protection of apartment group networks, fuse plugs of type Н-20 are still widely used.
Protective characteristics of fuses and circuit breakers.
The melting time of the fuse-link fuse-link (10.4) depends on the strength of the overload current (10.3). The more current, the faster, the melting of the fuse-link comes. The dependence of the total tripping time (the melting time of the fuse-link and arc burning) on the tripping current is called the time of the current or protective characteristic.
Figure (10.1) shows the protective characteristics of the PN-2 series fuses, at which the limiting values of the smallest and largest shut-off time are given for a given current, i.e., the so-called spread of the characteristic. As can be seen from the characteristics, the response time for the same current can fluctuate within a considerable range (up to ± 50%), which depends on the manufacturing tolerances, the insert material, its aging, the state of the contact connections, the influence of the environment,
The considerable variation in the melting times of the inserts is a serious drawback of the fuses (10.4), making it difficult to selectively operate the protection. It is recommended to ensure the selectivity of the fuses, so that each fuse-link inserted into the power supply side is two steps higher than the previous one, if this does not lead to an increase in the cross-section of the wires. A difference of at least one step is mandatory in all cases. For highly responsible buildings, the choice of fuse links for fuses should be made taking into account the variation in protective characteristics.
Benefits circuit breakers (10.5) with prefuse fuses (10.4) are as follows.
1. In case of overload (10.3) or short circuit (10.1), the circuit breaker (10.5) disconnects all three phases of the protected branch to the motor, preventing it from operating in two phases.
2. The circuit breakers (10.5) are soon ready again after operation, while in the fuse (10.4) a replacement of the calibrated insert or even the cartridge is required.
3. Circuit-breakers (10.5) have more accurate protective characteristics than fuses (10.4).
4. Circuit-breakers (10.5), in addition to the protection functions, can be used for infrequent switching of the circuits in which they are installed. Thus, they combine protection and switching functions.
5. Some types of circuit-breakers (10.5) have built-in block contacts used in the interlocking and signaling circuits, as well as independent releases that allow remote control.
6. Circuit-breakers (10.5) exclude the possibility of using uncalibrated elements, which, unfortunately, is often practiced in installations with fuses
The most commonly used circuit breakers (10.5) can be supplied with thermal, electromagnetic or combined releases (the latter are a combination of thermal and electromagnetic releases). The operating time of the thermal releases of the circuit breakers (10.5), as well as the fuses (10.4), decreases with increasing current. that is, they have an inverse current-dependent characteristic. Electromagnetic releases operate almost instantly at the current to which they are adjusted.
The releases are characterized by a rated current, i.e. The current that they can withstand indefinitely, which is guaranteed by the manufacturer.
The lowest current that causes the circuit breaker to trip (10.5) is called the pickup current or the pickup current. Under the setting of the trip unit is meant to adjust it to the selected current value, at which the release trips. The setting of the current of the electromagnetic release for instantaneous tripping is called a cutoff. It is important to emphasize that the rated current of the circuit breaker (10.5) characterizes the throughput of its contact parts and corresponds to the rated current of its largest thermal release.
The circuit breakers (10.5) are divided into unregulated and adjustable. The first include circuit breakers (10.5), the settings of the trip units are adjusted at the factory and no adjustments are required during installation and operation. These include the series AZ-100, AE-1000, AE-2000, AK-63, AB-25. Another group of circuit breakers (10.5) is equipped with devices for changing the installation current by acting on the mechanical system of the machine or on a special device that changes the operating time of the automaton (selective automata). Among the most common adjustable automatic circuit breakers (10.5) we mention the automatic machines of the АП50, А3700, АВ and АВМ series.
Selection of fuse links for fuses and trip units
With these requirements in mind, fusible fuse links (10.4) and releases of circuit breakers (10.5) should be selected according to the relationships given in Table (10.2), which are taken on the basis of catalog data and the current characteristics of protective devices.
When lines are protected by circuit breakers (10.5), which have combined releases (thermal and electromagnetic), electromagnetic releases are verified in terms of the expression for I uS to.a.o. Only at considerable multiplicities of starting currents of 6 and more. The installation of circuit breakers (10.5), which have only electromagnetic releases (cutoffs), in the networks of residential buildings is not recommended.
In a country house it is possible to arrange a grounding, that is, to ensure the electrical contact of the ground wire with the ground through the grounded earth. One of the variants of such a grounding device is shown in Fig. 6.
Fig. 6. Grounding arrangement: 1 - steel pipe ø 0.5 or 1 inch; 2 - dressing wire ø 2 mm; 3 - steel wire ø 5 mm, connected to the body of the switchboard.
For its manufacture, it is necessary to open a ditch depth of about 0.8 m and to hammer into it several grounding electrodes, for which segments of a water pipe with a diameter of about an inch are suitable. The ends of the pipes can be flattened with a sledge hammer to facilitate their immersion in the ground.
The top of each earthing must be connected by a 5 mm wire to the current lead.
These connecting wires are attached to the earthing switches and pulled into the harness by winding from a soft 2 mm iron wire. The same winding is attached to the harness to the current lead, which connects the earthing switches to the housing of the house electric switchboard.
If the circuit breaker or the general switch allows you to manually disconnect the home network (or workgroup), the protection devices make such a trip automatically, reacting to an emergency situation. Let's consider in which cases an emergency disconnection of the network is necessary.
SHORT CIRCUIT
If any part of the electrical wiring or appliance (light bulbs, iron, etc.) breaks the insulation and the phase wire touches zero, a short circuit will occur.
Since there is no load between the closed wires, in other words, electrical resistance the contact point is practically zero, the current through the contact will begin to grow until the wires melt, which, in particular, can lead to a fire. To protect against short circuits, fuses are used. A simple (in the form of a "plug") fuse is a fusible insert included in the phase wire, which, when the current rises, will burn and open the circuit long before serious troubles occur. Structurally, the fuse is designed in such a way that this micro crash does not lead to spoilage of the safety shoe. The sacrificed little heroine is thrown out and replaced with the next one.
The automatic fuses are designed so that in the event of a short circuit the increase in current results in the activation of an instantaneous electromagnetic release that disconnects the electrical circuit without harming itself. In order to turn on the electricity again after eliminating the short circuit, simply press the white button (red turns off) or throw the lever down when the fuse is triggered.
It is clear that the fuse should operate at current values selected with a solid reserve, otherwise random small fluctuations in the voltage in the network (and hence current) will lead to a permanent false alarm. On the other hand, the stock should not be too large to prevent the current from damaging the network before the cutoff occurs.
Note that the automatic fuses installed at the beginning of each home line (working group) protect from short circuit not only the house network, but also the outdoor one.
In fact, if they did not exist, an emergency short circuit would result in the failure of a transformer substation, or rather an electric power shield of a higher level, so that a significant number of users would lose electricity, and without calling the emergency service it would be not enough. And if there is an "automatic" it is enough to turn it on after it has been triggered (by removing, of course, the cause of the short circuit). It becomes clear and the need for several lines in the house: if one line has flown out, there are others in the reserve. By the way, the conclusion is this: it is convenient, if from each working group there is an emergency light bulb in the area of the meter or an emergency outlet, into which it is possible to turn on a portable lamp.
OVERLOAD
If a short circuit is an accident, a force majeure situation, then the overload, that is, the operation of the network and electrical appliances connected to it, at current values significantly exceeding the rated current, does not cause harm immediately and therefore is more insidious.
Overloading can be caused by both external causes and internal ones. External - this is an increased voltage in the network. Internal - the inclusion in the line of devices that consume an unacceptably high power for this line.
To protect against overload, so-called thermal releases are used, which are usually integrated into short-circuit protection devices, forming combined circuit breakers.
For sale, in addition to conventional fuses ("plugs"), there are domestic and imported circuit breakers: stationary (fixed with screws to the panel of the shield, or worn on a special mounting group plate) and "plug", intended for screwing into the shoe for ordinary plugs, rated for different rated operating currents.
When a person suffers an electric shock, the danger to life is created by the current. The magnitude of the current flowing through the body depends on very many circumstances: skin moisture, nervous excitation, contact with the soil. "Stress plays a major role, but the most important factor is the time of the current.
Correctly selected rated currents of tripping of both short-circuit and overload protection (as will be discussed below) are as follows:
- 16 A - for the line (group) with a total maximum consumption of 10 A (2.2 kW); it can be lighting devices or plug sockets for low-power devices;
- 25 A - for a line with a total power of up to 4 kW (about 18 A) - for example, for a network of plug sockets, with separate appliances with a power of about kW.
PROTECTION AGAINST CURRENT LEAKS
It should be borne in mind that circuit breakers that protect the mains and electrical appliances from short circuits and overloads do not help in a situation where a person touches live bare wires or current-carrying parts of devices: when a person is injured by current, the value of this current is too small to operate mentioned devices.
In these situations, protective devices that react to earth leakage currents are used.
Such devices are triggered when the current flows between the phase conductor and:
- earthing wire (not to be confused with zero);
- grounded housing;
- land.
- These currents are called leakage currents. They can occur in two cases:
- when the phase wire is closed with one of the three listed types of grounding;
- when a person touches a bare phase wire under voltage. In the second case, the leakage current flows through the human body, creating a threat to his life.
Thus, devices of this type perform two functions: fire protection in the case of a short-circuit "phase-earth" and disconnection of the network when the person comes in contact with bare live parts that are under voltage.