Fuses and circuit breakers are protection devices that automatically switch off the protected electrical circuit under abnormal conditions.
Fuses are used to protect electrical receivers, wires and cables from. They can also protect against significant overload if all elements of the protected network have a capacity of at least 25% higher than the fuse-link current. Since fuses withstand currents 30...50% higher than the rated currents of the fuse links for one hour or more, then at currents exceeding the rated currents of the fuse links by 60 - 100%. they melt in less than one hour.
Structurally, the fuse is a cartridge in which a fuse link is attached, which is an artificially weakened link in the electrical network.
In most fuses, blown fuse links are replaced with new ones.
Fuse classification
Fuses are divided into:
- inertial- with high thermal inertia, i.e. ability to withstand significant short-term current overloads. These are fuses with a screw thread and a lead conductive bridge;
- inertialess- with low thermal inertia, i.e. with limited overload capacity. These are fuses with a copper conductive bridge, as well as fuses with stamped inserts.
The most widely used fuses in electrical networks up to 1 kV are NGGN2-63, PN2, PR2.
- Fuses NPN2(non-separable with filler) are equipped with a glass non-separable cartridge filled with dry quartz sand and a copper wire insert with a tin ball. Such fuses cannot be recharged and must be replaced with new ones after tripping.
- Fuses PN2(collapsible with filler) consist of a porcelain body filled with fine-grained quartz sand, in which one or more copper plate fuse-links are located. When the fuse is actuated, the electric arc branches between the grains of quartz sand and is intensively cooled due to heat transfer to the filler.
- Fuses PR2(collapsible without filler) consist of a fiber tube in which a fusible insert of a special form of zinc alloy is located. When a fusible link burns out, the fiber tube releases gases, the pressure in the tube increases significantly and the arc deionizes.
Fuses type PR2 are used mainly in machine tools, switching boxes. In switchgears (panels, power cabinets), fuses NPN2 and PN2 are used, in distribution busbars - PN2.
In lighting networks, threaded (cork) fuses can be used, for example, type PD, PRS.
Watch an interesting video about the operation of fuses below:
Fuse characteristics
The fuse is characterized by:
- rated voltage at which the fuse operates for a long time;
- rated current of the cartridge, for which its current-carrying parts and contact connections are designed according to the condition of long-term heating;
- the rated current of the fuse-link, which it can withstand without melting for a long time;
- breaking capacity (limiting breaking current), determined by the maximum breaking current, at which the fusible insert burns out without dangerous ejection of flame or arc combustion products and without destruction of the cartridge;
- protective time-current characteristic, the dependence of the time of complete shutdown of the circuit on the magnitude of the switched current.
Basic technical data The most common fuses are shown in the table below:
The protective characteristics of fuse links of type PN2 for various rated currents are shown in Fig. 2.4.
Another interesting video about fuses:
Fuses along with the simplicity of their design and low cost have a number of significant disadvantages:
- inability to protect the circuit from overloads;
- scatter of protective characteristics caused by an increase in contact resistance as a result of weakening of contacts and aging of the insert material under operating conditions;
- in the event of a short circuit in a three-phase line, one of the three fuses may blow. Asynchronous electric motors with a squirrel-cage rotor connected to the line turn out to be included in two phases, and this can lead to their overload and failure.
Fig 2.4 Protective characteristics of fuses PN2
Purpose of circuit breakers
Protective characteristics of machines
Circuit breakers can have the following protective characteristics (Fig. 2.6):
- current-dependent characteristic - response time. Such switches have only a thermal release. They are rarely used due to insufficient limiting switching capacity and speed;
- current-independent response time characteristic. Such switches have only a current cut-off, made with the help of an electromagnetic or electronic release, operating without delay or with a time delay;
- current-limited two-stage response time characteristic. In the overload current zone, the circuit breaker trips with a current-dependent time delay, in the current zone - with a current cut-off with a current-independent, pre-set time delay (for selective switches) or without a time delay (for non-selective switches). The circuit breaker has either a thermal and electromagnetic (combined) release or an electronic release:
- three-stage protective characteristic. In the overload current zone, the circuit breaker trips with a current-dependent time delay, in the current zone - with an independent, pre-set time delay (selective cut-off zone), and with close ones - without a time delay (instantaneous trip zone).
The instantaneous operation zone is designed to reduce the duration of current exposure at close faults. Such switches have an electronic release and are used to protect the input to the transformer substation and outgoing lines.
The main technical data of some series of machines are given in table. P11.
Fuses are used to protect electrical circuits and electrical installations from short-circuit currents or overload currents.
The fuse is built into the electrical circuit. Its main task is to pass the operating current and break the electrical circuit when overcurrents appear. Distinguish between fuses low voltage(up to 1 kV) and high voltage(over 3 kV), however, in terms of purpose and principle of operation they are completely the same. There are also power and high-speed fuses.
Low-voltage fuses are structurally a fairly simple device. A conductive element (fuse link), under the influence of a current whose value is higher than the nominal value, heats up, melts in an arc-extinguishing medium (most often it is quartz sand SiO2) and evaporates, creating a break in the protected electrical circuit.
The insulator prevents the release of hot gases and liquid metal into the environment. It is made of high-grade technical ceramics and must withstand very high temperatures and internal pressures during shutdown.
The protective covers have strips for gripping with standardized handles for replacing fuse links of low-voltage fuses. Together with the ceramic housing, they create an explosion-proof enclosure for the switching electric arc.
Sand, in turn, is important for limiting the current. Typically, crystalline quartz sand with high mineralogical and chemical purity (SiO2 content > 99.5%) is used.
For the switching function, the specific size of the sand crystals and its optimal compaction are important.
The indicator allows you to quickly find burnt fuses. With increased spring stiffness, it can serve as a shock signaling device for actuating microswitches or disconnectors.
Solder shifts the characteristic curve to lower melting current values. It is selected in accordance with the material of the fusible element and must be in the right quantity and in the right place.
Contact knives mechanically and electrically connect the fuse link to the fuse base holder. They are made of copper or a copper alloy coated with tin or silver.
Traditional materials from which fusible inserts are made are: copper, zinc, silver, which have the necessary electrical resistivity.
The main advantage when using a fuse with a fuse link is the current-limiting effect. That is, the melting time of the fuse-link is quite short and, as a result, the short-circuit current does not have time to reach its maximum value.
Obviously, at the rated current level or less, the fuse link must conduct electricity for an unlimited amount of time.
To speed up the operating time of the fuse link, the following technical solutions are used:
· fuse links with sections of different widths (sections)
metallurgical effect in the design of fuse links
By reducing the cross-section (narrowing) of the fuse-link in certain places, the required is achieved - a shorter circuit opening time.
The metallurgical effect is as follows: certain low-melting metals (for example, lead and tin) are able to dissolve more refractory metals such as copper and silver in their structure.
To do this, drops of tin are applied to copper wires. When heated by supercurrent, the tin drops quickly melt, melting some of the wires. Further, the mechanism of operation of a fusible insert with a reduced cross section in certain places is used.
The main reason for the continued growth in the number of users of fuses, in addition to the extremely favorable price-performance ratio and small footprint, is their well-known reliability, which characterizes the fuses as the “last line of defense”. Only certified fuses with fuses that meet the declared characteristics will allow you to avoid fires that occur in electrical wiring and electrical installations.
TICKET No. 9
- Purpose and general design of the 1-PD4D diesel fuel system.
The fuel system is designed for storing, heating, cleaning and supplying fuel to diesel cylinders and ensures timely injection in the required sequence of certain portions of fuel under high pressure into the combustion chambers of diesel cylinders and spraying it into the smallest particles.
The system includes a fuel priming pump, a high pressure fuel pump, low and high pressure pipelines, a fuel tank, a fuel heater, coarse and fine filters, injectors, regulators. The fuel priming pump sucks fuel from the supply tank through the coarse mesh filter and delivers it under a pressure not exceeding 0.53 MPa (5.3 kgf/cm2) to the fine fuel filter installed on the diesel engine.
The unloader valve installed on the line from the fuel priming pump to the filter does not allow pressure in the fuel pipeline to rise above 0.53 MPa (5.3 kgf/cm2), bypassing excess fuel into the supply tank through the drain pipe.
From the fine fuel filter, the filtered fuel enters under pressure into the manifold of the high pressure fuel pump.
The pressure of 0.25 MPa (2.5 kgf/cm2) in the fuel manifold is maintained by a control valve that discharges excess fuel through the drain pipe into the tank. Valve 6 and tap 7 serve for emergency fuel supply to the diesel engine. The fuel pump pumps fuel under high pressure into the injectors according to the operating order of the diesel cylinders.
Leaked fuel from the injectors and high-pressure pump is drained into the supply tank.
- Purpose and design of the high-pressure fuel pump section of the TEM18DM diesel locomotive.
The fuel pump, designed to supply diesel cylinders under high pressure and in accordance with the load of strictly defined doses of fuel for each cycle, consists of the following main parts: crankcase, cam shaft, pushers, removable plunger sections and manifold.
The main parts of the fuel pump section (Fig. 30, a) are two precision pairs, made with high precision and mounted together with its other parts in housing 22, cast from cast iron. The first pair, the pump element, consists of a sleeve 10 and a plunger /7, and the second pair, the valve pair, consists of a discharge valve 5 and a seat 6. Both pairs are made of high-alloy heat-treated steel. The seal in each pair is achieved by carefully grinding one part to another. Therefore, if one of the parts is damaged, the pair is replaced with a new one.
Fig. 30 Section of the fuel pump (a) and its discharge valve (b): 1 - pressure fitting, 2, 8 - cavities communicating with the discharge pipeline, 3 - discharge valve spring, 4 - stop; 5 - discharge valve, 6 - discharge valve seat, 7 - rubber o-ring, 9 - space above the plunger, 10 - sleeve, 11 - plunger; 12 - vertical groove, 13 - annular groove; 14 - upper edge, 15 - lower edge, 16, 27 - locking screws, 17 - adjusting rack, 18 - plunger spring, 19 - guide cup, 20 - lower spring plate, 21 - retaining ring; 22 - section housing, 23 - spring ring, 24 - upper spring plate, 35 - gear; 26 - hole, 28 - groove, 29 - suction cavity of the body, 30-sealing copper ring; 31 - discharge valve; 32 - discharge valve seat, 33 - discharge valve spring (1 - before modernization! 11 - after modernization)
The sleeve 10 of the plunger of the pump pair is made in the form of a cylinder with a thickened upper part. Two through holes 26 in the upper part connect the above-plunger space 9 of the liner with the housing cavity 29, to which fuel is supplied. One of these holes on the outer surface of the sleeve has a conical countersink, and the other is equipped with a vertical groove into which a locking screw 27 fits, which keeps the sleeve from turning. In this case, the hole for the passage of fuel remains open. The lower flange of the sleeve is tightly ground to the annular groove of the body.
The plunger 11 consists of a cylindrical head and a shaped shank, made as one unit. On the surface of the head in the upper part there is an annular recess 13, connected by a vertical groove 12 to the space above the plunger 9. The lower edge 15 of the recess is made round, and the upper edge 14 is shaped along a helical line. At some distance from the end of the plunger head, it intersects with the edge of the vertical groove 12. The screw edge serves to cut off and regulate the amount of fuel supplied by the plunger. The plunger shank has two protrusions and a head. The protrusions fit into the vertical grooves of the shank of the gear 25, which is in mesh with the adjusting rack 17, and the head rests on the bottom of the guide cup 19, supported from below by the spherical surface of the adjusting bolt 28 of the pusher (see Fig. 29). A plate 20 (see Fig. 30, a) of a spring 18 is placed on the head, returning the plunger to the lower position.
The valve pair is installed on the upper end of the plunger sleeve. To ensure tightness, the seat of the valve pair is ground to the end of the liner and pressed against it by pressure fitting 1. The tightness with the section body is ensured by a rubber ring 7. In the center of the seat 6 there is a hole that serves as a seat for the discharge valve 5.
Valve 5 (Fig. 30, b) is hollow. In the lower part it has a needle-shaped landing cone, in the middle-side hole E, and in the upper part there is an annular collar P.
The shoulder P separates the discharge pipeline from the space above the plunger before the needle cone does this, and the hole E transfers fuel from the discharge pipeline into the space above the plunger 9 after they are separated by the shoulder P.
The valve is pressed against the seat cone by a spring 3, the other end of which rests against the stop 4, which serves to limit the lift of the discharge valve.
TICKET No. 10
- Purpose and design of the water system of the 1-PD4D diesel engine.
The diesel engine installed on diesel locomotives is water-cooled, the need for which is due to the high heating of its individual parts in contact with hot gases. Already at the end of the compression stroke, the air temperature in the cylinders rises to 500 - 700 °C, and during fuel combustion it reaches 2000 °C. Even the exhaust gases have a temperature of 430 - 480 °C. Such high heating of parts could cause significant deformation, destruction, burning of oil and, as a result, jamming of the pistons in the cylinders.
The intense heating of diesel parts requires intensive cooling with water, the temperature of which must be high enough to avoid the appearance of cracks in the block, cylinder liners, cylinder covers and turbocharger housing. The heated water is cooled in the radiator sections, and part of the heat removed from the diesel engine by water is used for auxiliary purposes (heating the fuel in the tank and the air in the driver’s cabin in the cold season).
On diesel locomotives, water is also used to cool diesel oil in the water-oil heat exchanger and charge air before it enters the diesel cylinders. Since oil and charge air must be cooled with water at a lower temperature compared to diesel cooling water, the water system has two independent water circulation circuits. The water temperature in the main circuit is maintained within 70 - 85 °C, and in the auxiliary circuit - 60 - 70 °C. Water circulation in each circuit is carried out by a special pump driven by the diesel crankshaft.
To cool the water in the main circuit, sixteen and the auxiliary circuit use eight water sections installed in the refrigerator shaft. Both circuits are combined by an expansion tank mounted above the refrigerator shaft
The closed-type diesel water system with forced water circulation has two independent cooling circuits (hot circuit, cold circuit), each of which has its own pipeline, water pump, refrigerator sections and a common cooling fan.
The system is designed to remove heat generated during diesel operation, to heat the driver's cabin and to warm up the diesel engine before starting from an external heat source.
The hot (main) circuit is designed to cool exhaust manifolds, turbocharger housings, bushings and diesel cylinder covers. In the cold season, hot circuit water is used to heat fuel in the fuel heater and heat the driver's cabin.
Water pump 46, left along the locomotive, pumps water into the cooling cavities of the diesel engine 42 and the turbocharger. Heated water is removed from the diesel engine in section 53 of the locomotive refrigerator and then into the suction
cavity of the water pump 46. In cold weather, part of the water from the water cavity of the left diesel exhaust manifold is diverted for heating to the fuel heater 29, heater 32, floor heaters of the driver's cabin 34 and 65.
The cold circuit is designed to remove heat from the charge air cooler and diesel oil coolers.
Water pump 63, on the right side of the locomotive, pumps water into the diesel oil cooler 22, section 3 of the refrigerator. The cooled water is then pumped through the oil cooler 59, the charge air cooler 64 and enters the suction pipe of the water pump 63.
The diesel water temperature is controlled by a remote thermometer 51, the meter of which is installed in the hot circuit at the water outlet of the diesel engine, and the indicator is on the driver's cab console. Temperature relay sensors 58 and 60 are installed on the water exit pipeline from the diesel engine (hot circuit) and the water inlet to the oil cooler (cold circuit), which send a signal to open the refrigerator shutters and to remove the load from the diesel engine (if the maximum permissible water temperature is exceeded).
66 thermostats (in hot and cold circuits) automatically
control the rotation speed of the refrigerator fan, maintaining the water temperature within optimal limits.
To control the water temperature in the cold circuit, a remote thermometer 4 is installed in front of the entrance to the oil cooler, and the indicator is installed on the remote control in the driver's cabin.
For periodic measurements of water temperature in the hot and cold circuits, fungi are installed under mercury thermometers. For periodic measurements of water pressure in the system, fungi are installed under pressure gauges and fungi under pressure-vacuum meters.
Steam and air are removed using steam-air tubes into the expansion tank 12, which is connected by feed pipes to the suction pipes of water pumps 46 and 63.
Water meter glass 13 is designed to monitor the water level in the expansion tank. On the side surface of the tank there are two lines with the inscriptions V.U. - upper water level and N.U. - lower water level. The water level in the tank should be between these marks. The filler neck 9, located in the upper part of the tank, is closed with a lid in which a steam-air valve 8 is mounted. To communicate the tank with the atmosphere when refueling from the bottom of a diesel locomotive or before removing the cover with the steam-air valve 8, there is a pilot pipe with a valve 6.
The position of the valves, taps and connecting heads when the diesel engine is running, heating is turned on, the fuel is warming up, the diesel engine is warming up from an external source, when filling the system with water and draining water from the system is indicated in the table in the figure.
Valves 11, 18, 19 and faucet 7 are installed on the make-up and steam-air pipes for the purpose of disconnecting the water tank from the system when testing the water cavities of the diesel engine.
2. Purpose and design of the 1-PD4D diesel injector.
The diesel injector (Fig. 32, a) is designed to atomize and distribute fuel in the combustion chamber. The main part of the nozzle is the nozzle, consisting of a precision pair - body 21 and needle 2. The nozzle is attached to the bottom of the nozzle body 4 with a nut 19. The upper end of the nozzle body and the mating end of the nozzle body have ground surfaces that ensure a tight joint. To inject fuel into the combustion chamber, a spherical head is made in the lower part of the nozzle body (Fig. 32, b) with nine holes with a diameter of 0.35 mm located around the circumference.
The locking cone of the needle 2 is ground into the seat of the nozzle body (see Fig. 32, a), which separates the cavity 24 of the nozzle from the combustion chamber. Rod 17 rests on the needle shank in the upper part with its spherical surface, transmitting to it the force from spring 7. The spring tension is adjusted (using bolt 10) to a fuel injection pressure of 275 kgf/cm2. After adjusting the spring tension, bolt 10 is secured with locknut II and sealed.
When the diesel engine is running, the fuel pumped by the fuel pump is supplied through a high-pressure pipeline into fitting 15, and from there, passing through the slot filter 16, channel 18, annular groove 20, through three inclined holes 22 enters cavity 24. Since the outlet of the nozzle body is closed needle 2, pressed to the seat by a spring, then the pressure in cavity 24 will increase sharply, acting on the large cone 1 of the guide part of the needle. When the force of fuel pressure, tending to lift the needle upward, exceeds the tightening force of spring 7, the nozzle needle rises. In this case, fuel will be injected at high speed from cavity 24 through the spray holes of the head of the atomizer housing into the combustion chamber.
Due to the high pressure in cavity 24, part of the fuel seeps between the needle and the nozzle body into the internal cavity of the nozzle, lubricating the rubbing surfaces.
The leaked fuel is discharged through drilling 13 and fitting 14 into the drain pipe. Fuel injection is interrupted as soon as the fuel supply from the pump stops.
Rice. 32. Diesel injector (a) and its sprayer (b):
Large needle cone; 2 - spray needle; 3 - cylinder cover; 4 - nozzle body; 5 - nozzle bushing; 6 - lower spring plate; 7-spring; « - upper plate of the spring; 9 - plug; 10 - adjusting bolt; 11- lock nut; 12 - seal; 13 - drilling; 14 - fuel outlet fitting; 15 - fuel supply fitting; 16 - slot filter; P - rod; 18 - fuel supply channel of the injector body; 19 - spray nut; 20 - annular recess of the spray body; 21 - spray body; 22 - inclined hole of the spray body; 23 - sealing ring; 24 - nozzle cavity; 1- sprayer before modernization; 11- sprayer after modernization
TICKET No. 11
- Purpose and design of the 1-PD4D diesel air purifier.
The air cleaner of a diesel locomotive (Fig. 23) is a continuous oil filter. Its cleaning efficiency is constant in all operating modes of the diesel locomotive and is 98.5% with a resistance of up to 20 mm of water. Art. The air purifier allows you to obtain technically clean air (dust content no more than 1 mg/m3) with a total dust content of 65 mg/m3. The filter elements of the air purifier are four mesh cassettes 21 (in the form of sectors), which are placed in a wheel 20. Each cassette has 16 meshes, of which six are No. 5 X 0.7, six are No. 3.2 X 0.5 and four are No. 7 X 1.2. The wheel 20, together with the cassettes 21, is mounted on a fixed axis 24, fixed in the walls of the housing, the lower part of which is an oil bath with a volume of 108 liters. The wheel rotates automatically using a pneumatic cylinder 12, to which air is supplied from the compressor. Air enters the pneumatic cylinder periodically as the 3RD pressure regulator operates. When the pressure regulator is activated, the air entering the pneumatic cylinder acts on its rod and, through rod 13, levers 15, 14, rod 27 and slider 16, moves the pawl 18, which engages with the ratchet band (teeth) of the wheel rim 20.
Rice. 22. Diesel locomotive air purifier:
Turbocharger suction pipe; 2, 4 - clamps; 3 - connecting sleeve; 5 - air cleaner frame; 6, 9 - hatches; 7 - mesh cassettes; 8 - blinds; 10 - main pipe; 11- clamps for fastening cassettes
The rotation speed of the air cleaner wheel depends on the response frequency of the air pressure regulator and is approximately 0.04 - 0.15 rpm. The cassettes are cleaned while they are in an oil bath. Trapped dust settles to the bottom of the bath. The dust holding capacity of the air cleaner is approximately 50 kg and is determined mainly by the capacity of the oil bath from the bottom of the body to the wheel rim 20. A tap with a hose 7 is provided for draining the oil, and hatches 26 are provided for removing dirt.
In the upper part of the air cleaner housing there are hatches 1, 5 and 17, which serve to draw air from the machine room in winter, while the blinds 22 are completely or partially closed.
A fuse is an electrical switching device that is used to disconnect a protected circuit. Its purpose is to protect the electrical network and electrical equipment from short circuits and significant overloads. The main parameters of the products are the rated and maximum switchable current, as well as the rated voltage. In this article we will take a detailed look at fuses: their purpose, types, design and principle of operation.
How does the device work?
The fuse operates in two modes, which differ significantly from each other.
- Normal network mode. In this mode, the device heats up as a steady process. At the same time, it completely heats up to a certain temperature and releases the generated heat to the environment. The so-called rated current strength is indicated on each element (as a rule, the largest current value of the structural element is indicated). The fuse can accommodate a fuse element of different rated current.
- Short circuit mode and . The device is designed in such a way that if the current in the network increases, it could burn out in the shortest possible time. To do this, the fusible element in certain areas is made with a smaller cross-section, where more heat is released than in wide areas. When almost all or completely all narrowed areas burn out. When an element melts, an electric arc is created around it, which is extinguished in the mechanism’s socket.
The current strength must be indicated on the device body, and the maximum permitted voltage at which the device will not fail must also be taken into account.
The graph below shows the dependence of the burnout time of the fuse element on the current:
Where l10 is the current at which the element melts and is disconnected from the network in 10 s.
Varieties and types of elements
Fuses are divided into two types: low voltage and high voltage. This division is explained by the voltage value of the working electrical network in which the fuse is used.
Low-voltage devices are labeled as PN or PR and are designed for voltages up to 1000 V. In low-voltage PN devices, there is a fine-grained filler around the copper insert. Their use is designed up to 630 Amperes.
The PR device is simpler (pictured below) than the PN, but in the event of a short circuit, they are also capable of extinguishing an electric arc. Designed for currents from 15 to 60 Amps.
Based on their design features, fuses are divided into cartridge, plug, plastic and tubular. Depending on the type of execution, collapsible and non-dismountable products are produced. Collapsible ones have the ability to access the insert. The structure is disassembled and the burnt insert is replaced with a new one. Non-separable ones are constructed from a glass bulb, therefore they are considered disposable and the inserts cannot be replaced.
Design
A modern fuse consists of two parts:
- a base made of electrical insulating material with metal threads (necessary for connection to an electrical circuit);
- replaceable insert that melts.
The basis of the device is an insert that burns or melts during a short circuit. In order to extinguish the arc, which is formed as a result of the burnout of the replaceable insert, arc extinguishing devices are installed.
The insert leads are connected to the terminals in such a way that the fuse is connected to the electrical circuit line. For this, special reliable fastening terminals (holders) are used, which should ensure good contact. If it is not there, then heating may occur in this place.
A design feature of fuses is that the device burns out before other parts of the mechanism are damaged. After all, it is easier to replace than a microcircuit or other hardware component. Therefore, such a part is chosen taking into account that its melting rate is greater than in the line wires. Their temperature should not reach a dangerous level, as this will lead to equipment failure.
The design of the plug-type mechanism has the form of a cartridge into which a fuse with a base is screwed. When an emergency occurs, the plug burns out. Today this plug looks like a button, similar to a regular switch. This button returns the device to working condition after an accident.
In addition to protecting the electrical circuit from damage, the fusible component also protects against fires and ignitions. After all, an ordinary wire can come into contact with combustible materials at the time of ignition, and the part burns out inside the device case.
The ratings of the device are selected according to the lowest rated currents of the electrical network or a separate part of the electrical circuit. The table of denominations is provided below:
If it is necessary to change such a component to AB (circuit breakers), then their rating should be one step larger than the component part. For example:
We talked about this in the corresponding article.
One of the important components of the conductive system that performs a protective function is the fuse. These devices come in various configurations and have many models. This article will talk about the fuse. Each block has its own current-carrying elements, so the conductive element takes an important part in the stable operation of electrical circuits. It should be noted that the concepts of fuse and fuse link have slightly different definitions. This article will help you understand this difference.
Operating principle
The basic feature of the fuse is that its combustion in the electrical circuit occurs much earlier than other elements. In the event of a current surge in an electrical circuit, it is much easier and faster to replace a fuse than to change live wires, microcircuits, etc.
This element received the name fusible because the main element of its design is a fusible insert. This component has a low melting point; according to the Joule-Lenz law, when current passes through a conductor, thermal energy is released in it, and the fuse burns out at a high current value, which is dangerous for other components. This leads to an open circuit. Thus, the fuse protects the remaining elements of the electrical circuit from damage.
Fuse operation modes:
- Short circuit:
- The fuse link burns out in the shortest possible time;
- Overload:
- The fuse-link burns out within a certain time, which depends on the current value in this mode. The higher the overload current, the faster the fuse burns.
- Normal mode. Heating of the device is a steady-state process in which:
- Full heating occurs to a specific temperature and the amount of heat released is released;
- Each fuse is labeled with its current rating;
- It is necessary to select a consumable element with a certain rated current.
When choosing the required fuse, you need to be guided not only by the current value indicated on the housing. But also the permissible operating voltage and time-current characteristics.
The time-current characteristic is necessary to indicate the magnitude of the change in the time of complete circuit break when a current of a certain value is supplied.
Design
The main element included in the fuse is the fuse link. These inserts have many configurations, but nevertheless have two basic elements:
- Fusible element - made of an alloy of various metals or made with specially selected metal alloys.
Fuse links are made of various materials:
- zinc;
- lead;
- copper;
- tin;
- silver.
- Housing - a block containing a set of fastening elements that allow the connection of the switching element to the electrical circuit.
The cases are made from varieties of durable ceramics such as:
- porcelain;
- corundum-mullite ceramics;
- steatite.
When using electrical fuses with low rated current, the housing is made of special glass.
The main parameters characterizing fuses include:
- Rated voltage;
- rated current;
- maximum power;
- response speed.
All these factors must be taken into account when calculating the fuse link.
Calculation of fusible rated current values is carried out according to formula 1:
From the formula, for calculation, you need to know U - voltage, Pmax - maximum load power.
Types of fuses
The main and most important step is the selection of fuse links. This is necessary, taking into account the various conditions in which the following types of electrical fuses are used:
- Electrical fuses are fork. This type of conductive devices often operates in a direct current circuit. The design is made in the form of an arrangement of electrical contacts on one side, and a fusible part on the reverse.
Fork safety elements are divided into:
- regular fork;
- forks of miniature sizes.
- Electrical fuses are cork. One of the most common species. The design is based on a body made of porcelain. In the inner part of the case there is a thin wire, which burns out in case of emergency mode. The housing block includes a weight that determines the condition of the safety component. Each weight has a specific color corresponding to the required current strength. If it hangs down on the wire section, it needs to be replaced.
Varieties of configurations and purpose:
- DIAZED – applicable in a system whose elements are designed to meet the most varied requirements of installation methods.
- NEOZED - this type allows you to safely replace fusible elements when de-energized.
The rated current of the fuse link is selected based on the maximum power of the network.
Current values according to the color of the checks
- Blade fuses. This type is used on electrical installation lines, with an operating current value of about 1200 - 1300 A. In turn, they are very dangerous to human health. The use of such types of components in a conductive system leads to very strict compliance with all safety requirements. Only suitably qualified personnel work at such facilities.
Blade electric fuse is divided according to current value:
- 000 (˂ 100 A);
- 00 (˂ 160 A);
- 0 (˂ 250 A);
- 1 (˂ 355 A);
- 2 (˂ 500 A);
- 3 (˂ 800 A);
- 4a (˂ 1250 A).
- Low current inserts. Their main purpose is to protect low-power electrical circuits. The design has a glass body made in the form of a cylinder with metal elements connected by conductive wire. When a short circuit occurs, the wire burns out, which in turn opens the circuit and keeps the remaining elements of the circuit intact.
Such enclosures are made with different overall dimensions (in mm):
- 3 x 15;
- 5 x 20;
- 7 x 15;
- 10 x 38.
To summarize the consideration of fuses, it is worth noting that fuses must be used in many electrical devices in order to avoid damage to their elements. In addition to the above, it makes sense to pay attention to their advantages and disadvantages.
Advantages:
- low cost;
- In the event of a high current surge, the electrical fuse completely opens the electrical circuit.
- In the event of a fuse failure, it is possible to simply replace the current-carrying element.
Flaws:
- use the fuse only once, then replace it;
- replacing the current-carrying element with an electrical fuse of a higher rating;
- When using three-phase electric motors, it is recommended to use a phase relay to avoid burning out one of the fuses.
Recently, many manufacturers have been using modern quality standards for development, so that the block of each conductive element can adequately compete with European and world analogues.
Thus, protecting electrical circuits using various fuses is one of the simplest, most reliable and cheapest ways.
Video about fuses
Protection devices are designed to ensure the safety of the operation of electrical networks, machines, electrical installations in the event of emergency conditions (short circuits, overloads). However, if installed and used incorrectly, they themselves can cause an accident, fire and explosion, because During their operation, electrical sparks and arcs occur.
The most common protection devices are:
fusible circuit breakers;
air circuit breakers;
thermal relay;
devices protective shutdown.
Fuse is a device in which, when a current exceeds the permissible value, the fuse-link melts and the electrical circuit opens. Fuses are single-use protective devices.
Compound:
A) fusible insert;
b) contact device;
V) frame(cartridge);
d) and sometimes filler(talc, quartz sand, etc.) to improve arc extinction and visual response rate.
Principle The action of fuses is based on the fact that the current passing through the fuse-link generates heat in accordance with the equality where I is the current passing through the fuse-link, R is the resistance of the fuse-link, t is the time of passage of the current: at a certain value of the current I and time t, heat is released enough to melt the fuse link and open the electrical circuit. This provides protection against overload current and short circuit.
Fuse parameters
A) rated current of fuse link I n.vst . – the current for which it is designed for long-term operation and is indicated on it.
b) rated fuse current I n.pr . – current equal to the largest of In.in and which is indicated on the fuse. All current-carrying contact parts of the fuse are designed for this current;
V) Rated voltage U n.pr . – voltage corresponding to the highest voltage at which it is permitted to be used and is indicated on the fuse.
G) maximum breaking current at a given voltage I pr.pr . – the highest value of short-circuit current at which reliable operation is guaranteed (without destruction of the housing).
(3 min) Full shutdown time of the electrical circuit, the fuse is determined by the time the insert is heated to the melting temperature, the time of its melting and combustion that appears when the arc melts.
Dependence of the total shutdown time of the circuit fuse off. from relative overload current or short circuit I/In.in. called protective characteristic, i.e. off =f(I/ In.vst.).
The dependence of the period of time during which the temperature of an element of an electrical installation reaches the maximum permissible on the ratio of the actual current in it I to the rated current Iн is called thermal characteristics of this element, i.e. load=f(I/ In).
Comparison of the protective characteristics of fuses with the thermal characteristics of the protected elements allows us to evaluate
possibility of reliable protection. (Fig.1)
I/I N.VST and I/I h
(5 min) It can be seen that the insert with the protective characteristic A protects the element of the electrical installation with a thermal characteristic IN at any multiplicity of current, and an insert with a protective characteristic WITH– only at multiplicity more than 4.
We need to strive for the shutdown time to be as short as possible under the action of short-circuit currents. and have a delay at overload currents. It can be done:
Right choose the material of the fusible insert;
use metallurgical effect;
choose rational design.
Inserts from low-melting metals (tin, lead, zinc, aluminum) have low thermal conductivity, so they heat up slowly; they are convenient for protecting elements from overload currents.
Inserts from refractory metals ( copper, silver) have low heat capacity and high thermal conductivity, therefore they heat up quickly, give a shorter delay time during overloads, which worsens their protective characteristics. But they have a large maximum shutdown current, so they are convenient for protecting elements from short-circuit currents.
To reduce the melting point (so that they heat up more slowly), inserts with metallurgical effect, for which a ball of low-melting metal (tin, an alloy of tin with cadmium, etc.) is soldered in the middle of an insert made of a refractory metal.
At the point where the ball is soldered, the more refractory metal dissolves into the low-melting one. This insert has better protective characteristics during overload currents and a lower melting temperature (2-3 times lower than the melting temperature of the base metal).
From point of view design protective performance is affected length (for fuses with U = 120 - 500V, the optimal insertion length is 70mm) and insert form(inserts are made with several parallel branches; inserts with 2–4 short isthmuses are used).
