As you know, fuses protect electrical networks from short circuits and overloads by destroying conductors specially designed for this purpose. Fuses are inexpensive and very simple in design. The main part of fuses is the fuse link. It is in it that the electric current is cut off and must be replaced after the fuse has tripped. Structurally, it is a housing, inside of which there is a fusible element that collapses after operation, and an arc extinguishing device, usually in the form of a dielectric filler, which extinguishes the resulting electric arc.
The main purpose of the fuse link is to be a section of the protected circuit with the smallest cross-section and greater resistance than other elements. As a result, the fuse-link, when a short-circuit current passes through the circuit, heats up faster and stronger than other areas, and therefore melts earlier, saving the electrical equipment from overheating and failure. In some cases, the melted section is built directly into the electrical appliance. For example, it is found in household lighting lamps to prevent overload of the electrical network when the filament burns out and the resulting electric arc. Usually this is a section of one of the input wires located in the lamp base. The specified section has a cross-section designed for a current not exceeding 7 amperes.
Physically, a fuse-link is a low-fusible conductor made in the form of a wire or plate of a special shape. For example, in radio-electronic devices, the fuse consists of a glass or porcelain tube in which the wire is placed, and in the widespread PR-2 fuses, the fuse link is made of a zinc plate, which is both a low-melting and corrosion-resistant material. The shape of the plate is a series of alternating wide and narrow areas(from one to four depending on the set voltage). The narrowed sections burn out first, preventing the short-circuit current from increasing to critical values and creating a current-limiting effect.
This configuration allows you to obtain a suitable time-current characteristic, which is the main parameter for determining the degree of protection of the circuit. In general, it is believed that the main indicator of a fuse link is the dependence of failure on the flowing current. When testing inserts, a batch of identical fuses is tested at different currents, measuring the time required for melting. Naturally, the higher the current, the shorter the burnout time. During the study, the lowest current is identified, at which the insert begins to melt for an indefinitely long time (an hour or two), the maximum current - the insert is destroyed in ten seconds, and the rated current, when the insert works for a long time without heating up. As a rule, the rated current is 2.5 times less than the maximum.
To reduce the melting threshold, the inserts are made of several parallel branches. This also contributes to better use of the arc extinguishing filler. For the same purposes, tin strips are applied to the copper strip of the insert, achieving a metallurgical effect (tin accelerates the melting of copper at low short-circuit currents). In some fuse models, the insert is made of several silver-plated copper wires in the form of an elongated spiral. Several turns have a smaller cross-section, which makes it possible to effectively extinguish an arc that occurs in several channels simultaneously. It is not recommended to replace the fuse link after the fuse has tripped with a self-made analogue. For reliable operation, only factory-made repair kits are required.
Fuse links are made of copper, zinc, lead or silver.
Today's most advanced fuses give preference to copper inserts with a tin solvent. Zinc inserts are also widespread.
Copper fuse inserts are the most convenient, simple and cheap. Improving their characteristics is achieved by fusing a tin ball
a certain place, approximately in the middle of the insert. Such inserts are used, for example, in the mentioned series of bulk fuses PN2. Tin melts at a temperature of 232°, significantly lower than the melting point of copper, and dissolves the copper of the insert at the point of contact with it. The arc that appears in this case already melts the entire insert and is extinguished. The current circuit turns off.
Thus, fusing a tin ball results in the following.
Firstly, copper inserts begin to react with a time delay to such small overloads, to which they would not react at all in the absence of a solvent. For example, a copper wire with a diameter of 0.25 mm with a solvent melted at a temperature of 280° in 120 minutes.
School for an electrician: articles, tips, useful information
Secondly, at the same sufficiently high temperature (i.e., under the same load), inserts with a solvent react much faster than inserts without a solvent.
For example, a copper wire with a diameter of 0.25 mm without a solvent at an average temperature of 1,000° melted in 120 minutes, and the same wire, but with a solvent at an average temperature of only 650°, melted in just 4 minutes.
The use of a tin solvent makes it possible to have reliable and cheap copper inserts that operate at a relatively low operating temperature, have a relatively small volume and weight of metal (which favors the switching ability of the fuse) and at the same time have greater speed at high overloads and react with a time delay to relatively small overloads.
Zinc is often used to make fuse links. In particular, such inserts are used in the mentioned series of PR-2 fuses.
Zinc inserts are more resistant to corrosion. Therefore, despite the relatively low melting point, for them, generally speaking, it would be possible to allow the same maximum operating temperature as for copper (250°C) and design inserts with a smaller cross-section. However, the electrical resistance of zinc is approximately 3.4 times greater than that of copper.
To maintain the same temperature, it is necessary to reduce energy losses in it, accordingly increasing its cross-section. The insert turns out to be much more massive. This, other things being equal, leads to a decrease in the switching capacity of the fuse. In addition, with a massive insert with a temperature of 250°, it would not be possible to maintain the temperature of the cartridge and contacts at an acceptable level within the same dimensions.
All this makes it necessary to reduce the maximum temperature of zinc inserts to 200°, and for this purpose, to increase the cross-section of the insert even more. As a result, fuses with zinc inserts of the same dimensions have significantly less resistance to short-circuit currents than fuses with copper inserts and tin solvents.
School for an electrician: articles, tips, useful information
Fuse - component power electronics single action, performing a protective function. The fuse is the weakest part of the protected electrical circuit, tripping in emergency mode, thereby breaking the circuit and preventing the subsequent destruction of more valuable elements of the electrical circuit by high temperatures caused by excessive current values.
In an electrical circuit, a fuse is a weak section of the electrical circuit that burns in emergency mode, thereby breaking the circuit and preventing subsequent destruction by high temperature.
Fuses are divided into the following types:
1. low current inserts(to protect small electrical appliances up to 6 amperes)
- 3x15 (the first number means the outer diameter, the second - the length of the insert)
- 10x30
2. fork(to protect electrical circuits of cars)
- miniature
- regular fork
3. cork(found in residential areas, up to 63 amperes)
- DIAZED (most common in the USSR)
- NEOZED
4. knife(up to 1250 amperes)
- size 000 (up to 100 amperes)
- size 00 (up to 160 amperes)
- size 0 (up to 250 amperes)
- size 1 (up to 355 amperes)
- size 2 (up to 500 amperes)
- size 3 (up to 800 amperes)
- size 4a (up to 1250 amperes)
5. quartz
6. gas generating
Fuses also differ in their response characteristics relative to the rated current. Due to the inertia of fuses, in the professional environment of electricians they are often used as selective protection in conjunction with circuit breakers. Selectivity between the fuse links themselves is achieved with a ratio of 1:1.6 [ibid.], the time-current characteristic of the fuses is established by the dependence, respectively, I²t; The PUE regulates the protection of overhead conductive lines so that the fuse trips within 15 seconds (the short circuit current at the end of the line must be equal to three rated currents of the fuse). An essential value is the time during which the conductor is destroyed when the set current is exceeded. To reduce this time, some fuses contain a pretension spring. This spring also separates the ends of the broken conductor, preventing an arc from occurring.
40-amp fuses with a "gG" response characteristic, equivalent to the Soviet "PPN" characteristic
- fuse link - an element containing a discontinuous part of an electrical circuit (for example, a wire that burns out when a certain current level is exceeded)
- a mechanism for attaching the fuse-link to the contacts, ensuring the inclusion of the fuse in the electrical circuit and installation of the fuse as a whole.
Fuse housings are usually made of high-strength grades of special ceramics (porcelain, soapstone or corundum-mullite ceramics). For fuse housings with low rated currents, special glass is used. The fuse body usually serves as a base part on which a fuse element with fuse contacts, an operation indicator, free contacts, devices for operating the fuse and a rating plate are mounted. At the same time, the housing performs the functions of an electric arc extinguishing chamber.
Fuse markings
The first letter means the protection range:
- a - partial range (short-circuit protection only)
- g - full range (protection from both short circuit currents and overload)
- h - high breaking ability (tubes are made of white or gray ceramics)
The second letter indicates the type of equipment being protected:
- G - universal fuse for protection various types equipment: cables, electric motors, transformers
- L - protection of cables and distribution devices
- B - protection of mining equipment
- F - protection of low-power circuits
- M - protection of electric motor circuits and disconnecting devices
- R - semiconductor protection
- S - fast combustion in case of short circuit and average combustion time in case of overload
- Tr - transformer protection
A fuse is the first device used in electrical circuits to protect against short circuits and overloads. The occurrence of these emergency operating conditions is inevitable. No matter how new and high-quality the electrical installation is, there is always a chance of damage to its insulation and connection of excess power to the power supply networks.
Fuse is a disposable component. After operation, either it itself or its fuse-link must be disposed of and replaced with new ones. These disadvantages are absent circuit breakers, disabling emergency network operation modes again and again, without destruction or failure. But fuses are still used in electrical installations.
Its advantages contribute to this:
- simple design, cheap to manufacture;
- ease of use;
- failure of the fuse is impossible - there is simply nothing in it to break. Therefore, there are no failures in their operation, which increases the reliability of the protection.
Fuse device
A fuse of any design consists of three parts: a body, a contact part and a fuse element.
Fusible element is a conductor made of low-melting material. When current passes through a fuse on a fusible element having electrical resistance, stands out electric power in the form of heat. If the current is below the rated current, then the heat is not enough to melt the metal from which the insert is made.
When the current exceeds the operating threshold, the insert melts, accompanied by a circuit break. The rupture occurs the faster the greater the current passes through the fuse. For each of them, manufacturing plants provide a time-current characteristic, which can be used to determine how long it will take to turn off the emergency mode with a given factor of exceeding the rated current. This information is used by designers to calculate the performance of fuse-based protection.
Fuse housing serves not only for the mechanical connection of its elements with each other. When a fuse link burns out, an electric arc inevitably occurs. The task of the fuse body is to prevent its spread and extinguish it as soon as possible.
Purpose contact system– ensure a reliable detachable connection of the protective device with the current conductors of the electrical installation. The contact area should be as large as possible in order to reduce the contact resistance and prevent heating of the connection. Anodized brass and copper are used for fuse contact systems.
Arc suppression in fuse housings
The simplest models contain nothing inside but air. But they are also designed for small currents, the shutdown of which is not accompanied by the formation of an arc with characteristics dangerous to electrical equipment. When the insert melts, it goes out on its own.
With an increase in the current cut off by the fuse, the need arises forced arc extinction inside the housing. Otherwise, it will not go out, continuing to feed the short circuit. The emergency circuit will not be turned off: the arc, having melted the contact system, will spray metal particles over the surface of the housing, forming a contact bridge. The short circuit current will continue to flow through it until the higher-level protection operates or the conductors finally melt. In the best case, the time to turn off the emergency operation mode will take several times longer.
The longer the short circuit opens, the more damage it will cause.. Therefore, special attention is paid to extinguishing the arc inside the fuse.
The first method to reduce short circuit tripping time was manufacturing the central part of the hollow fuse body from fiber. This laminate, consisting of cardboard compressed with cellulose mass, pre-impregnated with zinc chloride. Fiber products are resistant to gasoline, alcohol, kerosene, acetone, and also have insulating properties.
But the main advantage of fiber parts, which determined its spread in electrical engineering, is when exposed to an arc flame, it emits a mixture of gases that block its combustion process. Gases, mixing with the ionized plasma of the arc, impede the movement of charged particles in it. The resistance of the current-carrying channel increases sharply, and the arc goes out. Such fuses are called gas-generating, and in addition to fiber, they are also made from vinyl plastic.
The next method used to speed up the operation of the fuse is filling the body with quartz sand. The melting point of quartz is about 1700 degrees, and it is also an excellent dielectric. When the fuse link burns out, the arc, increasing in volume, spreads between the grains of sand. She has to go around them along an intricate and complex trajectory, as a result of which her length increases. Additionally, arc heat is removed from the filler material, which promotes deionization of the channel and rapid extinction of the discharge.
Quartz fuses are most widely used in electrical installations and are still used today. Gas-generating fuses are less common and are found only in older switchgear.
Application of fuses to protect high voltage electrical installations significantly simplifies and reduces the cost of their design. An alternative to this is a full relay protection device. And for its operation, sensors are required: current transformers and voltage transformers. Their task is to reduce the measured values to safe values with which relays and microprocessor terminals can operate. All this together turns out to be orders of magnitude more expensive than installing fuses.
But even more stringent requirements are imposed on the speed of fuses in electrical installations above 1000 V. To quickly turn off their fuse-link is attached to the spring connected to one of the contact terminals. The body is filled with quartz sand.
When the insert burns out, the spring is released and sharply contracts. Due to this, the length of the arc burning section quickly increases. Extinguishing occurs faster.
An additional and mandatory device for high-voltage fuses is serviceability monitoring unit. To safely test a low voltage fuse, you can use an indicator, voltage gauge, or tester. If necessary, you can turn off the switch and measure the resistance between the contacts of the protective device.
But it’s not possible to check the serviceability of a high-voltage fuse. You can't get close to him. The use of voltage indicators does not give reliable results. If the power transformer is protected by fuse links, the indicator will show behind the blown fuse the voltage induced on the winding that has lost power from the windings of other phases. When checking the serviceability of the inserts on cable line the indicator will light up from the residual charge remaining due to the large capacity of the cable.
To indicate that the protection has tripped, an indicator pops out from the fuse body and is clearly visible at a distance that is safe for inspection. For ease of maintenance, low-voltage fuses also use indicator devices that signal that the fuse link has burned out.
Another problem that exists when using fuses in networks above 1000 V is the occurrence of an open-phase mode due to burnout of the insert in one phase. Power transformers that remain in operation in two phases produce an asymmetrical voltage on the low-voltage winding, which threatens to damage consumers’ electrical appliances.
If the problem persists, if one insert burns out, turn off the power completely. To do this, use special fuses with strikers at one of its ends. The firing pin is spring-loaded and is released simultaneously with the fuse-link burning out. Paired with such devices they are used load switches having disconnecting strips. In the on position, the contact system of the switch is held by a latch. When the striker strikes the trip bar, the latch is knocked out. The circuit breaker's opening spring system pushes its contact system to the open position. The phase in which the shutdown occurred due to a short circuit is determined by the striker that has jumped out of the housing.
Semiconductor fuse
The development of power semiconductor technology has raised another problem. No mechanical protective device, including fuses, is capable of promptly shutting down the emergency operation of devices containing powerful diodes or transistors. Overloading of these devices is possible only for a limited time - tens of milliseconds. If this time is exceeded, the device is destroyed.
To minimize damage to electronics in frequency converters, inverters or soft starters use semiconductor fuses. Their pn junction burns out faster than any fuse link. But they have a peculiarity - when triggered, the semiconductor fuse does not fully guarantee disconnection of the circuit. The current through it stops, but not completely: a blown semiconductor fuse has some resistance. Therefore for safe operation another switching element is installed in front of it - a circuit breaker. They provide redundancy for semiconductor protection and are also used to guarantee voltage removal from the device to check the serviceability or replace fuses.
Self-resetting fuses
In some cases, after a circuit has been overloaded, it can be no harm turning it back on after a while. This is relevant in microprocessor and microcontroller technology. Self-resetting fuses are used to protect such circuits.
These devices include a polymer mass mixed with carbon. Carbon provides the required conductivity, but the device itself as a whole has resistance to the current passing through it. When this current exceeds the established threshold, the composition of the conductive mixture heats up, the polymer goes into an amorphous state, increasing in size. The connection between the carbon particles is broken, and the current through the fuse stops.
After the polymer cools, the conductive composition returns to its original form. Contact is restored and the device is ready for use again.
A fuse is an electrical switching device designed to disconnect the protected circuit by destroying live parts specially designed for this purpose under the influence of a current exceeding a certain value.
In fuses, the circuit is disconnected due to the melting of the fuse link, which is heated by the current of the protected circuit flowing through it. After disconnecting the circuit, it is necessary to replace the fuse-link with a working one.
The fuse is connected in series to the protected circuit, and to create a visible break in the electrical circuit and safe maintenance, non-automatic switches or circuit breakers are used together with the fuses.
Fuses are manufactured for voltage alternating current 42, 220, 380, 660 V and DC 24, 110, 220, 440 V.
The main elements of a fuse are the body, the fuse link (fuse element), the contact part, the arc extinguishing device and the arc extinguishing medium.
Fuses are characterized by the rated current of the fuse link, i.e. the current for which the fuse link is designed for long-term operation. Replaceable fusible elements for different rated currents can be inserted into the same fuse body, so the fuse itself is characterized by a rated current
fuse (base), which is equal to the highest rated current of the fuse links intended for this fuse design. For example, fuses of the PN2 and PR2 series have replaceable fuse links. Thus, the PN2-100 series fuse has a housing designed for a current of up to 100 A and replaceable fuse links for currents of 30, 40, 50, 60, 80, 100 A.
Fuses up to 1 kV are manufactured for rated currents up to 1000 A.
In normal mode, the heat generated by the load current in the fuse link is transferred to the environment, and the temperature of all parts of the fuse does not exceed the permissible limit. When overloaded or short circuited, the temperature of the insert increases and it melts. The greater the current flowing, the shorter the melting time. The dependence of the melting time of the fuse-link on the current value (multiplicity of the operating current in relation to the rated current of the fuse-link) is called the protective (time - current) characteristic of the fuse (Fig. 3.1.). At the same current, the melting time of the fuse-link depends on many reasons (material of the insert, condition of its surface, cooling conditions, etc.). To reduce the response time of the fuse, fuses made of different materials, special shapes are used, and the metallurgical effect is also used.
The most common fuse link materials are copper, zinc, aluminum, lead and silver.
Copper inserts are subject to oxidation, their cross-section decreases over time and the protective characteristics of the fuse change. To reduce oxidation, tinned copper inserts are usually used. The melting point of copper is 1080 °C, therefore, at currents close to the minimum melting current, the temperature of all fuse elements increases significantly.
Zinc and lead have low melting points (419 °C and 327 °C), which ensures slight heating of fuses in continuous operation.
Zinc is resistant to corrosion, so the cross-section of the fuse link does not change during operation, the protective characteristic remains constant. Zinc and lead have high resistivities, so fuse links have a large cross-section. Such fuse links are usually used in fuses without fillers. Fuses with zinc and lead inserts have long time delays during overloads.
Rice. 3.1. Time-current characteristics of the fuse
Silver inserts do not oxidize, and their characteristics are the most stable.
Aluminum inserts are used in fuses due to the shortage of non-ferrous metals. The high resistance of oxide films on aluminum makes it difficult to make reliable detachable contacts. Aluminum inserts are used in new designs of fuses of the PP31 series.
At high currents, fuse links are made of parallel wires or thin copper strips.
The main characteristic of a fuse is the time-current characteristic, which is the dependence of the melting time of the insert on the flowing current. For perfect protection, it is desirable that the time-current characteristic of the fuse (curve 1 in Fig. 1.1) at all points was slightly below the characteristics of the protected circuit or object (curve 2 in Fig. 3.1). However, the actual characteristics of the fuse (curve 3) crosses the curve 2. Let's explain this. If the fuse characteristic corresponds to the curve 1, then it will burn out due to aging or when starting the engine. The circuit will turn off in the absence of unacceptable overloads. Therefore, the melting current of the insert is selected greater than the rated load current. At the same time, the curves 2 And 3 intersect. In the area of high overloads (area B) The fuse protects the object. In area A The fuse does not protect the object.
At small overloads (l.5–2) I H 0 M fuse heating is slow. Most of the heat is lost to the environment. Complex heat transfer conditions make it difficult to calculate the fuse link.
The current at which the fuse-link burns out when it reaches a steady temperature is called the boundary current I POGR.
To accelerate the melting of inserts made of copper and silver, the metallurgical effect is used - the phenomenon of dissolution of refractory metals into molten, less refractory metals. If, for example, a ball of tin-lead alloy with a melting point of 182 °C is soldered onto a copper wire with a diameter of 0.25 mm, then at a wire temperature of 650 °C it will melt within 4 minutes, and at 350 °C - within 40 minutes . The same wire without a solvent melts at a temperature of at least 1000 ° C. To create a metallurgical effect on copper and silver inserts, pure tin, which has more stable properties, is used. In normal operation, the ball has virtually no effect on the temperature of the insert.
Figure 3.2. PR2 series fuse: A - cartridge; b - fuse link shapes
Acceleration of the melting of the insert is also achieved by using a specially shaped fuse insert (Fig. 3.2, b). With short-circuit currents, narrow areas heat up so quickly that almost no heat removal occurs. The insert burns out simultaneously in several narrowed places (section A - A and B - B, Fig. 3.2, b) before the short-circuit current reaches its steady-state value in a direct current circuit or shock current in an alternating current circuit (Fig. 3.3).
Rice. 3.3. Current-limiting effect of fuse links
fuses: A - at constant current;
b - at alternating current
The short-circuit current is limited to the value i limit (2-5 times). This phenomenon is called current-limiting action and improves the arc extinguishing conditions in fuses.
Extinguishing the electric arc that occurs after the fuse link has burned out must be carried out as quickly as possible. The arc extinction time depends on the design of the fuse.
The highest current that a fuse can break without causing any damage or deformation is called the breaking current limit.
Fuses are widely used to protect electric motors, electrical equipment, electrical networks in industrial and household electrical installations and have different designs.
Fuses, along with the simplicity of their design and low cost, have a number of significant disadvantages:
They cannot protect the line from overload because they allow
long-term overload until melting;
They do not always provide selective protection on the network following
the spread of their characteristics;
In the event of a short circuit in a three-phase network, the
blowing one of the three fuses and the line remains working
on two phases.
In this case, three-phase electric motors connected to the network turn on two phases, and this leads to overheating of the electric motor windings and their failure.
Fuses with closed collapsible housings (cartridges) without filler of the PR2 series (Fig. 3.2) are manufactured for voltages of 220 and 500 V and rated currents of 100-1000 A. Fuse holder PR2 (Fig. 3.2, A) for currents of 100 A and above consists of a thick-walled fiber tube 1, on which brass bushings are tightly fitted 3, having fine threads. Brass caps are screwed onto the tubes 4, which secure the fuse-link 2, screwed to the knives 6, before installing it in the cartridge. The fuses of this series are equipped with a washer 5, which has a groove for the knife and prevents rotation of the knives.
The cartridge is inserted into fixed contact posts mounted on an insulating plate. The necessary contact pressure is provided by springs.
Fuse links are made of zinc in the form of a plate with cutouts. Narrow areas generate more heat than wide areas. At rated current, excess heat due to the thermal conductivity of the zinc is transferred to the wide parts, so the entire insert has approximately the same temperature. During overloads, heating of narrow sections occurs faster, and the insert melts in the hottest place (section A - A, Fig. 3.2, b).
During a short circuit, the insert melts in narrow sections A - A and B - B. The resulting arc causes the formation of gases (50% CO 2, 40% H 2, 10% H 2 O vapor), since the walls of the cartridge are made of gas-generating material - fiber. The pressure, depending on the current being switched off, can reach 10 MPa or more, which ensures rapid extinguishing of the arc and the current-limiting effect of the fuse. To reduce the overvoltage that occurs when the short-circuit current is switched off, the fuse-link has several narrowed places. When they melt one by one, the full length of the arc gap is introduced into the circuit not immediately, but in steps.
Bulk fuses of the PN2 series (Fig. 3.4) are widely used to protect power circuits up to 500 V AC and 440 V DC and are available for rated currents of 100-1000 A.
 |
| 1 2 |
Rice. 3.4. Fuse series PN2
Porcelain, square outside and round inside, tube 1 has four threaded holes for screws to secure the cover 4 with sealing gasket 5. Fuse link 2 welded with electric contact spot welding to contact knife washers 3. Caps with asbestos gaskets hermetically seal the tube. The tube is filled with dry quartz sand 6. The fuse link is made of one or more copper strips with a thickness of 0.15-0.35 mm and a width of up to 4 mm. Slots 7 are made on the insert, reducing the cross-section of the insert by 2 times. To reduce the melting temperature of the insert, a metallurgical effect is used - tin balls are soldered onto copper strips 8, the melting temperature in this case does not exceed 475 °C, the arc occurs in several parallel channels (in accordance with the number of inserts); this ensures the least amount of metal vapor in the channel between the quartz grains and best conditions extinguishing the arc in a narrow gap. Bulk
fuses, just like fuses of the PR2 series, have a current-limiting property.
To reduce the resulting overvoltages, the fuse-link has slots along its length, and their number depends on the rated voltage of the fuse (based on 100-150 V per area between the slots). Since the insert burns out in narrow places, the long arc turns out to be divided into a number of short arcs, the total voltage of which does not exceed the sum of the cathode and anode voltage drops.
The filler in PN series fuses is pure quartz sand (99% SiO2). Instead of quartz, chalk (CaCO3) can be used; sometimes it is mixed with asbestos fiber. When an arc occurs, the chalk decomposes with the release of carbon dioxide CO 2 and CaO, a refractory material. The reaction occurs with energy absorption, which helps extinguish the arc.
The maximum switchable current of fuses of the PN2 series reaches 50 kA.
Bulk fuses of the NPN series have a non-removable glass cartridge without contact blades and are designed for currents up to 60 A.
Instead of PN2 fuses, fuses of the PP-31 series with aluminum inserts have been developed for rated currents of 63-1000 A and having a maximum shutdown current of up to 100 kA at a voltage of 660 V.
PP-17 series fuses are manufactured for currents of 500-1000 A, AC voltage 380 V and DC 220 V. The maximum breaking capacity of PP-17 fuses is 100-120 kA. The fuse consists of a fuse element placed in a ceramic housing filled with quartz sand, a trip indicator and a free contact. When the fuse element melts, the fusible element of the operation indicator burns out, releasing the striker introduced during assembly of the indicator, which switches the free contact, and the fuse operation signaling circuit is closed.
To protect semiconductor devices, high-speed fuses of the PP-41, PP-57, PP-59, PP-71 series have been developed. These fuses are made with fuse links made of silver foil in closed cartridges filled with quartz sand. They are designed for installation in alternating current circuits with voltage
380-1250 V and DC 230-1050 V. The electrical industry produces fuses for rated currents of 100-2000 A, maximum shutdown currents up to 200 kA. These fuses have an effective current-limiting effect.
In control circuits of machine tools, mechanisms, machines, as well as in power supply systems for residential and public buildings Plug fuses of the PRS series are widely used. Rated current of the case 6; 25; 63; 100 A.