When operating a household and industrial electrical network, there is always a risk of electrical injury or equipment damage. They can occur at any time when critical conditions appear. Protective devices can reduce such consequences. Their use significantly increases the safety of using electricity.

Electrical circuit protections operate on the basis of:

fuse;

mechanical circuit breaker.

Operating principle and fuse design

Two brilliant scientists, Joule and Lenz, simultaneously established the laws of mutual relationships between the amount of current passing in a conductor and the release of heat from it, revealing the dependence on the resistance of the circuit and the duration of the period of time.

Their findings made it possible to create the simplest protective structures based on the thermal effect of current on the metal wire. It uses a thin metal insert through which the full current of the circuit is passed.

At rated parameters for transmitting electricity, this “wire” reliably withstands the thermal load, and if its values ​​exceed the norm, it burns out, breaking the circuit and relieving the voltage from consumers. To restore the functionality of the circuit, it is necessary to replace the burnt-out element: the fuse-link.

It is clearly visible on the designs of fuses for household television and radio equipment with glass, transparent insert housings.

Special metal pads are mounted at its ends, creating electrical contact when installed in the sockets. This principle is embodied in electrical plugs with fusible links, which for many decades protected our parents and older generations from damage in electrical wiring.

Automatic structures were developed using the same form, which were screwed into sockets instead of plugs. But they did not need to be replaced when triggered components. To restore power supply, simply push the button inside the case.

Old electrical connections to the apartment were protected in this way. Then, along with fuses, they began to appear.

The choice of fuse is based on:

rated current values ​​of the fuse itself and its insert;

coefficients of minimum/maximum test current multiplicity;

limit switchable electric current and the possibility of interruption of transported power;

protective characteristics of the fuse link;

fuse rated voltage;

compliance with the principles of selectivity.

The fuses have a simple design. They are widely used in electrical installations including high voltage equipment up to 10 kV, for example, in the protection of instrument voltage transformers.

Operating principle and design of the circuit breaker

The purpose of a mechanical switching device called a circuit breaker is:

turning on, passing, turning off currents in normal circuit mode;

automatic removal of voltage from an electrical installation during emergency conditions, for example, metal short circuit currents. Circuit breakers operate in reusable short circuit and overload protection modes. The possibility of repeated use is considered their main difference from a fuse.

During the Soviet era, automatic circuit breakers of the AP-50, AK-50, AK-63, and AO-15 series were widely used in the energy sector.

In modern electrical diagrams Improved designs from foreign and domestic manufacturers are in use.

All of them are enclosed in dielectric housings and have common executive bodies that provide:

1. thermal tripping of the circuit when the permissible current value is slightly exceeded;

2. electromagnetic cut-off during sudden load surges;

3. arc suppression chambers;

4. contact systems.

In the case of heating by the energy of the generated heat, a bimetallic plate works, bending under the influence of temperature until the release mechanism is activated. This function depends on the amount of heat released and is extended over time until a certain point.

The cut-off operates as quickly as possible from the operation of the electromagnetic solenoid with the occurrence of an electric arc. To extinguish it, special measures are used.

Reinforced contacts are designed to withstand repeated breaks.

Operational differences between circuit breakers and fuses

The protective properties of both methods have been time-tested, and each method requires an analysis of specific operating conditions when assessing the cost of the structure, taking into account the duration and reliability of operation.

Circuit breakers simpler design, disable the circuit once, cheaper. They can relieve tension manually, but this is usually not very convenient. In addition, at slightly higher currents, they disconnect the load for a long time. This factor may cause increased fire danger.

Any fuse protects only one phase of the network.

Circuit breakers more complex, more expensive, more functional. But they are more accurately adjusted to the settings of the protected electrical circuit, selected according to the operating design current, taking into account the switched powers.

The casings of modern machines made of thermosets have increased resistance to thermal effects. They do not melt and are resistant to fire. For comparison, the polystyrene housing of old switches could withstand temperatures no higher than 70 degrees.

The design allows you to select models for simultaneous opening of one to four electrical circuits. If fuses are used in a three-phase circuit, they will remove voltage from the circuit with different time delays, which can become an additional reason for the development of an accident.

Fuses operate on current, without taking into account its characteristics. Circuit breakers are selected for the load and classified by letters:

A - electrical networks of increased length;

B - lighting of corridors and areas;

C - power and lighting systems with moderate starting currents;

D—predominant loads from turning on electric motors with high starting parameters;

K - induction furnaces and electric dryers;

Any electrical circuit consists of individual elements. Each of them is characterized by certain current values ​​at which this element is operational. Increasing the current above these values ​​may cause damage to the element. This occurs due to an unacceptably high temperature or due to a fairly rapid change in the structure of this element due to the influence of current. In such situations, fuses of various designs help avoid damage to electrical circuit elements.

Their classification is based on the method of breaking electrical circuit these fuses, and therefore we can list those that are most widely used as the following types of fuses:

• fusible,
• electromechanical,
• electronic,
• self-healing.

The method of breaking an electrical circuit covers the entire set of processes that occur in the fuse when it is triggered.

• Fuses break the electrical circuit as a result of the melting of the fuse link.
• Electromechanical fuses contain contacts that are switched off by a deformable bimetallic element.
• Electronic fuses contain an electronic key, which is controlled by a special electronic circuit.
• Self-resetting fuses are made using special materials. Their properties change when current flows, but are restored after the current in the electrical circuit decreases or disappears. Accordingly, the resistance first increases and then decreases again.

Fusible

The cheapest and most reliable are fuses. Fuse link, which, after increasing the current above the set value, melts or even evaporates, guaranteed to create a break in the electrical circuit. The effectiveness of this method of protection is determined mainly by the rate of destruction of the fuse-link. For this purpose, it is made of special metals and alloys. These are mainly metals such as zinc, copper, iron and lead. Since the fuse link is essentially a conductor, it behaves like a conductor, which is characterized by the graphs shown below.

Therefore for proper operation fuse, the heat generated in the fuse-link at the rated load current should not lead to its overheating and destruction. It dissipates into the environment through the elements of the fuse body, heating the insert, but without destructive consequences for it.



But if the current increases, the heat balance will be disrupted and the temperature of the insert will begin to increase.



In this case, an avalanche-like increase in temperature will occur due to an increase in the active resistance of the fuse-link. Depending on the rate of temperature rise, the insert either melts or evaporates. Evaporation is facilitated by a voltaic arc, which can occur in a fuse at significant values ​​of voltage and current. The arc temporarily replaces the destroyed fuse-link, maintaining current in the electrical circuit. Therefore, its existence also determines the timing characteristics of fuse-link disconnection.



• The time-current characteristic is the main parameter of a fuse-link, by which it is selected for a particular electrical circuit.

In emergency mode, it is important to break the electrical circuit as quickly as possible. For this purpose, special methods are used for fuse links, such as:

• local reduction in its diameter;
• "metallurgical effect".



In principle, these are similar methods that allow, one way or another, to cause local, faster heating of the insert. A variable cross-section with a smaller diameter heats up faster than with a larger cross-section. To further speed up the destruction of the fuse-link, it is made composite of a pack of identical conductors. As soon as one of these conductors burns out, the total cross-section will decrease and the next conductor will burn out, and so on until the entire pack of conductors is completely destroyed.

The metallurgical effect is used in thin inserts. It is based on obtaining a local melt with a higher resistance and dissolving the base material of the low-resistance insert in it. As a result, local resistance increases and the insert melts more quickly. The melt is obtained from drops of tin or lead, which are applied to a copper core. Such methods are used for low-power fuses for currents up to several units of ampere. They are mainly used for various household electrical appliances and devices.

The shape, dimensions and material of the housing may vary depending on the fuse model. The glass case is convenient because it allows you to see the state of the fusible insert. But the ceramic case is cheaper and stronger. Under certain tasks other designs have been adapted. Some of them are shown in the image below.



Conventional electrical plugs are based on tubular ceramic bodies. The plug itself is a body that is specially made to fit the cartridge for convenient use of the fuse. Some designs of plugs and ceramic fuses are equipped with a mechanical indicator of the status of the fuse link. When it burns out, a semaphore-type device is triggered.

When the current increases beyond 5 - 10 A, it becomes necessary to extinguish the voltage arc inside the fuse body. To do this, the internal space around the fusible insert is filled with quartz sand. The arc quickly heats the sand until gases are released, which prevent further development of the voltaic arc.

Despite certain inconveniences caused by the need for a supply of fuses for replacement, as well as slow and insufficiently accurate operation for some electrical circuits, this type of fuses is the most reliable of all. The higher the rate of increase in current through it, the greater the reliability of operation.

Electromechanical

Fuses of electromechanical design are fundamentally different from fuses. They have mechanical contacts and mechanical elements to control them. Since the reliability of any device decreases as it becomes more complex, for these fuses, at least theoretically, there is a possibility of such a malfunction in which the set tripping current will not be turned off. Repeated operation is a significant advantage of these devices over fuses. Disadvantages can be identified as:

• the appearance of an arc when turned off and the gradual destruction of contacts due to its influence. It is possible that the contacts may be welded together.
• Mechanical contact drive, which is expensive to fully automate. For this reason, re-enabling has to be done manually;
• insufficiently fast response, which cannot ensure the safety of some “perishable” electricity consumers.

An electromechanical fuse is often referred to as a “circuit breaker” and is connected to the electrical circuit either by a base or by wire terminals stripped of insulation.

Electronic

In these devices, mechanics are completely replaced by electronics. They have only one drawback with its several manifestations:

• physical properties of semiconductors.

This disadvantage manifests itself:

• in irreversible internal damage to the electronic key from abnormal physical influences (excess voltage, current, temperature, radiation);
• false operation or failure of the control circuit electronic key from abnormal physical influences (excess temperature, radiation, electromagnetic radiation).



Self-healing

A bar is made of a special polymer material and equipped with electrodes for connection to an electrical circuit. This is the design of this type of fuse. The resistance of a material in a given temperature range is small, but increases sharply starting from a certain temperature. As it cools, the resistance decreases again. Flaws:

• dependence of resistance on ambient temperature;
• long recovery after triggering;
• breakdown by excess voltage and failure for this reason.

Choosing the right fuse provides significant cost savings. Expensive equipment, timely switched off by a fuse in the event of an accident in the electrical circuit, remains operational.

Modern electrical networks and devices are very complex and require reliable protection against possible overloads and short circuits. The main protective role in such cases is played by various safety devices. Among the variety of these devices, the most common are fuses, which have a high degree of reliability, ease of operation and relatively low cost.

Despite the widespread use of automatic protective devices, fuse links remain relevant in protecting electronic equipment, automotive electrical networks, industrial electrical installations and power supply systems. They are still used in the distribution boards of many residential buildings due to their reliable operation, small size, stable performance and quick replacement.

What are fuses used for?

If two wires connected to a current source are connected, the well-known short circuit effect will occur. The reason may be damaged insulation, incorrect connection of consumers, etc. With a relatively low resistance of the wires, at this moment a very high current will flow through them. As a result of overheating of the wires, the insulation catches fire, which can lead to a fire.

To avoid negative consequences quite possibly by incorporating fuses, also known as plugs. If the current exceeds the permissible value, the wire inside the fuse becomes very hot and quickly melts, breaking the electrical circuit at this point.

The design of fuses can be tubular or plug. Tubular elements are manufactured in a closed fiber casing with gas generation properties. If the temperature rises, high pressure is created inside the tube, causing the circuit to break. Plug fuses have a standard design, equipped with a wire that melts under the influence of high electric current.

There is another type of so-called self-healing fuses, made of polymer materials that change their structure at different temperatures. Significant heating leads to a sharp change in resistance towards an increase, as a result of which the circuit breaks. Further cooling causes a decrease in resistance, so the circuit closes again. These fuses are mainly used in complex digital devices. They are not used in conventional power networks due to their high cost.



Sometimes some craftsmen try to replace a blown fuse, using instead so-called bugs, which are a piece of thick wire or thin wires twisted into a common bundle. It is strictly forbidden to use such homemade devices, since the current during a short circuit will be unacceptably high. Extreme heating of the wiring will cause damage, ignition and fire.

Fuse device

The composition includes a housing or cartridge with electrical insulating properties, and the fuse link itself. Its ends are connected to terminals that connect the fuse in series with the electrical circuit, together with the protected device or electrical line. The material of the fuse link is selected so that it can melt before the temperature indicator of the wires reaches a dangerous level, or the consumer fails as a result of overload.



Based on their design features, fuses can be cartridge, plate, plug and tube. The calculated current strength that the fuse link can withstand is indicated on the device body.

Low-voltage fuses have a fairly simple design. Under the influence of high current, the fuse-link or conductive element is subjected to intense heating, after which, upon reaching a certain temperature, it melts in the arc-extinguishing medium and evaporates, breaking the protected circuit. This is how a fuse works in an electrical circuit.

To prevent hot gases and liquid metal from entering the environment, a ceramic insulator is used, also known as the device body, which is resistant to high temperatures and significant internal pressure. The protective covers located at the edges of the fuse are equipped with special strips for unified handles that grip fuse-links when replacing unusable elements. With the help of protective covers and a ceramic housing, an explosion-proof shell is created that limits the switching electric arc.



Sand filling the internal space limits the current. The material is selected with certain crystal sizes, after which it is compacted properly. As a rule, fuses are filled with quartz crystalline sand, which has high chemical and mineralogical purity. The connection of the fuse-link with the base-holder is carried out mechanically, using contact knives. They are made from copper or copper alloys coated with tin or silver.

Fuse characteristics

The main characteristic is the direct dependence of the melting time on the current strength. Therefore, the time during which the fuse link blows out corresponds to a certain current. This parameter better known as the time-current characteristic.



In addition to the time indicator, there are other characteristics that are used to determine the types of fuses. Among them, first of all, it should be noted. This is the most permissible load current under the conditions of heating the fuse body for a long time. When choosing a device based on this indicator, the load of the electrical circuit must be taken into account, as well as the operating conditions of the fuse.

In some cases, the current rating may be higher than the current in the electrical circuit itself. For example, in electric motor starters to avoid the fuse blowing during starting. It should be taken into account that the rated current of the fuse must correspond to the rated current of the element being replaced.

In turn, the rated current of the element being replaced represents the maximum permissible load current for a long time when this element is installed in the holder or contacts. In addition, there are base and fuse holder current ratings that must be taken into account when selecting a protective device. In addition, an indicator such as rated voltage is used. This parameter represents the interpole voltage, which coincides with the rated phase-to-phase voltage of the protected electrical networks.



In order for fuses to provide reliable protection, the value of this value must be greater than or equal to the voltage of the protected object. For example, a fuse rated 400 volts can be used to protect 220 volt circuits, but not vice versa. Thus, this value characterizes the ability of the fuse to promptly break the electrical circuit and extinguish the arc.

Therefore, when choosing a fuse as a protective device, it is imperative to take into account the parameters that make it possible to ensure reliable protection of the object.

Types of fuses

For all devices of this type, there is a general classification according to their basic properties.



Fuse links can close in different ways, and therefore the external effects that occur when the current is turned off are also different. Such fuses are divided into the following types:

• An open fuse-link in which there are no devices to limit the volume of the arc, the emission of molten metal particles and flame.
• A semi-closed cartridge with a shell open on one or both sides. It creates a certain danger for people nearby.
• Closed cartridge. It is the most reliable because it does not have all of the above disadvantages. Almost all modern fuses are produced with a closed cartridge.

Arc extinction can be performed different ways. Depending on this, fuses are available with or without filler. In the first case, powdery, fibrous or granular components are used, and in the second, due to the movement of gases or high pressure in the cartridge. The designs of the cartridges themselves are divided into collapsible and non-collapsible. The first option involves replacing the melted insert, and in the second case the entire element will have to be replaced. In some cases, non-separable cartridges can be reloaded in special workshops.

Fuses may or may not be replaced while energized. In the first case, replacement can be done directly by hand, without touching live parts. In the second case, the device must be disconnected from the voltage.

Fuse markings

Each fuse in the diagram is indicated by a specific symbol. The standard marking consists of two letter characters. The first letters determine the protective interval: a - partial (protection against short circuits only) and g - complete (protection against short circuits and overloads is provided).

The second letter indicates the types of protected devices:

• G - protects any equipment.
• F - only low current circuits are protected.
• Tr - transformer protection.
• M - electric motors and disconnecting devices.

More detailed information Information about the marking of fuses can be found in reference books intended for electrical engineers.

The disposable component protects the power source from excessive load and is the weakest link in the electrical circuit. Fuses are included in almost all electrical systems. This device consists of a piece of wire, the cross-section of which is designed to carry a certain amount of current. When excessive load occurs in the circuit, the fuse element melts and breaks the circuit.

The main properties of a fuse are: rated voltage, rated current, maximum permissible current.

Some people believe that the quality of a fuse depends on the thickness of the wire in it. But it is not so. An unqualified calculation of the thickness of the fuse link can easily cause a fire, since in addition to the fuse itself, the wires that make up the circuit also heat up. If you install a fuse with a wire that is too thin, it will not ensure normal operation and will quickly break the circuit.

Operating principle

Fuses are included in the gap of an electrical circuit in such a way that the total load current of this circuit passes through them. Until the upper limit of the current is exceeded, the wire element is warm or cold. But, when a significant load appears in the circuit or a short circuit occurs, the current increases significantly, melts the fuse wire element, which leads to an automatic break in the circuit.

Fuses operate in 2 different modes:

• Normal mode , when the device is heated in a steady process in which it is completely heated to operating temperature and releases heat outside. Each fuse indicates the highest current value at which the wire element melts. The insert body may contain fusible elements designed for different current strengths.
• Overload and short circuit mode . The device is designed in such a way that when the current increases to the upper permissible limit, the fusible element burns out very quickly. To achieve this property, the fuse element in some places is made with a smaller cross-section. They generate more heat than other places. During a short circuit, all narrow sections of the fusible element melt and open the circuit. At this time, an electric arc is formed around the melting point, which goes out in the fuse housing.

Marking

The designation of fuses is represented by two letters. Let's take a closer look at the marking of fuses.

The first letter determines the protection interval:

• a— partial interval (short circuit (short circuit) protection).
• g— full interval (protection against short circuit and overload).

The second letter determines the type of protected device:

• G— universal type for protecting various equipment.
• L— protection of wires and switchgears.
• B— protection of mining equipment.
• F— protection of low current circuits.
• M— protection of disconnecting devices and electric motors.
• R— protection of semiconductor devices.
• S- fast response during short circuit and medium response during overload.
• Tr— protection of transformers.

types and device

Low current inserts



These fuses are used to protect low-power electrical devices with a current consumption of up to 6 A.

The first number is the outer diameter, the 2nd is the length of the fuse.

• 3 x 15.
• 4 x 15.
• 5 x 20.
• 6 x 32.
• 7 x 15.
• 10 x 30.

Fork fuses

They are used for use in cars and protect their circuits from overloads. Plug inserts are manufactured for voltages up to 32 V. Appearance their designs are shifted to the side, since the contacts are on one side and the fusible part on the other.

• Miniature inserts.
• Regular.

Cork inserts

They are used in residential buildings and operate at currents up to 63 A.

• DIAZED.
• NEOZED.

Such fuses are used for lighting, protection devices household devices, meters, low-power electric motors. They differ from tubular inserts in the method of fastening.

Tubular inserts



Such inserts are made in closed form with housings made of material - fiber, which forms a gas that creates high pressure, breaking the chain. Contacts.

1. Caps.
2. Rings.
3. Fiber.
4. The insert is fusible.

Blade fuses

The operating current reaches 1.25 kA. Standard sizes of knife types:

• 000 – up to 100 A.
• 00 – up to 160 A.
• 0 – up to 250 A.
• 1 – up to 355 A.
• 2 – up to 500 A.
• 3 – up to 800 A.
• 4 – up to 1250 A.

Quartz



This type of insert is current-limiting, does not produce gases, and is used for internal installation. Quartz fuses are designed for voltages up to 36 kilovolts.

1 – Cartridge (ceramics, glass).
2 – Fusible insert.
3 – Caps (metal).
4 - Filler.
5 – Index.

The cartridge is closed with caps, ensuring tightness. The filler has certain requirements:

• Durability (electrical).
• High thermal conductivity.
• Should not form gases.
• Should not absorb moisture.
• The filler particles must be of strictly required size to avoid sintering or the inability to extinguish the arc.

Quartz sand meets these requirements. The fusible element is made of copper coated with silver. Due to its considerable length, the fusible element is wound in the form of a spiral.

Gas generating



This type includes collapsible PR fuses, firing inserts for external installation PSN, exhaust PVT for transformers.

The PR insert is used for indoor installation in devices up to 1000 volts. It consists of:

1. The cartridge is made of fiber with brass rings around the edges. Brass caps are screwed onto the ends.
2. Caps.
3. Fuse element in the form of a zinc plate.
4. Contacts.

When the insert burns under the influence of an electric arc, a significant amount of gas is formed. Its pressure increases, the arc goes out in the gas flow. The insert is made in a V-shape, since during combustion of the bottleneck, a smaller amount of metal vapor is formed, which prevents the arc from extinguishing.

Thermal fuses

This type of insert is a disposable device. It serves to protect expensive equipment elements from overheating above the set temperature limit. Temperature-sensitive materials are placed inside the housing, which ensures the installation of inserts in circuits with high current.



The principle of operation is as follows. In normal mode, the insert has a resistance equal to zero. When the housing from the protected device heats up to the operating temperature, the heat-sensitive jumper is damaged, which breaks the power supply circuit of the device. After tripping, you need to replace the thermal fuse and eliminate the cause of the breakdown.

Such fuses have become popular in household electrical devices: toasters, coffee makers, irons, as well as in climate control equipment.

General Features

Fuses differ in their tripping properties from their rated current. Fuses have an inert response, so professionals often use them for selective protection together with electrical circuit breakers.

The rules regulate the protection of overhead lines so that the insert operates within 15 s. An important value is the destruction time of the conductor when working with a current exceeding the set value. To reduce this time, some fuse designs have a pre-tensioned spring. It separates the edges of the destroyed conductor to prevent the occurrence of an electric arc.

Fuse housings are made from durable ceramics. For low currents, inserts with glass housings are used. The insert body plays the role of the main part. A fusible element, an operation indicator, contacts, and a table with data are attached to it. The housing also acts as an arc extinction chamber.

Disadvantages of fuses

• Can be used once.
• A significant disadvantage of fuse links is their design, which allows unscrupulous specialists to perform shunting (use “bugs”). This may cause the wiring to catch fire.
• In 3-phase electric motor circuits, when one fuse trips, one phase disappears, which most often leads to engine malfunctions. In this case, it is advisable to use a phase control relay.
• It is possible to illegally install a fuse at a higher current rating.
• Phase imbalance may occur in 3-phase networks at significant currents.

Advantages of fuses

• In asymmetrical 3-phase circuits in emergency cases on the 1st phase, electricity disappears only in this phase, other phases will continue to power consumers. At high currents, this situation should not be allowed, as this will lead to phase imbalance.
• Due to their low speed of action, fuses can be used for selectivity.
• The selectivity of the inserts themselves in a series circuit is much simpler to calculate compared to automatic fuses, since the rated currents of fuses connected in series must differ from each other by a factor of 1.6.
• The design of the fuse is much simpler than that of an electric circuit breaker, so damage to the mechanism is excluded. This provides a complete guarantee that the circuit will be disconnected during an accident.
• After replacing the fuse with a fusible element, protection is restored in the circuit with properties that satisfy the device manufacturer, in contrast to the use of a machine whose contacts may burn out, thereby changing the protection characteristics.

Fuses are switching electrical products used to protect the electrical network from overcurrents and short circuit currents. The principle of operation of fuses is based on the destruction of specially designed current-carrying parts (fuse links) inside the device itself when a current flows through them, the value of which exceeds a certain value.




Fuse links are the main element of any fuse. After burning out (cutting off the current), they must be replaced. Inside the fuse link there is a fusible element (it is this that burns out), as well as an arc extinguishing device. The fuse link is most often made of a porcelain or fiber body and is attached to special conductive parts of the fuse. If the fuse is designed for low currents, then the fuse for it may not have a housing, i.e., be frameless.




The main characteristics of fuse ratings include: rated current, rated voltage, breaking capacity.




Fuse elements also include:




The fuse holder is a removable element, the main purpose of which is to hold the fuse;




Fuse contacts are the part of the fuse that provides electrical communication between the conductors and the fuse contacts;




The fuse striker is a special element whose task, when the fuse trips, is to influence other devices and contacts of the fuse itself.




All fuses are divided into several dozen types:




According to the design of fuse links, fuses are either collapsible or non-removable. With collapsible fuses, you can replace the fuse link after it burns out; with non-removable fuses, this cannot be done;




Presence of filler. There are fuses with and without filler;




Designs for manufacturing fuse links. There are fuses with blade, bolt and flange contacts;




Fuses for the fuse-link body are divided into tubular and prismatic. In the first type of fuses, the fuse link has a cylindrical shape, in the second type it has the shape of a rectangular parallelepiped;




Type of fuse links depending on the range of tripping currents. There are fuses with a breaking capacity in the full range of shutdown currents - g and with a breaking capacity in part of the range of shutdown currents - a;




Speed. There are slow-acting fuses (used in most cases in transformers, cables, electrical machines) and high-speed fuses (used in semiconductor devices);




Fuse base designs can be with a calibrated base (in such fuses it will not be possible to install a fuse link designed to work with a rated current greater than the fuse itself) and with an uncalibrated base (in such fuses it is possible to install a fuse link whose rated current is greater than the rated current the fuse itself);




Voltage fuses are divided into low-voltage and high-voltage;




Number of poles. There are one-, two-, three-pole fuses;




The presence and absence of free contacts. There are fuses with and without free contacts;




Depending on the presence of a striker and an indicator, there are fuses - without a striker and without an indicator, with an indicator without a striker, with a striker without an indicator, with an indicator and a striker;




By the method of fastening the conductors, fuses are divided into fuses with front connection, rear connection, universal (both rear and front);




Installation method. There are fuses on their own base and without it.




Historically, the mechanical design of fuse boxes and their overall and connection dimensions have varied from country to country. There are four main national standards for fuse mounting sizes: North American, German, British and French. There are also a number of fuse housings that are the same from country to country and are not national standards. Most often, such cases refer to the standards of the manufacturer that developed a specific type of device, which turned out to be successful and gained a foothold in the market. In recent decades, as part of the globalization of the economy, manufacturers have gradually joined the international system of fuse housing standards to simplify the conditions for the interchangeability of devices. When choosing, you should try to use fuses of international standards: IEC 60127, IEC 60269, IEC 60282, IEC 60470, IEC60549, IEC 60644.




It should be noted that according to the type of fuse-links, depending on the range of shutdown currents and operating speed, fuses are divided into usage classes. In this case, the first letter indicates the functional class, and the second indicates the object to be protected:




1st letter:




a - protection with breaking capacity in part of the range (accompanied fuses): fuse links capable of at least long-term passing currents not exceeding the rated current specified for them, and disconnecting currents of a certain multiple relative to the rated current up to the rated breaking capacity;




g - protection with breaking capacity over the entire range (general purpose fuses): fuse links capable of at least continuously passing currents not exceeding the rated current specified for them, and disconnecting currents from the minimum melting current to the rated breaking capacity.




2nd letter:




G - protection of cables and wires;




M - protection of switching devices/motors;




R - protection of semiconductors/thyristors;




L - protection of cables and wires (in accordance with the old, no longer valid DIN VDE standard);




Tr - transformer protection.




A general view of the time-current characteristics of fuses of the main categories of use is shown in Figure 2.1.




Fuse links with the following usage classes provide:




gG (DIN VDE/IEC) - protection of cables and wires over the entire range;




aM (DIN VDE/IEC) - protection of switching devices in part of the range;




aR (DIN VDE/IEC) - protection of semiconductors in part of the range;




gR (DIN VDE/IEC) - protection of semiconductors over the entire range;




gS (DIN VDE/IEC) - protection of semiconductors, as well as cables and lines over the entire range.




Fuses with breaking capacity over the entire range (gG, gR, gS) reliably switch off both short-circuit currents and overloads.





Rice. 2.1.




Fuses with partial breaking capacity (aM, aR) serve exclusively for short-circuit protection.




To protect installations for voltages up to 1000 V, electric, tubular and open (plate) fuses are used.




The electrical fuse consists of a porcelain body and a plug with a fuse link. The supply line is connected to the fuse contact, the outgoing line to the screw thread. In the event of a short circuit or overload, the fuse link burns out and the current in the circuit stops. The following types of electrical fuses are used: Ts-14 for current up to 10 A and voltage 250 V with a rectangular base; Ts-27 for current up to 20 A and voltage 500 V with a rectangular or square base and Ts-33 for current up to 60 A and voltage 500 V with a rectangular or square base.




For example, electrical fuses threaded, PRS series, designed to protect against overloads and short circuits of electrical equipment and networks. Rated voltage before




keepers - 380 V alternating current frequency 50 or 60 Hz. Structurally, PRS fuses (Fig. 2.2) consist of a body, a fuse-link PVD, a head, a base, a cover, and a central contact.




PRS fuses are produced for rated fuse-link currents from 6 to 100 A. The designation of the fuse indicates what connection it is: PRS-6-P - 6 A fuse, front wire connection; PRS-6-Z - 6A fuse, rear wire connection.




Cylindrical fuses PTSU-6 and PTSU-20 with a threaded base Ts-27 and fuse-links for currents of 1, 2, 4, 6, 10, 15, 20 amperes are produced in a plastic case. PD fuses have a porcelain base, while PDS fuses have a base material of steatite. In domestic conditions, automatic plug fuses are used, where the protected circuit is restored by a button.




Tubular fuses are produced in the following types: PR-2, NPN and PN-2. The PR-2 fuse (dismountable fuse) is intended for installation in networks with voltages up to 500 V and for currents of 15, 60, 100, 200, 400, 600 and 1000 A.




In the fuse holder PR-2 (Fig. 2.3), the fuse link 5, attached with screws 6 to the contact blades 1, is placed in a fiber tube 4, onto which threaded bushings 3 are mounted. Brass caps 2 are screwed onto them, securing the contact knives, which fit into fixed spring contacts installed on the insulating plate.










Rice. 2.2.










Rice. 2.3.




Under the influence of an electric arc that occurs when a fuse blows, the inner surface of the fiber tube decomposes and gases are formed that help quickly extinguish the arc.




Closed fuses with fine-grained filler include fuses of the NPN, NPR, PN2, PN-R, and KP types. Fuses of the NPN type (filled, non-removable fuse) have a glass tube. The rest have porcelain pipes. NPN type fuses are cylindrical in shape, PN type are rectangular.




The NPN fuse set consists of: fuse link - 1 piece; contact bases - 2 pcs.




NPN fuses are manufactured for voltages up to 500 V and currents from 15 to 60 A, fuses PN2 (bulk fuse, collapsible) - for voltages up to 500 V and currents from 10 to 600 A. Bulk fuses have fuse links made of several parallel copper or silver-plated wires are placed in a closed porcelain cartridge filled with quartz sand. Quartz sand promotes intensive cooling and deionization of gases produced during arc combustion. Since the tubes are closed, splashes of molten metal from the fuse links and ionized gases are not emitted outside. This reduces fire hazards and increases the safety of fuse servicing. Fuses with filler, like PR type fuses, are current-limiting.




Open plate fuses consist of copper or brass plates - tips into which calibrated copper wires are soldered. The tips are connected to the contacts on the insulators using bolts.




NPR type fuses are a closed, collapsible (porcelain) cartridge filled with quartz sand for rated currents up to 400 A.




PD fuses (PDS) - 1, 2, 3, 4, 5 - with filler for installation directly on busbars for currents from 10 to 600 A.




To protect power valves of semiconductor converters of medium and high power during external and internal short circuits, high-speed fuses are widely used, which are the cheapest means of protection. They consist of contact blades and a silver foil fusible link placed in a closed porcelain socket.




The fuse link of such fuses has narrow calibrated isthmuses, which are equipped with radiators made of a ceramic material that conducts heat well, through which heat is transferred to the fuse body. These radiators also serve as arc-extinguishing chambers with a narrow slot, which significantly improves the extinction of the arc that occurs in the isthmus region. A signal cartridge is installed parallel to the fuse-link, the blinker of which signals the melting of the fuse-link and, acting on the microswitch, closes the signal contacts.




For a long time, the industry produced two types of high-speed fuses designed to protect converters with power semiconductor valves from short-circuit currents:




1) fuses of the PNB-5 type (Fig. 2.4, a) for operation in circuits with a rated voltage of up to 660 V DC and AC for rated currents of 40, 63, 100, 160, 250, 315, 400, 500 and 630 A;




2) PBV type fuses for operation in alternating current circuits with a frequency of 50 Hz with a rated voltage of 380 V for rated currents from 63 to 630 A.










Rice. 2.4.




Currently, the industry produces fuses of the PNB-7 type (Fig. 2.4, b) for a rated current of 1000 A and for a rated electrical circuit voltage of 690 V AC. The fusible elements of the PNB-7 fuse are made of pure silver (speed and durability). The contacts (terminals) of the fuse are made of electrotechnical copper with galvanic coating (high conductivity and durability).




The fuse housing is made of high-strength ultra-porcelain. The design of the fuse allows the use additional devices- operation indicator, free contact.




Structure symbol fuses PNB7-400/100-X1-X2:




PNB-7 - series designation;




400 - rated voltage, V;




100 - rated current;




X1 - symbol of the type of installation and type of connection of conductors to the terminals: 2 - on its own insulating base with base contacts; 5 - on the bases of complete devices with base contacts; 8 - without base, without contacts (fuse link);




X2 - symbol for the presence of an operation indicator: 0 - without alarm; 1 - with striker and free contact; 2 - with operation indicator; 3 - with striker.




Industrial fuses of the PP series are designed to protect electrical equipment of industrial installations and electrical circuits from overloads and short circuits.




Fuses of this series are produced in the following main types: PP17, PP32, PP57, PP60S. Fuses are manufactured with a trip indicator, with a trip indicator and free contact, or without signaling. Depending on the type, fuses are designed for voltages up to 690 V and rated currents from 20 A to 1000 A. Design features allow the installation of free contacts, normally open or closed, as well as the installation method - on their own base, on the base of complete devices, on conductors of complete devices .




Designation structure for fuses of types PP17 and PP32 - Х1Х2 - Х3 - Х4 - ХХХХ:




1) X1X2 - size designation (rated current, A): 31 -100A; 35 - 250A; 37 - 400A; 39 - 630A.




2) X3 - symbol of the type of installation and type of connection: 2 - on its own base, 5 - on the base of complete devices, 7 - on conductors of complete devices (bolt connection), 8 - without a base (fuse link), 9 - without a base ( The fuse link is unified in size with fuses PN2-100 and PN2-250).




3) X4 - symbol for the presence of an operation indicator, striker, free contact: 0 - without signaling, 1 - with striker and free contact, 2 - with operation indicator, 3 - with striker.




4) ХХХХ - climatic version: UHL, T and placement category 2, 3.




Currently, semiconductor converters are equipped with fuses of the PP57 (Fig. 2.5, a) and PP60S (Fig. 2.5, b) series.









Rice. 2.5.




The first ones are designed to protect converter units during internal short circuits of AC and direct current at voltages of 220 - 2000 V for currents of 100, 250, 400, 630 and 800 A. The second - for internal short circuits of alternating current at voltages of 690 V for currents of 400, 630, 800 and 1000 A.




Designation structure for fuses type PP57 - ABCD - EF:




Letters PP - fuse;




The two-digit number 57 is the conditional series number;




A - two-digit number - symbol of the rated current of the fuse;




B - number - symbol of the rated voltage of the fuse;




C - number - symbol according to the installation method and type of connection of conductors to the fuse terminals (for example, 7 - on the conductors of the converter device - bolted with angled terminals);




D - number - symbol for the presence of an operation indicator and an auxiliary circuit contact:




0 - without operation indicator, without auxiliary contact





1 - with operation indicator, with auxiliary contact





2 - with operation indicator, without auxiliary circuit contact;




E - letter - symbol of climatic version;






An example of a fuse symbol: PP57-37971-UZ.




PPN fuses are intended to protect cable lines and industrial electrical installations from overload and short circuit currents. Fuses are used in electrical networks alternating current with a frequency of 50 Hz with a voltage of up to 660 V and are installed in low-voltage complete devices, for example, in distribution panels ShchO-70, input distribution devices VRU1, power distribution cabinets ShRS1, etc.




Advantages of PPN fuses:




1) the fuse body and the base of the holder are made of ceramics;




2) the fuse and holder contacts are made of electrical copper;




3) the fuse housing is filled with fine quartz sand;




4) overall dimensions of fuses are ~15% smaller than PN-2 fuses;




5) power losses are ~40% less than those of PN-2 fuses;




6) presence of an operation indicator;




7) fuses are mounted and removed using a universal puller.




The design features of the PPN series fuses are shown in Fig. 2.6.




Fuses of the PPNI series (Fig. 2.7) for general use are designed to protect industrial electrical installations and cable lines from overload and short circuit and are available for rated currents from 2 to 630 A.




Used in single-phase and three-phase networks with voltages up to 660 V, frequency 50 Hz. Areas of application of PPNI fuses: input distribution devices (IDU); cabinets and distribution points (ShRS, ShR, PR); equipment of transformer substations (KSO, ShchO); low voltage cabinets (ShR-NN); control cabinets and boxes.











Rice. 2.6.




Due to the use of high-quality modern materials and a new design, PPNI fuses have reduced power losses compared to PN-2 fuses. The data presented in Table 2.1 shows the efficiency of PPNI fuses compared to PN-2.











Rice. 2.7.











The fuse and holder contacts are made of electrical copper with galvanic coating with a tin-bismuth alloy, which prevents their oxidation during operation.










The base of the holder (insulator) is made of reinforced thermosetting plastic, resistant to corrosion, mechanical stress, temperature changes and dynamic shocks that occur during short circuits up to 120 kA.










The fuse-link contacts are knife-shaped (sharpened), which allows them to be installed in holders with less effort.










All dimensions of PPNI fuse-links can be conveniently installed or dismantled using the universal removal handle RS-1, the insulation of which can withstand voltages up to 1000 V.










For fast and effective arc extinguishing, the fuse body is filled with highly chemically purified quartz sand.










The fusible element is made of phosphor bronze (an alloy of copper and zinc with the addition of phosphorus) and is securely connected by spot welding to the fuse terminals.










The design of the fuse link has a special indicator, made in the form of a retractable rod, which allows you to visually determine tripped fuses.










PPNI fuses with a breaking capacity over the entire “gG” range operate reliably both under short-circuit currents and overloads.










The design, technical parameters, overall and installation dimensions of fuse-links and PPNI holders comply with modern IEC and GOST standards, and, therefore, these fuses can replace other domestic and imported fuses.

Selection of fuse links




Fuses are installed on all branches if the cross-section of the wire on the branch is smaller than the cross-section of the wire in the main line, at the inputs and in the head sections of the network in input distribution devices, power distribution cabinets and power boxes complete with switches or on separate panels. For selectivity of action, it is necessary that each subsequent fuse in the direction of the current source has




the rated current of the fuse link is at least one step higher than the previous one.




To calculate the protection of networks and equipment using fuses, the following data is required:




Rated fuse voltage;




Maximum short circuit current switched off by fuse;




Rated fuse current;




Rated current of fuse link;




Protective characteristic of the fuse.




The rated voltage of the fuse (Unom, pr) is called




the voltage indicated on it for continuous operation at which it is intended. The actual mains voltage (Uc) should not exceed the rated fuse voltage by more than 10%:




Uс ≤ 1.1 Unom,pr (2.1)




The rated current of a fuse (Inom, pr) is the current indicated on it, equal to the largest of the rated currents of the fuse links (Imax nom, PV) intended for this fuse. This is the maximum long-term current passed by the fuse under the condition of heating its parts, except for inserts.




Inom,pr = Imax nom,PV (2.2)




The maximum switchable current (breaking capacity) of a fuse (Imax,pr) is the largest value (effective) of the periodic component of the current that is switched off by the fuse without destruction and dangerous emission of flame or combustion products of an electric arc. This fuse size for each type may vary depending on the voltage, rated current of the fuse, the value of cosph in the disconnected circuit and other conditions.




The rated current of a fuse link (Inom, PV) is the current indicated on it for continuous operation at which it is intended. In practice, this is the maximum long-term current passed by the insert (Imax, PB), according to the condition of the permissible heating of the insert itself.




Inom,PV = Imax,PV (2.3)




Usually, in addition to the rated current of the insert, two more values ​​of the so-called test currents are indicated, by which the inserts are calibrated. The lower value of the test current the fuse link must withstand certain time, usually 1 hour, without melting; at the upper value of the test current, the insert should burn out in no more than a certain time, usually also 1 hour.




The main data for determining the burnout time of the insert, and, consequently, the selectivity of fuses connected in series, are their protective characteristics.




The protective characteristic of a fuse is the dependence of the total shutdown time (the sum of the melting time of the insert and the arc burning time) on the value of the switched off current.




Protective characteristics are usually given in the form of a graph, in rectangular coordinates. Time is plotted along the vertical coordinate axis, and the multiplicity of the current switched off by the fuse to the rated current of the insert, or the switched current, is plotted along the horizontal axis.




The selectivity of fuse protection is ensured by selecting fuse links in such a way that if a short circuit occurs, for example, on a branch to an electrical receiver, the nearest fuse protecting this electrical receiver will trip, but the fuse protecting the head section of the network will not trip.




The selection of fuse links according to the selectivity condition should be made using the standard protective characteristics of the fuses, taking into account the possible spread of actual characteristics according to the manufacturer.




A typical time-current characteristic of a modern double-action fuse is shown in Figure 2.8.




With a rated current of 200 A, the fuse should operate indefinitely. The characteristic shows that as the current decreases, the response time in the region of low currents increases rapidly and the dependence curve should ideally asymptotically tend to the straight line I = 200 A, for time t = + ∞. In the area of ​​operating overloads, that is, in the case when the current through the fuse is within the range of (1-5)⋅In, the response time of the fuse is quite long - it exceeds a few seconds (at a current of 1000A, the response time is 10 s).




This type of dependence allows the protected equipment to operate freely over the entire range of operating overload characteristics. With a further increase in current, the slope of the time-current characteristic (Fig. 2.8) quickly increases, and already with an eleven-fold overload, the response time is only 10 ms. A further increase in the overload current reduces the response time to an even greater extent, although not as quickly as in the area between five and ten times the overload. This is explained by the finite rate of arc extinction due to the finite heat capacity of the filler material, the finite heat of fusion of the fusible bridge material, and the certain mass of the melting and evaporating bridge metal. With a further increase in current (more than 15-20 times the rated value), the response time of the fuse element can be 0.02-0.5 ms, depending on the type and design of the fuse.









Rice. 2.8.




With a rated current of 200 A, the fuse should operate indefinitely. The characteristic shows that as the current decreases, the response time in the region of low currents quickly increases, and the dependence curve should ideally asymptotically tend to the straight line I = 200 A, for time t = + ∞. In the area of ​​operational overloads, i.e. in the case when the current through the fuse is within the range of (1-5)⋅In, the response time of the fuse is quite long - it exceeds a few seconds (at a current of 1000 A, the response time is 10 s).




This type of dependence allows the protected equipment to operate freely over the entire range of operating overload characteristics. With a further increase in current, the slope of the time-current characteristic (Fig. 2.8) quickly increases, and already with an eleven-fold overload, the response time is only 10 ms. A further increase in the overload current reduces the response time to an even greater extent, although not as quickly as in the area between five and ten times the overload. This is explained by the finite rate of arc extinction due to the finite heat capacity of the filler material, the finite heat of fusion of the fusible bridge material, and the certain mass of the melting and evaporating bridge metal. With a further increase in current (more than 15-20 times the rated value), the response time of the fuse element can be 0.02-0.5 ms, depending on the type and design of the fuse.




Siemens produces a wide range of fuses (combinations gG, gM, aM, gR, aR, gTr, gF, gFF), six standard sizes - 000(00С), 00, 1, 2, 3, 4а (designations according to IEC) for rated currents from 2 to 1600 A and voltages (~ 400V, 500V and 690V; - 250V, 440V) with the most commonly used knife type (NH) contacts in practice, predominantly in a vertical installation position.




NH type fuses have high breaking capacity and stable characteristics. The use of NH type fuses allows for selectivity of protection during short circuit.




Knife-type fuses NH (analogue of PPN) are intended for installation in contact holders PBS, PBD, in PVR series APC and RBK, as well as in load switches of type RAB. It is possible to use these fuses in protective devices designed for the use of domestic PPN-type inserts.




NH type fuses are arc extinguishing fuses in a closed volume. The fusible link is stamped from zinc, which is a low-melting and corrosion-resistant metal. The shape of the fuse-link makes it possible to obtain a favorable time-current (protective) characteristic. The insert is located in a sealed insulating ceramic housing. Filler - quartz sand with a SiO content of at least 98%, with grains (0.2-0.4)⋅10 -3 m and humidity not higher than 3%.




When disconnected, the narrowed isthmuses of the fuse-link burn out, after which the resulting arc is extinguished due to the current-limiting effect that occurs when the narrowed sections of the fuse-link burn out. The average arc extinction time is 0.004 s.




The time-current characteristics of NH type fuses for use class gG are shown in Figure 2.9.





2 10 100 1 000 10 000 100 000




Expected short-circuit current IP, A




Rice. 2.9.




NH type fuses operate silently, with virtually no emission of flame or gases, which allows them to be installed at close distances from each other.




One more important characteristic fuse as a protective device is the so-called protective indicator, called I 2 ⋅t in foreign sources. For a protected electrical circuit, the protective indicator is the amount of heat generated in the circuit from the moment of occurrence emergency situation until the circuit is completely switched off by the protective device. The value of the protective indicator specific device, in fact, determines the limit of its resistance to thermal destruction in emergency modes. When calculating the value of the protective index, the effective value of the current in the circuit is used.




For example, the effective value of the current flowing through the fuse can be calculated for commonly used AC rectifier circuits from the (smoothed) direct current Id or from the phase current IL, the values ​​of which are given in Table 2.2.




During a short circuit, the fuse current (Fig. 2.10) increases during the melting time tS to the short circuit current IC (melting current peak).


Table 2.2 Effective value of current flowing through the fuse

AC Rectifier Circuit	Effective value of phase current (phase fuse)	Branch current rms value (fuse in branch)
Single-pulse with midpoint
Two-pulse with midpoint
Three-pulse with midpoint
Six-pulse with midpoint
Double three-phase half wave
with midpoint (parallel)
Two-pulse bridge circuit
Six-pulse bridge circuit
Single-phase bidirectional circuit

During the arc extinguishing time tL, an electric arc is formed and the short circuit current is extinguished (Fig. 2.10).




The integral of the quadratic value of the current (∫l 2 dt) over the entire operating time (tS + tL), briefly called the total Joule integral, determines the heat that is supplied to the semiconductor element to be protected during the opening process.




To achieve a sufficient protective effect, the total Joule integral of the fuse insert must be less than the value of I 2 ⋅t (ultimate load integral) of the semiconductor element. Since the total Joule integral of the safety insert with increasing temperature, and, consequently, with increasing preload, practically decreases in the same way as the value of I 2 ⋅t of the semiconductor element, it is enough to compare the values ​​of I 2 ⋅t in an unloaded (cold) ) condition.









Rice. 2.10.




The total Joule integral (I 2 ⋅tA) is the sum of the melting integral (I 2 ⋅tS) and the arc integral (I 2 ⋅tL). In general, the value of the total Joule integral semiconductor device must be greater than or equal to the value of the fuse protection indicator:




((∫I 2 t) (semiconductor, t = 25 °C, tP = 10 ms) ≥ ((∫I 2 ⋅tA) (fuse link).




The melting integral I 2 ⋅tS can be calculated for any time values, based on pairs of values ​​of the time-current characteristic of the fuse insert.




As the melting time decreases, the melting integral tends to a lower limit value, at which during the melting process practically no heat is removed from the bridges of the melting conductor into the surrounding space. The melting integrals specified in the selection and ordering data and in the characteristics correspond to a melting time tS = 1 ms.




While the melting integral I 2 ⋅tS is a property of the fuse link, the arc integral I 2 ⋅tL depends on the characteristics of the electrical circuit, namely:




From recovery voltage UW;




From the power factor cosф of the short-circuited circuit;




From the expected current IP// (current at the installation location of the fuse link if it is short-circuited).




The maximum arc integral is achieved for each type of fuse at a current from 10⋅IP to 30⋅IP.




When protecting networks with fuses of types PN, NPN and NPR with given protective characteristics, the selectivity of the protection action will be carried out if between the rated current of the fuse-link protecting the head section of the network (Inom G, PV) and the rated current of the fuse-link at the branch to the consumer (Inom O , PV) certain ratios are maintained.




For example, at small fuse-link overload currents (about 180-250%), selectivity will be maintained if Inom G, PV > Inom O, PV by at least one step of the standard scale of rated currents of fuse-links.




In the event of a short circuit, selectivity of protection with NPN type fuses will be ensured if the following ratios are maintained:




I(3)SC / Inom O, PV ≤ …50; 100; 200;




Inom G, PV / Inom O, PV…2.0; 2.5; 3.3,




where I(3)SC is the three-phase short circuit current of the branch, A.




The relationships between the rated currents of fuse links Inom G, PV and Inom O, PV for fuses of the PN2 type, ensuring reliable selectivity, are given in Table 2.3.




If the protective characteristics of fuse links are unknown, a method of checking the selectivity in relation to the cross-sections of the inserts, adjusted for the material of the insert and the design of the fuse, is recommended. In this case, the cross-sections of the fuse links of the fuses connected in series (SK and SH) are determined; the ratio SP/SK is calculated and compared with the value SP/SK = a, which ensures selectivity.





SK - cross-section of the fuse insert installed closer to the short circuit; SP - cross-section of the fuse insert installed closer to the power source.




The value of a is determined from Table 2.4; if the calculated value Sn/SK ≥ a, then selectivity is ensured.




The main condition determining the choice of fuses for protection asynchronous motors with a squirrel-cage rotor, is the detuning from the starting current.


Table 2.3 Rated currents of series-connected fuse links PN2, providing reliable selectivity

Rated current of the smaller fuse-link Inom O, PV A	Rated current of the larger fuse-link Inom G, PV, A, with the ratio I(3)SC / Inom O, PV
				100 or more




Note. 1(3) Short circuit - short circuit current at the beginning of the protected section of the network.

The detuning of fuse-links from starting currents is carried out according to time: the start of the electric motor must be completely completed before the insert melts under the influence of the starting current.




Operating experience has established a rule: for reliable operation of inserts, the starting current should not exceed half the current, which can melt the insert during the start-up.




All electric motors are divided into two groups according to start time and frequency. Motors with easy starting are considered to be motors of fans, pumps, metal-cutting machines, etc., the start of which ends in 3-5 s; these motors are started rarely, less than 15 times in 1 hour.




Engines with heavy starting include engines of cranes, centrifuges, ball mills, the start of which lasts more than 10 s, as well as engines that are started very often - more than 15 times in 1 hour. This category also includes engines with easier starting conditions, but especially responsible ones, for whom false burnout of the insert during startup is completely unacceptable.


Table 2.4 Sn/SK insert cross-section ratio ensuring selectivity

Metal fuse link	Metal fuse link,
fuse located	located closer to the short circuit.
closer to the power source
Fuse with filler
Fuse without filler

The selection of the rated current of the fuse link for detuning from the starting current is made according to the expression:




Inom,PV ≥ I start,DV / K, (2.4)




where Ipus, DV is the starting current of the motor, determined from the passport, catalogs or direct measurement; K is a coefficient determined by the starting conditions and is equal to 2.5 for engines with easy starting, and 1.6-2 for engines with heavy starting.




Since the insert heats up and oxidizes when starting the engine, the cross-section of the insert decreases, the condition of the contacts deteriorates, and it can falsely burn out during normal engine operation. An insert selected in accordance with (2.4) can also burn out when




Starting or self-starting of the engine is delayed compared to the estimated time.




Therefore, in all cases, it is advisable to measure the voltage at the motor inputs at the time of start-up and determine the start-up time.




To prevent the inserts from burning out during startup, which may result in the engine operating in two phases and causing damage, it is advisable in all cases where this is permissible due to sensitivity to short-circuit currents, to select inserts that are coarser than according to condition (2.1).




Each engine must be protected by its own separate protection device. A common device is allowed to protect several low-power motors only if the thermal stability of the starting devices and overload protection devices installed in the circuit of each motor is ensured.

Selection of fuses to protect lines supplying several asynchronous electric motors




Protection of lines supplying several motors must ensure both starting the motor with the highest starting current and self-starting of the motors, if it is permissible under safety conditions, technological process and so on.




When calculating protection, it is necessary to accurately determine which motors are switched off when the voltage drops or completely disappears, which remain switched on, and which are switched on again when voltage appears.




To reduce disruptions to the technological process, special circuits are used to turn on the holding electromagnet of the starter, which ensures immediate inclusion of the motor in the network when voltage is restored. Therefore, in the general case, the rated current of the fuse link, through which several self-starting motors are powered, is selected according to the expression:




Inom, PV ≥ ∑Ipus, DV / K, (2.5)




where ∑Ipus, DV is the sum of the starting currents of self-starting electric motors.

Selecting fuses to protect lines in the absence of self-starting electric motors




In this case, fuse links are selected according to the following ratio:




Inom, PV ≥ Imax, TL / K, (2.6)




where Imax, TL = Ipus, DV + Idolt, TL - maximum short-term line current; Ipus, DV - the starting current of an electric motor or a group of simultaneously switched on electric motors, when starting which the short-term line current reaches highest value; Idlit, TL - long-term calculated line current until the electric motor (or group of electric motors) is started - this is the total current consumed by all elements connected through a fuse, determined without taking into account the operating current of the started electric motor (or group of motors).

Selection of fuses to protect asynchronous electric motors from overload


Since the starting current is 5-7 times the rated current of the motor, the fuse-link selected according to expression (2.4) will have a rated current 2-3 times the rated current of the motor and, while withstanding this current for an unlimited time, cannot protect the motor from overload . To protect motors from overload, thermal relays are usually used, built into magnetic starters or circuit breakers.




If a magnetic starter is used to protect the motor from overload and control it, then when choosing fuse-links it is also necessary to take into account the condition of preventing damage to the contactors of the starter.




The fact is that during short circuits in the engine, the voltage on the holding electromagnet of the starter decreases, it falls off and breaks the short circuit current with its contacts, which, as a rule, are destroyed. To prevent this short circuit, the motors must be switched off by a fuse before the starter contacts open.




This condition is ensured if the shutdown time of the short circuit current by the fuse does not exceed 0.15-0.2 s; for this, the short circuit current must be 10-15 times greater than the rated current of the fuse insert protecting the electric motor, i.e.:




I(3) Short circuit / Inom, PV ≥ 10–15. (2.7)

Protection by fuses of networks up to 1000 V from overload




PUE 3.1.10 specifies networks with voltages up to 1000 V, which require, in addition to short circuit protection, overload protection. These include:




1. All networks laid openly with unprotected insulated wires with a flammable sheath, inside any premises.




2. Everything lighting networks regardless of the design and method of laying wires or cables in residential and public buildings, in retail premises, in service and amenity premises of industrial enterprises, in fire hazardous industrial premises, all networks for powering household and portable electrical appliances.




3. All power networks in industrial enterprises, residential and public premises, if, due to the conditions of the technological process, long-term overload of wires and cables may occur.




4. All networks of all types in explosive premises and explosive outdoor (outside buildings) installations, regardless of the operating mode and purpose of the network.




The rated current of the fuse-link must be selected as low as possible, subject to the condition of reliable transmission of the maximum load current. Almost at a constant, without shocks, load, the rated current of the insert 1nom, PV is taken approximately equal to the maximum continuous load current Imax, TN, namely:




Inom, duty cycle ≥ Imax, TN. (2.8)




Based on the rated current of the insert, the permissible continuous load current 1dlit,TN for the conductor (laid under normal conditions) protected by the selected insert is determined:




kк⋅Inom, PV ≤ kп⋅Idlit, TN, (2.9)




where kk is a coefficient that takes into account the design of the conductors protected by the insert, equal to 1.25 according to PUE 3.1.10 for conductors with rubber and similar flammable insulation, laid in all rooms except non-explosive industrial ones. For any conductors laid in non-explosive industrial premises, and paper-insulated cables in any premises, kк = 1:




kп = kп1⋅kп2⋅kп3, (2-10)




where kп is a general correction factor corresponding to the case when the actual laying conditions differ from normal ones.




If the load is of the nature of shocks, for example, a crane electric motor, and the duration of the load is less than 10 minutes, then a correction factor kп1 is introduced. This coefficient is introduced for copper conductors with a cross-section of at least 6 mm2 and aluminum conductors with a cross-section of at least 10 mm2. The value kп1 is taken according to the expression




kп1 = 0.875/ √PV,




where PV is the on-time duration expressed in relative units, equal to the ratio of the on-time of the receiver, for example an electric motor, to the total cycle time of the intermittent mode. The kP1 coefficient is introduced if the duration of the switching on is no more than 4 minutes, and the break between switching on is at least 6 minutes. Otherwise, the load current value is taken as for the continuous mode.




If the ambient temperature differs from normal, a correction factor kP2 is introduced, determined from the PUE tables.




When laying more than one cable in one trench, a correction factor kP3 is introduced, which is also determined from the PUE tables.




In secondary switching circuits (operating current, instrumentation, voltage measuring transformers, etc.), fuse-links are selected according to short-circuit currents based on the condition:




I(3)SC / Inom,PV ≥ 10 (2.11)




Fuses are installed on distribution boards and power points. The fuse link is installed vertically. After tightening all the fasteners, check the contact between the contacts of the knife or cartridge cap and the jaws of the racks. The “springing” of the contact jaws of the racks when a knife or cartridge cap enters them should be noticeable to the eye. Fuse holders must not fall out of the contact posts when a force is applied to them, equal to for fuses rated for current: 40A - force 30N; 100A - 40N; 250A - 45N; 400A - 50N; 600A - 60N.




When switching on again, the fuses are checked to the following extent:




1. Visual inspection, cleaning, checking contact connections.




2. Checking the correct choice of the rated current of the fuse link.




In production conditions, reasons arise when it is necessary, in the absence of a standard fuse link, to replace it with a conductor whose properties will be equivalent to the fuse link.




Table 2.5 shows the cross-sectional area of ​​various conductor materials suitable for use as a fuse link.

Selecting fuses to protect semiconductor elements




Fuses for protecting the semiconductor elements of the insert are selected according to the rated voltage, rated current, total Joule integral I2⋅tA and load cycle factor, taking into account other specified conditions.




The design voltage Uр of a fuse link is the voltage given as the effective value of the alternating voltage when generating ordering and design data, as well as indicated on the fuse link itself.




The design voltage of the fuse link is selected in such a way that it reliably switches off the voltage that initiates the short circuit. This voltage should not exceed the value of Uр +10%. In this case, it is also necessary to take into account the fact that the supply voltage Upc of the AC rectifier can increase by 10%. If in a short-circuited circuit two branches of the AC rectifier circuit are located in series, then if the short-circuit current is sufficiently large, one can count on uniform voltage distribution.


Table 2.5 The value of the wire cross-section for the fuse link depending on the load current

Current value, A	Lead, mm2	Alloy, mm2: 75% - lead, 25% - tin		Iron, mm2

Straightening mode. For AC rectifiers that operate only in rectification mode, the supply voltage Uпc acts as the exciting voltage.




Invert mode. For AC rectifiers that also operate in inverting mode, the failure may be caused by the inverter stalling. In this case, the sum from the supply voltage acts as the exciting voltage Uin in the short-circuited circuit DC voltage(for example, the electromotive force of a DC machine) and the voltage of the three-phase current of the supply network. When selecting a fuse insert, this amount can be replaced by alternating voltage, the effective value of which corresponds to 1.8 times the value of the three-phase voltage of the supply network (Uin = 1.8 Upc). Fuse links must be designed in such a way that they reliably interrupt the voltage Uin.




The rated current, load capacity Ip of the fuse link is the current given in the selection and ordering data and characteristics, and also indicated on the fuse link as the effective value of the alternating current for the frequency range 45-62 Hz.




For operation of a fuse link with rated current, the normal operating conditions are:




Natural air cooling at ambient temperature +45°C;




The cross-sections of the connections are equal to the control cross-sections when operating in NH fuse bases and disconnectors;




The half-cycle current cut-off angle is 120°;




Constant load is maximum at rated current.




For operating conditions different from those listed above, the permissible operating current Ip of the fuse link is determined by the following formula:




Ip = ku ⋅ kq ⋅ kl ⋅ ki ⋅ kwl ⋅ Ip, (2.12)




where Ip is the calculated current of the fuse link;




ku - correction factor for ambient temperature;




kq - correction factor of the connection cross-section;




kl - correction factor for current cut-off angle;




ki is the correction factor for intensive air cooling;




kwl - load cycle coefficient.




The load cycle factor kwl is a reduction factor that can be used to determine the time-invariant load capacity of fuse links under any load cycle. Safety inserts have different load cycle coefficients due to their design. The characteristics of the fuse links indicate the corresponding load cycle factor kwl for > 10,000 load changes (1 hour "On", 1 hour "Off") over the expected service life of the fuse links.




With a uniform load (there are no load cycles and shutdowns), you can take the load cycle factor kwl = 1. For load cycles and shutdowns that last more than 5 minutes and occur more than once a week, you should choose the load cycle factor kwl specified in characteristics of individual safety links from manufacturers.




Residual coefficient - krw.




Preloading the safety insert reduces the permissible overload and melting time. Using the residual coefficient krw, it is possible to determine the time during which the fuse link, with a periodic or non-periodic load cycle in excess of the pre-calculated permissible load current Ip, can operate with any overload current Ila without losing its original properties over time.




The residual coefficient kRW depends on the preload V= Ieff/Ip - (the ratio of the effective value of the current Ieff flowing through the fuse during the load cycle to the permissible load current Ip), as well as on the overload frequency F. Graphically, this dependence is represented by two curves (Fig. 2.11): kRW1 = f (V), with F = frequent shock currents / load cycle currents > 1/week; kRW2 = f (V), with F = rare surge currents / load cycle currents


After defining graphically coefficient kRW1 (kRW2), the reduced duration of permissible load tsc can be determined by the expression:




tsc = kRW1 (kRW2) ⋅ ts




The reduction in the melting time of the safety insert tsy during preload is determined from the calculated value of V using the given curve kR3 = f (V) (Fig. 2.11) according to the expression:




tsy = kR3 ⋅ ts






Rice. 2.11.

AC rectifiers often operate not with continuous loads, but with alternating loads, which may also briefly exceed the rated current of the AC rectifier.




For the case of variable load, four typical types of load are classified for the operating mode of fuse links that does not change over time:




Unknown variable load, but with a known maximum current (Fig. 2.13);




Variable load with a known load cycle (Fig. 2.14);




Random shock load from a preload with an unknown sequence of shock pulses (Fig. 2.15).




Determining the required rated current IP of the fuse link for each of the four types of load is carried out in two stages:




1. Determination of the design current IP based on the effective value Ieff of the load current:




IP > Ieff ⋅(1/ ku ⋅ kq ⋅ kl ⋅ ki ⋅ k). (2.13)




2. Checking the permissible duration of overload by current blocks that exceed the permissible operating current of the IP/ fuse, using the expression:




kRW ⋅ ts ≥ tk, (2.14)




where tK is the duration of the overload.




If the obtained overload duration is shorter than the corresponding required overload duration, then select a fuse link with a higher rated current Ip (taking into account the rated voltage Up and the permissible total Joule integral) and repeat the test.




Fuse selection example