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 considered fuses, possessing 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?
In the case of connecting two wires connected to a current source, a well-known effect will occur short circuit. 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.
Avoid negative consequences quite possible 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. Such homemade devices It is strictly forbidden to use it, 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 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 is 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. Please note 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 pole-to-pole 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 quantity 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 necessary to mandatory take into account the parameters that allow you 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 in 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 on fuse markings can be obtained from reference books intended for electrical engineers.
Any electrical circuit consists of individual elements. Each of them is characterized by certain current values at which the 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 various designs allow you to avoid damage to electrical circuit elements.
Their classification is based on the way these fuses break the electrical circuit, 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. A fuse link, which melts or even evaporates after increasing the current above the set value, is 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 of the fuse, the heat that is released 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. Other designs are adapted for specific tasks. 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 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 activation or breakdown of the electronic key control circuit due to abnormal physical influences (excess of 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 temperature environment;
- 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.
The fuse is a classic of electrical engineering in the field of protecting networks from overloads and short circuits. Although in our time it is successfully replaced by circuit breakers, there are a huge number of examples where the fuse link is an indispensable safety link in the electrical circuit: electronics, automotive electrical network, industrial electrical installations, power supply systems.
Plug type fuses
Plug fuses still operate in many residential distribution boards in the post-Soviet space. Due to their miniature size, reliability, low cost, ability to quickly replace, and constant characteristics during operation, fuses have not lost their relevance, and this article will be useful in selecting fuses that have the following basic parameters:
- Un – rated operating voltage;
- Ivs – rated current of the fuse-link, above which it burns out;
- Iп – rated current of the fuse.
Terminology
In electrical engineering, a fuse is an overcurrent protection device that has a disposable component called a fuse link, which opens the electrical circuit when the specified parameters are reached by melting the conductor.
In other words, the electric fuse is a reusable holder into which a disposable insert is inserted, which melts when Ivs is exceeded. In everyday life, these two terms are considered identical, but in technical descriptions Ip equals the maximum possible Ivs, since certain types of fuses require the use of plug-in elements with different Ivs.
For example, fuse NPN2-60 can be inserted into fuse links with Ivs from 6 to 60A, respectively, its Ip is equal to 60A.
fuses of the NPN series of different currents Operating principle
Structurally, the disposable element is made in the form of a small cross-section conductor enclosed in a protective glass, porcelain or plastic shell. At values close to Ibc, heat is released that is insufficient to heat the conductor to the melting temperature due to heat dissipation. When Ibc is exceeded, the conductive material melts and the electrical circuit breaks.
These components come in a wide variety, from thin wires used to protect electronic devices to massive plates designed to operate in circuits carrying currents in excess of thousands of amperes.
The fuse operates in several stages: heating, melting and evaporation of the metal, electric arc, arc extinguishing. The last stage means a complete shutdown, and in order for the arc to go out, the rated voltage of the fuse must not be less than the mains voltage.
terms of Use
The heating temperature of the fuse-link should not exceed permissible values during long-term operation of the fuse. Therefore, Ivs and Ip must be selected with a value equal to or one value greater than the rated load current of the protected network. But it should also be taken into account that the circuit should not be broken during starting overloads of connected electrical appliances.
For example, to start an asynchronous electric motor with a squirrel-cage rotor, a current exceeding seven times the rated value is required, which drops as the rotor accelerates to operating speed. The startup time depends on the characteristics of each specific electrical appliance.
Time current characteristic
The use of fuses in circuits with short-term overloads is possible due to the fact that when the IBC is exceeded, the shutdown does not occur immediately, but after some time necessary for heating the melted wire. The response period depends on the ambient temperature and the purpose of the fuse, which can be found out from the current time dependence graphs. During the short overload time, the material of the consumable element does not have time to overheat before the load returns to its normal value.
Time current characteristic for fuses of the PPN series, where the time of their burnout is indicated depending on the current value
Time current characteristics of fuses Various shutdown times
Branching graphs means working in hot (left) and cold (right) environments. For PPN with Ivs=25A, with I=100A the shutdown will occur in one second (red lines). At I=50A it will take approximately 40s. to trigger ( green on the graph).
At I=30A (blue segments), the fuse will hold the load for about half an hour (2000s/60m) at high temperatures. The graph shows that in cold conditions at I=30A it will never actually burn out. Therefore, the choice of fuses should be made by checking its time-current characteristic, finding out the shutdown time under certain conditions.
Calculation of Ivs according to PUE 5.3.56.
Ratio of starting current Ip.ed. to Ivc should not exceed 2.5, otherwise the fuse will not withstand starting overloads. This coefficient is adopted for engines with easy starting, and for difficult conditions (frequent starts, long acceleration times) a ratio of 2.0-1.6 is used.
That is,
The starting current of the electric motor is indicated in its passport, as well as on the housing itself. Let's say Ip.ed = 60A. In order for the fuse to withstand this current and properly protect against short circuits and long-term overloads, using the above formula you need to calculate Ivs = 60/2.5 = 24A. We select the closest value from the PPN series - 25A.
Selection table for some types of fuses Let's look at the time current characteristic, where we can see that the shutdown time at 60A is in the range of 10-20 s, which is quite enough for the engine to gain speed.
Let's say you have several electric motors and you need to protect the line, for this you need:
where — — the sum of all currents of simultaneously operating electric motors is equal to the calculated current in the line;
— starting current el. engine of the highest power;
— calculated current of the highest power from the number of operating electric power. engines.
After the calculation, this condition must be met:
Temporary fuse (“bug”)
Another wonderful feature of fuses is the possibility of repairing them using improvised means, but only for temporary replacement, having made calculations according to complex formulas, or by selecting the conductor diameter from the table:
Table for selecting temporary fuse links You need to measure the thickness of the wire with a micrometer or caliper. If there are none, you can wind the wire around a pencil, measure the length of the winding, dividing it by the number of turns to get its approximate diameter.
Protection devices are designed to ensure the safety of the operation of electrical networks, machines, electrical installations in the event of emergency conditions (short circuits, overloads). However, if installed and used incorrectly, they themselves can cause an accident, fire and explosion, because During their operation, electrical sparks and arcs occur.
The most common protection devices are:
fusible fuses;
air circuit breakers;
thermal relay;
devices protective shutdown.
Fuse is a device in which, when a current exceeds the permissible value, the fuse-link melts and the electrical circuit opens. Fuses are single-use protective devices.
Compound:
A) fusible insert;
b) contact device;
V) frame(cartridge);
d) and sometimes filler(talc, quartz sand, etc.) to improve arc extinction and visual response rate.
Principle The action of fuses is based on the fact that the current passing through the fuse-link generates heat in accordance with the equality where I is the current passing through the fuse-link, R is the resistance of the fuse-link, t is the time the current passes: at a certain value of the current I and time t, heat is released enough to melt the fuse link and open the electrical circuit. This provides protection against overload current and short circuit.
Fuse parameters
A) rated current of fuse link I n.vst . – the current for which it is designed for long-term operation and is indicated on it.
b) rated fuse current I n.pr . – current equal to the largest of In.in and which is indicated on the fuse. All current-carrying contact parts of the fuse are designed for this current;
V) rated voltage U n.pr . – voltage corresponding to the highest voltage at which it is permitted to be used and is indicated on the fuse.
G) maximum breaking current at a given voltage I pr.pr . – the highest value of short-circuit current at which reliable operation is guaranteed (without destroying the housing).
(3 min) Full shutdown time of the electrical circuit, the fuse is determined by the time the insert is heated to the melting temperature, the time of its melting and combustion that appears when the arc melts.
Dependence of the total shutdown time of the circuit fuse off. from relative overload current or short circuit I/In.in. called protective characteristic, i.e. off =f(I/ In.vst.).
The dependence of the period of time during which the temperature of an element of an electrical installation reaches the maximum permissible on the ratio of the actual current in it I to the rated current Iн is called thermal characteristics of this element, i.e. load=f(I/ In).
Comparison of the protective characteristics of fuses with the thermal characteristics of the protected elements allows us to evaluate
possibility of reliable protection. (Fig.1)
I/I N.VST and I/I h
(5 min) It can be seen that the insert with a protective characteristic A protects an element of an electrical installation with a thermal characteristic IN at any current ratio, and the insert with a protective characteristic WITH– only for multiplicities of more than 4.
We need to strive for the shutdown time to be as short as possible under the action of short-circuit currents. and have a delay during overload currents. This can be done:
Right select the material of the fuse link;
use metallurgical effect;
choose rational design.
Inserts from low-melting metals (tin, lead, zinc, aluminum) have low thermal conductivity, so they heat up slowly; they are convenient for protecting elements from overload currents.
Inserts from refractory metals ( copper, silver) have low heat capacity and high thermal conductivity, therefore they heat up quickly, give a shorter time delay during overloads, which worsens their protective characteristics. But they have a large maximum shutdown current, so they are convenient for protecting elements from short-circuit currents.
To reduce the melting point (so that they heat up more slowly), inserts with metallurgical effect, for which a ball of low-melting metal (tin, an alloy of tin with cadmium, etc.) is soldered in the middle of the insert of a refractory metal.
At the point where the ball is soldered, the more refractory metal dissolves into the low-melting one. This insert has better protective characteristics during overload currents and a lower melting temperature (2-3 times lower than the melting temperature of the base metal).
From the point of view design influences the protective characteristics length (for fuses with U = 120 – 500V, the optimal insertion length is 70mm) and insert form(inserts are made with several parallel branches; inserts with 2–4 short isthmuses are used).
Fuse links are made of copper, zinc, lead or silver.
In today's most advanced fuses, preference is given 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.
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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 further increase the cross-section of the insert. 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.
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