What are fuses made of? Fuses - their purpose, types and types, design and principle of operation

–This is an element of an electrical circuit, the main purpose of which is to protect it from damage..

Operating principle

The fuse is designed in such a way that it burns out before other elements are damaged. After all, it is easier to insert a new fuse than to replace wires, microcircuits and other elements that can burn out when there is a surge in current in the circuit.

A fuse is called a fuse because it is based on a fuse link. This fuse link consists of an alloy that has low temperature melting and when a current dangerous for the circuit occurs, the amount of heat that is released when such a current flows through this insert is enough to melt it. When the insert melts - “burns out”, the circuit is open.

The reasons for a blown fuse can be short circuit, overload and sudden surges in current.

Not only does the fuse protect the circuit from damage, but it also serves as protection against fires and fires, since the fuse link burns out in the fuse body, unlike the wire, which may come into contact with flammable materials during combustion.

It happens that people make the so-called bug. Usually this is an ordinary piece of wire that is inserted in place of the fuse. This is done because there is no fuse of the required rating at hand or to bypass the protection. Often, such bugs lead to fires, since it is not known at what current such a bug will burn out or whether it will burn out at all.

Fuse device

As mentioned above, the simplest fuse consists of its main part - a fuse link (wire) and a housing, which is intended to be connected to the electrical circuit and serves as a fastener for the insertion.

Advantages and disadvantages

The advantages of fuses include their relatively low cost.

The main disadvantage of the fuse is that it takes a relatively long time to operate compared to automatic fuses. During the time a fuse blows in high-voltage networks, equipment may fail. In addition, a fuse is a disposable element, that is, once it burns out, it cannot be used for further use, while automatic fuses can serve for quite a long time, since the principle of their operation is based on opening the circuit without damaging the structure of the fuse itself.

Main settings

The parameters that characterize a fuse are rated current, rated voltage, power, response speed.

Where U– network voltage, and Pmax– maximum load power with a margin of about 20%.

The speed at which fuses operate varies. For example, in circuits where there are semiconductor devices, it is better if the fuse burns out faster so as not to damage the devices, but if it is a powerful fuse that is used in the electric motor circuit, then it will be much more useful if it does not break the circuit every time at the moment of inrush currents.

Fuse– the first device used in electrical circuits to protect against short circuits and overloads. The occurrence of these emergency operating conditions is inevitable. No matter how new and high-quality the electrical installation is, there is always a chance of damage to its insulation and connection of excess power to the power supply networks.

Fuse is a disposable component. After operation, either it itself or its fuse-link must be disposed of and replaced with new ones. Automatic switches do not have these shortcomings; they turn off emergency modes of network operation again and again, without destruction or failure. But fuses are still used in electrical installations.

Its advantages contribute to this:

• simple design, cheap to manufacture;
• ease of use;
• failure of the fuse is impossible - there is simply nothing in it to break. Therefore, there are no failures in their operation, which increases the reliability of the protection.

Fuse device

A fuse of any design consists of three parts: a body, a contact part and a fuse element.

Fusible element is a conductor made of low-melting material. When current passes through a fuse on a fuse element that has electrical resistance, a electric power in the form of heat. If the current is below the rated current, then the heat is not enough to melt the metal from which the insert is made.

When the current exceeds the operating threshold, the insert melts, accompanied by a circuit break. The rupture occurs the faster the greater the current passes through the fuse. For each of them, manufacturing plants provide a time-current characteristic, which can be used to determine how long it will take to turn off the emergency mode with a given factor of exceeding the rated current. This information is used by designers to calculate the performance of fuse-based protection.

Fuse housing serves not only for the mechanical connection of its elements with each other. When a fuse link burns out, an electric arc inevitably occurs. The task of the fuse body is to prevent its spread and extinguish it as soon as possible.

Purpose contact system– ensure a reliable detachable connection of the protective device with the current conductors of the electrical installation. The contact area should be as large as possible in order to reduce the contact resistance and prevent heating of the connection. Anodized brass and copper are used for fuse contact systems.

Arc suppression in fuse housings

The simplest models contain nothing inside but air. But they are also designed for small currents, the shutdown of which is not accompanied by the formation of an arc with characteristics dangerous to electrical equipment. When the insert melts, it goes out on its own.

With an increase in the current cut off by the fuse, the need arises forced arc extinction inside the housing. Otherwise, it will not go out, continuing to feed the short circuit. The emergency circuit will not be disconnected: the arc has melted contact system, will spray metal particles over the surface of the housing, forming a contact bridge. Current will continue to flow through it short circuit, until the higher-level protection is triggered, or the current conductors completely melt. In the best case, the time to turn off the emergency operation mode will take several times longer.

The longer the short circuit opens, the more damage it will cause.. Therefore, special attention is paid to extinguishing the arc inside the fuse.

The first method to reduce short circuit tripping time was manufacturing the central part of the hollow fuse body from fiber. This is a layered material consisting of cardboard, pressed with cellulose mass, pre-impregnated with zinc chloride. Fiber products are resistant to gasoline, alcohol, kerosene, acetone, and also have insulating properties.

But the main advantage of fiber parts, which determined its spread in electrical engineering, is when exposed to an arc flame, it emits a mixture of gases that block its combustion process. Gases, mixing with the ionized plasma of the arc, impede the movement of charged particles in it. The resistance of the current-carrying channel increases sharply, and the arc goes out. Such fuses are called gas-generating, and in addition to fiber, they are also made from vinyl plastic.

The next method used to speed up the operation of the fuse is filling the body with quartz sand. The melting point of quartz is about 1700 degrees, and it is also an excellent dielectric. When the fuse link burns out, the arc, increasing in volume, spreads between the grains of sand. She has to go around them along an intricate and complex trajectory, as a result of which her length increases. Additionally, arc heat is removed from the filler material, which promotes deionization of the channel and rapid extinction of the discharge.

Quartz fuses are most widely used in electrical installations and are still used today. Gas-generating fuses are less common and are found only in older switchgear.

Use of fuses to protect electrical installations high voltage significantly simplifies and reduces the cost of their design. An alternative to this is a full relay protection device. And for its operation, sensors are required: current transformers and voltage transformers. Their task is to reduce the measured values ​​to safe values ​​with which relays and microprocessor terminals can operate. All this together turns out to be orders of magnitude more expensive than installing fuses.

But even more stringent requirements are imposed on the speed of fuses in electrical installations above 1000 V. To quickly turn off their fuse-link is attached to the spring connected to one of the contact terminals. The body is filled with quartz sand.

When the insert burns out, the spring is released and sharply contracts. Due to this, the length of the arc burning section quickly increases. Extinguishing occurs faster.

An additional and mandatory device for high-voltage fuses is serviceability monitoring unit. To safely test a low voltage fuse, you can use an indicator, voltage gauge, or tester. If necessary, you can turn off the switch and measure the resistance between the contacts of the protective device.

But it’s not possible to check the serviceability of a high-voltage fuse. You can't get close to him. The use of voltage indicators does not give reliable results. If the power transformer is protected by fuse links, the indicator will show behind the blown fuse the voltage induced on the winding that has lost power from the windings of other phases. When checking the serviceability of the inserts on the cable line, the indicator will light up due to the residual charge remaining due to large capacity cable.

To indicate that the protection has tripped, an indicator pops out from the fuse body and is clearly visible at a distance that is safe for inspection. For ease of maintenance, low-voltage fuses also use indicator devices that signal that the fuse link has burned out.

Another problem that exists when using fuses in networks above 1000 V is the occurrence of an open-phase mode due to burnout of the insert in one phase. Power transformers that remain in operation in two phases produce an asymmetrical voltage on the low-voltage winding, which threatens to damage consumers’ electrical appliances.

If the problem persists, if one insert burns out, turn off the power completely. To do this, use special fuses with strikers at one of its ends. The firing pin is spring-loaded and is released simultaneously with the fuse-link burning out. Paired with such devices they are used load switches having disconnecting strips. In the on position, the contact system of the switch is held by a latch. When the striker strikes the trip bar, the latch is knocked out. The circuit breaker's opening spring system pushes its contact system to the open position. The phase in which the shutdown occurred due to a short circuit is determined by the striker that has jumped out of the housing.

Semiconductor fuse

The development of power semiconductor technology has raised another problem. No mechanical protective device, including fuses, is capable of promptly shutting down the emergency operation of devices containing powerful diodes or transistors. Overloading of these devices is possible only for a limited time - tens of milliseconds. If this time is exceeded, the device is destroyed.

To minimize damage to electronics in frequency converters, inverters or devices soft start semiconductor fuses are used. Their pn junction burns out faster than any fuse link. But they have a peculiarity - when triggered, the semiconductor fuse does not fully guarantee disconnection of the circuit. The current through it stops, but not completely: a blown semiconductor fuse has some resistance. Therefore, for safe operation, another switching element is installed in front of it - a circuit breaker. They provide redundancy for semiconductor protection and are also used to guarantee voltage removal from the device to check the serviceability or replace fuses.

Self-resetting fuses

In some cases, after a circuit has been overloaded, it can be no harm turning it back on after a while. This is relevant in microprocessor and microcontroller technology. Self-resetting fuses are used to protect such circuits.

These devices include a polymer mass mixed with carbon. Carbon provides the required conductivity, but the device itself as a whole has resistance to the current passing through it. When this current exceeds the established threshold, the composition of the conductive mixture heats up, the polymer goes into an amorphous state, increasing in size. The connection between the carbon particles is broken, and the current through the fuse stops.

After the polymer cools, the conductive composition returns to its original form. Contact is restored and the device is ready for use again.

Fuse links are made of copper, zinc, lead or silver.

Today's most advanced fuses give preference to copper inserts with a tin solvent. Zinc inserts are also widespread.

Copper fuse inserts are the most convenient, simple and cheap. Improving their characteristics is achieved by fusing a tin ball

a certain place, approximately in the middle of the insert. Such inserts are used, for example, in the mentioned series of bulk fuses PN2. Tin melts at a temperature of 232°, significantly lower than the melting point of copper, and dissolves the copper of the insert at the point of contact with it. The arc that appears in this case already melts the entire insert and is extinguished. The current circuit turns off.

Thus, fusing a tin ball results in the following.

Firstly, copper inserts begin to react with a time delay to such small overloads, to which they would not react at all in the absence of a solvent. For example, a copper wire with a diameter of 0.25 mm with a solvent melted at a temperature of 280° in 120 minutes.

School for an electrician: articles, tips, useful information

Secondly, at the same sufficiently high temperature (i.e., under the same load), inserts with a solvent react much faster than inserts without a solvent.

For example, a copper wire with a diameter of 0.25 mm without a solvent at an average temperature of 1,000° melted in 120 minutes, and the same wire, but with a solvent at an average temperature of only 650°, melted in just 4 minutes.

The use of a tin solvent makes it possible to have reliable and cheap copper inserts that operate at a relatively low operating temperature, have a relatively small volume and weight of metal (which favors the switching ability of the fuse) and at the same time have greater speed at high overloads and react with a time delay to relatively small overloads.

Zinc is often used to make fuse links. In particular, such inserts are used in the mentioned series of PR-2 fuses.

Zinc inserts are more resistant to corrosion. Therefore, despite the relatively low melting point, for them, generally speaking, it would be possible to allow the same maximum operating temperature as for copper (250°C) and design inserts with a smaller cross-section. However, the electrical resistance of zinc is approximately 3.4 times greater than that of copper.

To maintain the same temperature, it is necessary to reduce energy losses in it, accordingly increasing its cross-section. The insert turns out to be much more massive. This, other things being equal, leads to a decrease in the switching capacity of the fuse. In addition, with a massive insert with a temperature of 250°, it would not be possible to maintain the temperature of the cartridge and contacts at an acceptable level within the same dimensions.

All this makes it necessary to reduce the maximum temperature of zinc inserts to 200°, and for this purpose, to increase the cross-section of the insert even more. As a result, fuses with zinc inserts of the same dimensions have significantly less resistance to short-circuit currents than fuses with copper inserts and tin solvents.

School for an electrician: articles, tips, useful information

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 quite quick change the structure of this element from the influence of current. In such situations, fuses of various designs help 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 the following types 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 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. 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 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 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: electronic equipment, automotive electrical networks, 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 electrical 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, you can insert into the NPN2-60 fuse fuse links with Ivs from 6 to 60A, respectively, its Ip is equal to 60A.

Principle of operation

Structurally, the disposable element is made in the form of a conductor of small cross-section, 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 complete shutdown, and 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 acceptable values during long-term operation 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

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. for activation (green color 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 Ivs should not exceed 2.5, otherwise the fuse will not withstand starting overloads. This coefficient is accepted 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.

We 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 itself high 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:

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.