Thermal release- provides protection only against overcurrent.

Electromagnetic release- provides protection only against short circuits.

Thermal-magnetic (magnetic-thermal, combined) release- consists of two types of releases - thermal and electromagnetic. Provides protection against both overcurrent and short circuits.

Thermal-magnetic (magnetic-thermal, combined) release, with protection against leakage currents- in addition to protection against overloads and short circuits, it protects people and electrical installations from ground faults.

Electronic release(electronic protection unit - Overcurrent Release) - (depending on the version) provides the maximum number of types of protection.

Release device

Thermal release

The thermal release is a bimetallic plate that, when heated, bends and acts on the free release mechanism. A bimetallic plate is made by mechanically joining two metal strips. Two materials with different coefficients of thermal expansion are selected and connected to each other by soldering, riveting or welding.

Advantages:

• no moving parts;
• undemanding to pollution;
• simplicity of design;
• low price.

Flaws:

• high own energy consumption;
• sensitive to temperature changes environment;
• when heated from third-party sources, they can cause false alarms.

Electromagnetic release

The electromagnetic release is an instantaneous device. It is a solenoid, the core of which acts on the free release mechanism. When a supercurrent flows through the solenoid winding, a magnetic field is created that moves the core, overcoming the resistance of the return spring.

The EM release can be configured (at the manufacturer's factory or by the consumer) to operate at short-circuit currents ranging from 2 to 20 In. The setting error varies approximately ±20% of the set current value for molded case switches.
For power circuit breakers The short-circuit setpoint (the current value at which tripping is initiated) can be indicated either in amperes or as a multiple of the rated current.
There are settings: 3.5In; 7In, 10In; 12In and others.

Advantages:

• simplicity of design;

Flaws:

• creates a magnetic field.

Thermomagnetic release

Often a series connection of thermal and electromagnetic release. Depending on the manufacturer, this connection of two devices is called a combined or thermomagnetic release.

Thermomagnetic or combined release

Thermomagnetic release with leakage current protection

The machine with these releases, in addition to thermal and electromagnetic releases, has a unit capable of detecting fault current to the ground using toroidal transformer, which covers all live parts, as well as the neutral, if it is distributed. Earth leakage releases can be used in combination with a circuit breaker to provide two main functions in one device:

• protection against overloads and short circuits;
• protection against indirect contact (appearance of voltage) on conductive parts due to insulation damage).

Electronic release

A release connected to measuring current transformers (three or four, depending on the number of protected conductors), which are installed inside the circuit breaker and provide a dual function: supplying power for normal control of the release and detecting the value of the current that passes in live parts. Therefore they are only compatible with networks alternating current.

The signal from the transformers is processed by an electronic part (microprocessor), which compares it with the specified settings. When the signal exceeds the threshold, the circuit breaker release acts directly on the breaker tripping assembly via a trip coil.

The release control unit allows you to build a user-defined program according to which the circuit breaker will trip the main contacts.

Advantages:

• a varied selection of settings needed by the user;
• high accuracy of execution of a given program;
• performance indicators and reasons for operation;
• logic selectivity with upstream and downstream switches.

• high price;
• fragile block management;
• exposure to electromagnetic fields.

A circuit breaker release (automatic) is an electrical device that turns off the network if a large electric current occurs in it. This device is used to ensure that if the wires overheat, there is no fire in the house, and expensive household appliances do not fail.

Types of switches

All machines are divided according to the type of release. They are divided into 6 types:

• thermal;
• electronic;
• electromagnetic;
• independent;
• combined;
• semiconductor.

They very quickly recognize emergency situations, such as:

• the occurrence of overcurrents - an increase in the current strength in the electrical network that exceeds the rated current of the circuit breaker;
• voltage overload – short circuit in the circuit;
• voltage fluctuations.

At these moments, the contacts in the automatic releases open, which prevents serious consequences in the form of damage to wiring and electrical equipment, which very often leads to fires.

Thermal switch

It consists of a bimetallic plate, one of the ends of which is located next to the release device of the automatic release. The plate is heated by the current passing through it, hence the name. When the current begins to increase, it bends and touches the trigger bar, which opens the contacts in the “machine”.

The mechanism operates even with slight excesses of the rated current and an increased response time. If the load increase is short-term, the switch does not trip, so it is convenient to install it in networks with frequent but short-term overloads.

Advantages thermal release:

• absence of contacting and rubbing surfaces;
• vibration stability;
• budget price;
• simple design.

The disadvantages include the fact that its work largely depends on temperature regime. It is better to place such machines away from heat sources, otherwise there is a risk of numerous false alarms.

Electronic switch

Its components include:

• measuring devices (current sensors);
• Control block;
• electromagnetic coil (transformer).

At each pole of the electronic circuit breaker there is a transformer that measures the current passing through it. The electronic module that controls the trip processes this information, comparing the obtained result with the specified one. In the event that the resulting indicator is greater than the programmed one, the “machine” will open.

There are three trigger zones:

1. Long delay. Here electronic release serves as a thermal one, protecting circuits from overloads.
2. Short delay. Provides protection against minor short circuits that usually occur at the end of the protected circuit.
3. The working area “instantly” provides protection against high-intensity short circuits.

Pros - big choice settings, maximum accuracy of the device to a given plan, the presence of indicators. Cons: sensitivity to electromagnetic fields, high price.

Electromagnetic

This is a solenoid (a coil of wound wire), inside of which there is a core with a spring that acts on the release mechanism. This is an instant action device. As the supercurrent flows through the winding, a magnetic field is generated. It moves the core and, exceeding the force of the spring, acts on the mechanism, turning off the “automatic machine”.

Pros: resistance to vibration and shock, simple design. Cons – forms a magnetic field, triggers instantly.

This is an additional device to automatic releases. With its help you can turn off both single-phase and three-phase machine, located at a certain distance. To put into action independent release, it is necessary to apply voltage to the coil. To return the machine to initial position You need to manually press the “return” button.

Important! The phase conductor must be connected from one phase from under the lower terminals of the switch. If it is connected incorrectly, the independent switch will fail.

Basically, independent circuit breakers are used in automation panels in highly ramified power supply devices of many large objects, where control is transferred to the operator's console.

Combination switch

It has both thermal and electromagnetic elements and protects the generator from overloads and short circuits. To operate the combined automatic release, the current of the thermal circuit breaker is indicated and selected: the electromagnet is designed for 7–10 times the current, which corresponds to the operation of heating networks.

The electromagnetic elements in the combination switch provide instantaneous protection against short circuits, and the thermal elements protect against overloads with a time delay. The combined machine is switched off when any of the elements is triggered. During short-term overcurrents, none of the types of protection are triggered.

Semiconductor switch

Consists of AC transformers, magnetic amplifiers for direct current, a control unit and an electromagnet that performs the functions of an independent automatic release. The control unit helps set the selected contact release program.

Its settings include:

• regulation of the rated current in the device;
• setting the time;
• triggered at the moment of occurrence short circuit;
• protective switches against overcurrent and single-phase short circuit.

Pros - a large selection of regulation for different power supply schemes, ensuring selectivity to series-connected circuit breakers with fewer amperes.

Minuses - high price, fragile control components.

Installation

Many home-grown electricians believe that installing a machine is not difficult. This is fair, but certain rules must be followed. Circuit breaker releases, as well as plug fuses, must be connected to the network so that when the plug of the circuit breaker is turned out, its screw sleeve is without voltage. Connection of the supply conductor at one-way feeding with a machine should be carried out to fixed contacts.

Installation of an electric single-phase two-pole circuit breaker in an apartment consists of several stages:

• securing the switched-off device to the electrical panel;
• connecting wires without voltage to the meter;
• connecting voltage wires to the machine from above;
• turning on the machine.

Fastening

We install a DIN rail in the electrical panel. Cutting off right size and fasten it with self-tapping screws to the electrical panel. Snap it in automatic release network onto the DIN rail using a special lock, which is located on the back of the machine. Make sure that the device is in shutdown mode.

Connection to the electricity meter

We take a piece of wire, the length of which corresponds to the distance from the meter to the machine. We connect one end to the electric meter, the other to the terminals of the release, observing the polarity. We connect the supply phase to the first contact, and the neutral supply wire to the third. Wire cross-section – 2.5 mm.

Connecting voltage wires

From the central electrical distribution panel, the supply wires are connected to the apartment panel. We connect them to the terminals of the machine, which must be in the “off” position, observing the polarity. The wire cross-section is calculated depending on the energy consumed.

Turning on the machine

Only after all the wires have been installed correctly can the automatic current release be put into operation.

It happens that the constant shutdown of the machine becomes big problem. Do not try to solve this problem by installing a trip unit with a higher current rating. Such devices are installed taking into account the cross-section of the wires in the house, and, perhaps, a large current in the network is unacceptable. The problem can only be solved by inspecting the electrical supply system of the apartment by professional electricians.

What is a circuit breaker?

Circuit breaker(automatic) is switching device designed to protect the electrical network from overcurrents, i.e. from short circuits and overloads.

The definition of “switching” means that this device can turn on and off electrical circuits, in other words, switch them.

Automatic circuit breakers come with an electromagnetic release that protects the electrical circuit from short circuits and a combined release - when in addition to the electromagnetic release a thermal release is used to protect the circuit from overload.

Note: In accordance with the requirements of the PUE, household electrical networks must be protected from both short circuits and overload, therefore, to protect home wiring Automatic machines with a combined release should be used.

Automatic switches are divided into single-pole (used in single-phase networks), two-pole (used in single-phase and two-phase networks) and three-pole (used in three-phase networks), there are also four-pole circuit breakers (can be used in three-phase networks with a TN-S grounding system).

1. Design and principle of operation of a circuit breaker.

The figure below shows circuit breaker device with a combined release, i.e. having both an electromagnetic and thermal release.

1,2 - respectively lower and upper screw terminals for connecting the wire

3 - moving contact; 4—arc chamber; 5 - flexible conductor (used to connect moving parts of the circuit breaker); 6 - electromagnetic release coil; 7 - core of the electromagnetic release; 8 — thermal release (bimetallic plate); 9 — release mechanism; 10 — control handle; 11 — clamp (for mounting the machine on a DIN rail).

The blue arrows in the figure show the direction of current flow through the circuit breaker.

The main elements of the circuit breaker are electromagnetic and thermal releases:

Electromagnetic release provides protection of the electrical circuit from short circuit currents. It consists of a coil (6) with a core (7) located in its center, which is mounted on special spring, the current in normal operation mode passing through the coil according to the law of electromagnetic induction creates an electromagnetic field that attracts the core inside the coil, but the strength of this electromagnetic field is not enough to overcome the resistance of the spring on which the core is installed.

During a short circuit, the current in the electrical circuit instantly increases to a value several times higher than the rated current of the circuit breaker; this short circuit current, passing through the coil of the electromagnetic release, increases the electromagnetic field acting on the core to such a value that its retraction force is enough to overcome the resistance springs, moving inside the coil, the core opens the moving contact of the circuit breaker, de-energizing the circuit:

In the event of a short circuit (i.e., with an instantaneous increase in current several times), the electromagnetic release disconnects the electrical circuit in a fraction of a second.

Thermal release provides protection of the electrical circuit from overload currents. Overload can occur when electrical equipment is connected to the network with a total power exceeding the permissible load of this network, which in turn can lead to overheating of the wires, destruction of the insulation of the electrical wiring and its failure.

The thermal release is a bimetallic plate (8). Bimetallic plate - this plate is soldered from two plates of different metals (metal “A” and metal “B” in the figure below) having different coefficients of expansion when heated.

When a current exceeding the rated current of the circuit breaker passes through the bimetallic plate, the plate begins to heat up, while metal “B” has a higher expansion coefficient when heated, i.e. when heated, it expands faster than metal “A”, which leads to curvature of the bimetallic plate; as it bends, it affects the release mechanism (9), which opens the moving contact (3).

The response time of the thermal release depends on the amount of excess current in the electrical network of the rated current of the machine; the greater this excess, the faster the release will operate.

As a rule, the thermal release operates at currents 1.13-1.45 times higher than the rated current of the circuit breaker, while at a current 1.45 times higher than the rated current, the thermal release will turn off the circuit breaker in 45 minutes - 1 hour.

Whenever the circuit breaker is turned off under load, an electric arc is formed on the moving contact (3), which has a destructive effect on the contact itself, and the higher the switched current, the more powerful the electric arc and the greater its destructive effect. effect. To minimize damage from an electric arc in a circuit breaker, it is directed to the arc-extinguishing chamber (4), which consists of separate, parallel-installed plates; when the electric arc falls between these plates, it is crushed and extinguished.

3. Marking and characteristics of circuit breakers.

VA47-29- type and series of circuit breaker

Rated current— the maximum current of the electrical network at which the circuit breaker is capable of operating for a long time without emergency shutdown of the circuit.

Rated voltage— the maximum network voltage for which the circuit breaker is designed.

PKS— ultimate breaking capacity of the circuit breaker. This figure shows the maximum short circuit current that can turn off a given circuit breaker while maintaining its functionality.

In our case, the PKS is indicated at 4500 A (Ampere), this means that with a short circuit current (short circuit) less than or equal to 4500 A, the circuit breaker is able to open the electrical circuit and remain in good condition, if the short circuit current. exceeds this figure, there is a possibility of the movable contacts of the machine melting and welding them to each other.

Triggering characteristics— determines the range of operation of the circuit breaker protection as well as the time during which this operation occurs.

For example, in our case, a machine with characteristic “C” is presented; its response range is from 5·I n to 10·I n inclusive. (I n - rated current of the machine), i.e. from 5*32=160A to 10*32+320, this means that our machine will provide instantaneous disconnection of the circuit already at currents of 160 - 320 A.

4. Selecting a circuit breaker

The choice of machine is carried out according to the following criteria:

— By number of poles: single- and double-pole are used for single-phase network, three- and four-pole - in three-phase network.

— By rated voltage: The rated voltage of the circuit breaker must be greater than or equal to the rated voltage of the circuit it protects:

Unom. AB⩾ Unom. networks

— By rated current:The required rated current of the circuit breaker can be determined in one of the following four ways:

1. With the help of our .
2. With the help of our .
3. Using the following table:

1. Calculate yourself using the following method:

The rated current of the circuit breaker must be greater than or equal to the rated current of the circuit it protects, i.e. the current for which this electrical network is designed:

Inom. AB⩾ Icalc. networks

The calculated current of the electrical network (I rated network) can be determined using ours, or you can calculate it yourself using the formula:

Icalc. networks= Pnetworks/(U network *K)

where: P network - network power, Watt; U network - network voltage (220V or 380V); K - coefficient (For a single-phase network: K=1; For a three-phase network: K=1.73).

Network power is defined as the sum of the powers of all electrical receivers in the house:

Pnetworks=(P 1 + P 2 …+ Pn)*K s

Where: P1, P2, Pn— power of individual electrical receivers; K s— demand coefficient (K c = from 0.65 to 0.8) if only 1 power receiver or a group of power receivers that are connected to the network at the same time is connected to the network K c = 1.

The maximum power allowed for use can also be taken as the network power, for example from technical specifications, project or electricity supply agreement, if any.

After calculating the mains current, we take the nearest larger standard value of the rated current of the machine: 4A, 5A, 6A, 8A, 10A, 13A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, etc.

NOTE: In addition to the method described above, it is possible to simplify the calculation of the circuit breaker; for this you need:

1. Determine the network power in kiloWatts (1 kiloWatt=1000Watt) using the formula given above:

P network =(P 1 + P 2 ...+ P n)*K s, kW

2. Determine the network current by multiplying the calculated network power by the conversion factor ( K p) equal: 1,52 -for 380 Volt network or 4,55 — for a 220 Volt network:

Inetworks= Pnetworks*K p, Ampere

3. That's all. Now, as in the previous case, we round the resulting value of the network current to the nearest higher standard value of the rated current of the machine.

And in conclusion select the response characteristic(see characteristics table above). For example, if we need to install a circuit breaker to protect the electrical wiring of the whole house, we select characteristic “C”; if the electric lighting and socket group are divided into two different circuit breakers, then for lighting we can install a circuit breaker with characteristic “B”, and for sockets - with characteristic “C”, if you need a circuit breaker to protect the electric motor, select characteristic “D”.

Here's an example of a calculation: There is a house in which there are the following pantographs:

• Washing machine with a power of 800 watts (W) (equal to 0.8 kW)
• Microwave oven - 1200W
• Electric oven - 1500 W
• Refrigerator - 300 W
• Computer - 400 W
• Electric kettle - 1200W
• TV - 250W
• Electric lighting - 360 W

Mains voltage: 220 Volt

Let us take the demand coefficient to be 0.8

Then the network power will be equal to:

How to choose the right circuit breaker?

A circuit breaker (in the language of electricians, “machine”) is the basis of protection in low-voltage (up to 1000 Volt) power electrical circuits. This is a combined electrical device that combines the functions of a switch and a protective device. Almost the entire distribution and protection system for household electrical wiring is built on automatic devices. I would like to immediately note that the main use of the machine is to protect that section of electrical wiring that is located between the outlet of the machine and the consumer. If there is another machine further along the line, then our machine must defend the area between these two machines. If an overload or short circuit occurs in any section of the circuit, only one circuit breaker should operate, protecting that particular section of the circuit.

How to choose a machine?

Let's take a classic example. We are making repairs in an apartment (or in a private house), changing the electrical wiring and want to protect it from overloads and short circuits. A common practice these days is to divide the wiring into several branches and protect each of them with a separate machine. In apartments, lighting and sockets are often separated into separate lines. In addition, a separate line can be allocated for an electric stove, another for kitchen sockets and utility room sockets, which usually include the most powerful electrical appliances in the apartment: an electric kettle, a microwave oven, etc. It should be noted that standard electrical outlets used in our homes are usually designed for a maximum current of 10 or 16A, and are often the weakest link in the electrical wiring. Therefore, the rating of the circuit breaker protecting the line with such sockets cannot be higher than 16A, no matter how thick the wire is.

About the material and thickness of the wire - this is a separate topic, here I’ll just say briefly: copper and only copper, for apartments and private houses we take a cross-section of 1.5 sq. mm for lighting, 2.5 sq. mm for standard sockets. Accordingly, the ratings of circuit breakers for lighting lines are 10A, for lines feeding sockets, 16A (provided that the sockets are also 16-amp). This raises a number of questions. It turns out that each socket can withstand 16 Amps alone, but the total current of the entire group of sockets should also not exceed the same 16 Amps.

Some people don’t like this situation, and they install machines with a higher current - 25A and even higher. For some reasons, this should not be done, even if the cross-section of the wire allows such a current to pass long time. Let's imagine a situation where some powerful power tool is plugged into one of the sockets, which consumes current up to 25-30A. It is clear that with such a current, unpleasant processes can occur in the outlet, including fire, but a 25-amp circuit breaker will not feel this overload. Well, or he will feel it, but only when everything is already burning with a blue flame. Someone may argue that there is no standard power tool with such a current consumption, but the tool can be non-standard and faulty. Or it may happen that several powerful electrical appliances are connected to the outlet through an extension cord at the same time, with the same result.

Therefore, if it is assumed that the total current of equipment simultaneously plugged into sockets will be more than 16A, then the right decision will divide the sockets into several groups and power each group through a separate machine. It must be borne in mind that both 16 and 10 amp outlets are available for sale. I will not say that they are of poor quality, they are simply designed for a maximum load current of 10 A. For such sockets, it is permissible to lay wiring with a cross-section of 1.5 mm 2, but the machine in this case must also be 10-amp. Regarding extension cords. Very often you can find cheap options, the cross-section of the cord of such an extension cord is 1 mm 2, sometimes even smaller. Extension cords themselves usually do not have any protection. Therefore, use such extension cords with extreme caution, understanding that the machine does not protect them.

Marking of circuit breakers

We can see some mysterious inscriptions on the body of the machine gun. The main ones are indicated by numbers below:

Explanation:

1. Rated current of the machine
2. Triggering characteristics
3. Maximum breaking current
4. Trip class.

In addition to the above inscriptions, the case usually contains the manufacturer’s logo and the type of machine, as well as a brief schematic designation, showing where the fixed contact is located (when positioned vertically, it is usually placed on top) and how the releases are located relative to the contacts. The clamping contact screws can be closed with curtains (see the machine on the far left), this is convenient for sealing. The body is usually made of polystyrene - in my opinion, not the most suitable material for a device that can get quite hot.

Rated current of the machine

The time has come to figure out what the rated current of the machine actually means and what the protection operation current will be. A common mistake is that people often think that the rated current is the tripping current. In fact, a working circuit breaker will never trip at its rated current. Moreover, it will not work even at 10% overload. If there is a large overload, the machine will turn off, but this does not mean that it will turn off quickly. A conventional modular circuit breaker has 2 releases: a slow thermal one and a fast-reacting electromagnetic one. The thermal release basically contains a bimetallic plate, which is heated by the current passing through it. When heated, the plate bends and, at a certain position, acts on the latch and the switch turns off. The electromagnetic release is a coil with a retractable core, which, at high current, also acts on the latch that turns off the circuit breaker. If the purpose of a thermal release is to turn off the circuit breaker during overloads, then the task of an electromagnetic release is to quickly turn off during short circuits, when the current value is several times higher than the rated value.

Range of rated currents

I had to install circuit breakers with a rating of 0.2A. In general, I have come across modular machines of the following denominations: 0.2, 0.3, 0.5, 0.8, 1, 1.6, 2, 2.5 3, 4, 5, 6, 6.3, 8, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125 Amp. That is, I cannot say that the ratings correspond to any single standard series, such as E6, E12 for resistors or capacitors. They sculpt whatever they want. With machines above 100A the situation is approximately the same. The maximum rating of a machine designed to operate in 0.4 kV networks that I have seen is 6300A. This corresponds to a transformer with a capacity of 4 MVA, but we don’t make more powerful transformers for this voltage, this is the limit.

Triggering characteristics

The sensitivity of electromagnetic releases is regulated by a parameter called the response characteristic. This is an important parameter, and it’s worth dwelling on it a little. The characteristic, sometimes called a group, is denoted by one Latin letter; on the body of the machine it is written right before its nominal value, for example, the inscription C16 means that the rated current of the machine is 16A, characteristic C (the most common, by the way). Less popular are machines with characteristics B and D; current protection of household networks is mainly based on these three groups. But there are machines with other characteristics.

According to Wikipedia, circuit breakers are divided into the following types (classes) based on instantaneous tripping current:

• type B: over 3 I n up to 5 I n inclusive (where I n- rated current)
• type C: over 5· I n to 10· I n inclusive
• type D: over 10 I n up to 20 I n inclusive
• type L: over 8· I n
• type Z: over 4 I n
• type K: over 12· I n

At the same time, Wikipedia refers to GOST R 50345-2010. I specifically re-read this entire standard, but it never mentions any types L, Z, K. And for some reason I don’t see such machines on sale. For European manufacturers, the classification may be slightly different. In particular, there is an additional type A(over 2· I n until 3· I n). Some manufacturers have additional shutdown curves. For example, at ABB there are circuit breakers with curves K(8 - 14 I n) and Z (2 - 4· I n), complying with IEC 60947-2 standard. In general, we will keep in mind that, in addition to B, C and D, there are other curves, but in this article we will consider only these. Although the curves themselves are the same - they generally show the dependence of the response time of the thermal release on the current. The only difference is the point to which the curve reaches, after which it abruptly ends to a value close to zero. And here are the graphs themselves:

These are average graphs; in fact, some variation in the response time of thermal protection is allowed. What should we keep in mind when choosing a shutdown characteristic? Here the starting currents of the equipment that we are going to turn on through this machine come to the fore. It is important for us that the starting current in sum with other currents in this circuit does not exceed the operating current of the electromagnetic release (cut-off current). It’s easier when we know exactly what will be connected to our machine, but when the machine protects a group of sockets, then we can only guess what and when it will be turned on. Of course, we can take it with a reserve - install group D machines. But it is far from a fact that the short circuit current in our circuit somewhere on a distant outlet will be sufficient to trigger the cutoff. Of course, after ten seconds the thermal release will heat up and turn off the circuit, but this will be a serious test for the wiring, and a fire may occur at the point of the circuit. Therefore, we need to look for a compromise. As practice has shown, to protect sockets in residential premises, offices - where the use of powerful power tools is not expected, industrial equipment, - it is best to install machines of group B. For the kitchen and utility unit, for garages and workshops, machines with characteristic C are usually installed - where there are sufficiently powerful transformers, electric motors, there are also starting currents. Group D machines should be installed where there is equipment with difficult starting conditions - conveyors, elevators, lifts, machine tools, etc.

Look at the following picture, very similar in meaning to the previous one; here you can see the spread of thermal protection parameters of circuit breakers:

Notice the two numbers at the top of the graph. These are very important numbers. 1.13 is the multiplicity below which no serviceable machine will ever work. 1.45 is the multiplicity at which any working machine is guaranteed to work. What do they actually mean? Let's look at an example. Let's take a 10A machine. If we pass a current of 11.3A or less through it, it will never turn off. If we increase the current to 12, 13 or 14 A, our machine may turn off after some time, or it may not turn off at all. And only when the current exceeds 14.5A can we guarantee that the machine will turn off. How fast depends on the specific instance. For example, with a current of 15A, the response time can range from 40 seconds to 5 minutes. Therefore, when someone complains that his 16-amp circuit breaker does not work at 20 amperes, he does it in vain - the circuit breaker is absolutely not obliged to work at such a multiplicity. Moreover, these graphs and figures are normalized for an ambient temperature of 30°C; at lower temperatures the graph shifts to the right, at higher temperatures - to the left.

Current limiting class

Let's move on. An electromagnetic release, although called instantaneous, also has a certain response time, which reflects such a parameter as the limitation class. It is indicated by one number and for many models this number can be found on the device body. Basically, machines with current limiting class 3 are now produced - this means that from the time the current reaches the response value until the circuit is completely broken, no more than 1/3 of the half-cycle will pass. With our standard frequency of 50 Hertz, this turns out to be about 3.3 milliseconds. Class 2 corresponds to a value of 1/2 (about 5 ms), there are probably others, but I am not aware of their existence. According to some sources, the absence of marking of this parameter is equivalent to class 1. I would call this parameter not a current limiting class, but a cutoff speed. It would seem that the faster, the better. In fact, sometimes it makes sense to install a machine with a slower response - this applies to group machines, so that during a short circuit on some outgoing line they do not trip together with the machine of this line, i.e. so that there is selectivity. Although there is no guarantee that a machine with a smaller class will work slower than a machine with a larger class. Therefore, build selectivity based on this parameter, I wouldn’t, and there are no official recommendations about this.

Maximum breaking current

A very important parameter is the maximum shutdown current. This parameter largely reflects the quality of the power part of the machine. Usually in the retail network we are offered machines with a shutdown current of up to 4.5 or 6 kA. Sometimes you come across cheap models with a breaking capacity of 3 kA. And although in living conditions The short-circuit current rarely reaches such values; nevertheless, I do not recommend using circuit breakers with a breaking capacity of less than 4.5 kA. Because if the breaking capacity is small, then we should expect smaller area contacts, worse arc chutes, etc.

Where to buy machines?

It is usually not a problem to buy a circuit breaker with characteristic C - they are presented in sufficient assortment in construction and hardware stores and markets. Machines with characteristics B and D are also found in these places, but quite rarely. They can be ordered from companies or small specialized stores. Or you can buy it in the ABC-electro online store. This store has almost all machines of all denominations and characteristics. It’s nice that there are not only the usual ratings of 6, 10, 16, 25, but also 8, 13, 20 Amperes, which are often not enough to ensure good selectivity.

Dependence of response on ambient temperature

Another point that is often forgotten is the dependence of the thermal protection of the machine on the ambient temperature. And it is very significant. When the machine and the protected line are in the same room, it’s usually okay: as the temperature drops, the sensitivity of the machine decreases, but the load capacity of the wire increases, and the balance is more or less maintained. Problems can arise when the wire is warm and the machine is cold. Therefore, if such a situation occurs, then an appropriate amendment must be made. Examples of such dependencies are shown in the graph below. More accurate information on a specific model should be found in the manufacturer's data sheet.

Number of poles of the machine. Series and parallel connection of poles and circuit breakers

The machine can have from 1 to 4 poles. Each pole has its own thermal and electromagnetic release. When one of them is triggered, all poles are switched off simultaneously. It is also possible to turn on only all poles together with one common handle. There is another type of slot machine - the so-called 1p+n. This machine synchronously switches 2 wires: phase and neutral, but it has only one release - only on the phase contact. When the release is triggered, both contacts open. Despite the fact that 2 wires pass through such a machine, it is not considered two-pole.

Can poles be connected in parallel or in series? Can. But you need to have good reasons for this. For example, when disconnecting an inductive load or simply in cases of overload or short circuit - that is, when a large current has to be broken, an electric arc occurs. To break it, there are arc-extinguishing chambers, but still this does not pass without a trace - the contacts may burn, soot may appear. If we connect the poles in series, the arc will be divided between them, it will be extinguished faster, and there will be less wear on the contacts. The disadvantages of this method include increased losses - after all, there is some kind of voltage drop on the contacts, and the higher the current, the more power is lost on them (usually several watts at currents of 10-100A, usually the manufacturer includes this information in the passport) . Parallel connection of poles is usually used when there is no machine of the required rating, but there is a machine of a lower rating, but with “extra” poles. In this case, usually, to calculate the total rated current, it is recommended to multiply the rated current of one pole by 1.6 for 2 parallel poles, for 3 parallel poles by 2.2, for 4 parallel poles by 2.8. Perhaps in some emergency cases this is a way out, but at the first opportunity you need to replace such a surrogate with a machine of the required denomination.

The situation is even more complicated when connecting machines in parallel and in series. Of course, you can come up with a situation and somehow even justify the parallel connection of two or more machines, but I would not recommend even considering this option. How the currents will be distributed, what will happen after one of the machines is turned off - all this is doubtful and difficult to predict. It makes more sense to turn on the machines sequentially. For example, this can be considered as increasing the reliability of protection: if one of the machines malfunctions, the other will cover it. But usually they don’t do this, and a group machine is considered as insurance. In addition, the circuit breaker itself consumes a certain amount of electricity, so an additional circuit breaker also means additional losses.

Circuit breaker power dissipation

As an example, I will give the passport values ​​of this parameter for VA 47-63 automatic machines (the values ​​are given for new automatic machines at current values ​​equal to the rated one):

As you can see, the circuit breaker also wants to eat. Therefore, you should not get carried away and stick machine guns wherever possible. Where do the losses occur? The main part falls on the thermal release. But there is no need to overdramatize the situation. These losses are proportional to the current flowing. Therefore, if, for example, the load is 2 times less than the rated load, then the losses will be correspondingly half as much, and if there is no load, there will be no losses. If they are presented as a percentage, then the values ​​will be on the order of 0.05-0.5%, with the smallest percentage for the most powerful machines. In the contacts themselves, while the machine is new, losses are insignificant. But during operation, the contacts will burn out, the contact resistance will increase, and with it the losses will increase. Therefore, with an old machine, losses may be noticeably greater. By the way, measuring losses is quite simple - you need to measure the voltage drop across the machine and the current passing through it. At home, I do this using this very inexpensive device that combines a multimeter and a clamp meter:

Yes - cheap Chinese consumer goods, but quite suitable for household purposes.

Selecting a machine based on load power (current)

Although the main purpose of the machine is to protect electrical wiring, under certain conditions it is advisable to calculate the machine based on the load current. This is possible in cases where the line extending from the machine is intended to power one specific electrical appliance. In household networks, this could be an electric stove or air conditioner, some kind of machine, electric boiler, etc. As a rule, we know the rated current of an electrical appliance, or we can calculate it by knowing the load power. Since the wiring is selected with a certain margin, in this case the rating of the machine is usually less than what we would get by calculating by permissible current wires. Therefore, in case of any short circuits inside the electrical device or its overload, our protection will work, protecting it from further destruction.

Selecting a machine for an electric drive (electric motor, solenoid valve, etc.)

If the load in the circuit is an electric motor, then you need to remember that the starting current of the motor is several times higher than the rated current, so in this case you need to use machines with characteristic C, and in some cases (non-household) even D. We select the rating of the machine according to the rated current of the motor . It can be read on the plate or measured with the aforementioned pliers. You need to measure the current with a loaded engine, don’t forget. It is clear that the machine cannot exactly match the motor current; choose the closest value. Some manufacturers claim machines with special characteristics, especially for electric motors. Although, upon closer examination, these characteristics are usually somewhere between C and D. Of course, such an automatic machine will not protect the engine properly and, if, for example, the shaft jams, the following will happen: the cutoff will not work, because the current will not be higher than the starting current, and the thermal protection may not be in time - overheating of the windings in the motor occurs very quickly. Therefore, the electric motor needs additional protection in the form of a special high-speed thermal (or electronic) relay. The same rules should be followed when choosing a machine for electromagnetic drive(various valves, curtains, etc.).

Circuit breaker manufacturers

Large machines are a separate topic; here we consider manufacturers exclusively in the context of modular products. In the post-Soviet space, brands such as ABB, Legrand, Shneider Electric have proven themselves well. Usually the products of these companies will be recommended to you when you ask for something more reliable. From Russian manufacturers, quite decent devices are made by KEAZ, Kontaktor, DEKraft. IEK received the most unflattering reviews - probably rightly so, although they are perhaps the most popular on sale due to their low price.

The fuse is electrical appliance, providing protection of the electrical network from emergency situations associated with the current parameters (current, voltage) going beyond the specified limits. The simplest fuse is a fuse link.

This is a device connected in series to the protected circuit. As soon as the current in the circuit exceeds a predetermined one, the wire melts, the contact opens, and the protected section of the circuit thus remains undamaged. The disadvantage of this method of protection is that the protective device is disposable. Burnt out - needs to be replaced.

Circuit breaker device

A similar problem is solved using so-called circuit breakers (AB). Unlike disposable fuses, automatic machines are quite complex devices; when choosing them, several parameters should be taken into account.

They are also connected in series in the circuit. When the current increases, the circuit breaker breaks the circuit. Circuit breakers are produced in a wide variety of design and with various parameters. The most common machines today are those for mounting on a DIN rail (Fig. 1).

AP-50 assault rifles (Fig. 3-5) and many others are widely known from Soviet times. The machines are produced with the number of poles (lines for connection) from one to four. At the same time, two- and four-pole circuit breakers can include not only protected, but also unprotected contact groups, which are usually used to break the neutral.

Composition and structure of AB

Most circuit breakers include:

• manual control mechanism (used to manually turn the machine on and off);
• switching device (set of moving and fixed contacts);
• arc extinguishing devices (grid of steel plates);
• releases.

Arc extinguishing devices provide extinguishing and blowing of the arc, which is formed when the contacts through which the overcurrent passes are opened (Fig. 2)

Release - a device (part of a machine or additional device), mechanically connected to the AB mechanism and ensuring the opening of its contacts.

The circuit breaker usually contains two releases.

The first release - reacts to long-term, but small network overload (thermal release). Usually this device is based on a bimetallic plate, which, under the influence of a current passing through it, gradually heats up and changes its configuration. Eventually she presses down on the retaining mechanism, which releases and opens the spring-loaded contact.

The second release is the so-called “electromagnetic” one. It provides a quick response of the AV to a short circuit. Structurally, this release is a solenoid, inside the coil of which there is a spring-loaded core with a pin that rests on a movable power contact.

The winding is connected in series. During a short circuit, the current in it increases sharply, due to which the magnetic flux increases. In this case, the resistance of the spring is overcome, and the core opens the contact.

AB parameters

The first parameter is the rated voltage. Automatic machines are produced for direct current only and for alternating and direct current. DC circuit breakers for general use quite rare. In household and industrial networks AVs are mainly used for alternating and direct current. Most often, AVs with a rated voltage of 400V, 50Hz are used.

The second parameter is the rated current (In). This is the operating current that the machine passes through itself in a long-term mode. The usual range of ratings (in amperes) is 6-10-16-20-25-32-40-50-63.

The third parameter is the breaking capacity, the ultimate switching capacity (UCC). This is the maximum short circuit current at which the machine can open the circuit without being destroyed. The usual series of PKS passport values ​​(in kiloamperes) is 4.5-6-10. At a voltage of 220 V, this corresponds to a network resistance (R=U/I) of 0.049 Ohm, 0.037 Ohm, 0.022 Ohm.

As a rule, the resistance of household electrical wires can reach 0.5 Ohm; a short circuit current of 10 kA is possible only in the immediate vicinity of an electrical substation. Therefore, the most common PKS are 4.5 or 6 kA. Circuit breakers with PKS 10 kA are used mainly in industrial networks.

The fourth parameter characterizing the AB is the setting current (setting) of the thermal release. This parameter for various machines ranges from 1.13 to 1.45 of the rated current. We noted that when the rated current passes, long-term operation of the circuit with AV is guaranteed.

The setting of the thermal release is greater than the nominal value; it is the actual current reaching the set value that will cause the machine to turn off. It should be noted that automatic machines of the Soviet period provide for manual adjustment of the thermal protection setting (Fig. 5). Access to the adjusting screw is not possible in machines installed on a DIN rail.

The fifth parameter of the circuit breaker is the setting current of the electromagnetic release. This parameter determines the multiple of excess of the rated current at which the AV will operate almost instantly, reacting to a short circuit.

An important characteristic of the machine is the dependence of the response time on the current (Fig. 6). This dependence consists of two zones. The first is the area of ​​responsibility of thermal protection. Its peculiarity is a gradual decrease in the time it takes for the current to pass before tripping. This is understandable - the higher the current, the faster the bimetallic plate heats up and the contact opens.

If the current is very high (short circuit), the electromagnetic release is triggered almost instantly (within 5–20 ms). This is the second zone on our chart.

According to the setting of the electromagnetic release, all automatic machines are divided into several types:

• A Primarily for protection electronic circuits and long chains;
• B For conventional lighting circuits;
• C For circuits with moderate starting currents (motors and transformers of household appliances);
• D For circuits with large inductive loads, for industrial electric motors;
• K For inductive loads;
• Z For electronic devices.

The most common are B, C and D.

Characteristic B - used for general purpose networks, especially where it is necessary to ensure selectivity of protection. The electromagnetic release is configured to operate at a current ratio of 3 to 5 relative to the nominal value.

When connecting purely active loads (incandescent light bulbs, heaters...), the starting currents are almost equal to the operating currents. However, when connecting electric motors (even refrigerators and vacuum cleaners), the starting currents can be significant and cause false operation of the machine with the characteristic in question.

The most common are machines with characteristic C. They are quite sensitive, and at the same time do not give false positives when starting engines of household appliances. Such a switch operates at 5-10 times the nominal value. Such machines are considered universal and are used everywhere, including industrial facilities.

Characteristic D is the setting of the electromagnetic release for 10 - 14 current ratings. Typically such values ​​are needed when using asynchronous motors. As a rule, circuit breakers with characteristic D are used in a three- or four-pole design to protect industrial networks.

At sharing circuit breakers, you need to have an idea of ​​the concept of selective protection. The construction of selective protection ensures that circuit breakers located closer to the accident site are triggered, while more powerful circuit breakers located closer to the voltage source should not operate. To achieve this, more sensitive and fast-acting machines are installed closer to consumers.

Good day, dear friends!

Today I will continue to talk about circuit breakers in the light of measuring the resistance of the phase-zero loop.

In the last article devoted to measuring the resistance of the phase-zero loop, I mentioned the time-current characteristics of circuit breakers. Today I will give as an example the following characteristics for an assault rifle of the VA47-29 type:

Each circuit breaker has its own characteristic. Usually it is given in the passport for the machine in the form shown in the figure. Those. there is some variation in the parameters. As you can see, this spread is quite large.

For characteristic “B”, the cut-off current (current of the electromagnetic release) can be in the range from 3In to 5In;

For characteristic “C” - from 5In to 10In;

For characteristic “D” - from 10In to 14In.

This means that the short circuit current measured or calculated by us for a specific line may either satisfy the parameters of the circuit breaker (be sufficient to turn it off) or not.

The real characteristic of the dependence of the response time of a circuit breaker on the current flowing through it for each specific machine can only be obtained by checking the parameters of this machine.

But many laboratories do not have the equipment to test circuit breakers. and accordingly, they do not have this type of work. They do it simply. To check the compliance of the circuit breaker with the line parameters (possible short circuit current), use upper value cut-off current, i.e. for characteristic “C” it ​​is 10In. This approach is quite justified, because the machine will probably turn off at a current greater than the possible tripping current of the release, but in some cases it is not reliable enough. Because if the measured short circuit current is less than 10In, then, of course, if the line wires are in good condition, it is necessary to replace the circuit breaker with a suitable one. Although, when checking the circuit breaker, it may become clear. that its operation current is, for example, 7In and in this case, even with the short-circuit current we measured, the machine should reliably turn off, i.e. There was no need to replace the machine.

Let's return to the time-current characteristic. Let's say we checked the machine and, based on the measured parameters, obtained its individual characteristics (displayed by the green line in the figure).

What does it give us?

According to the PUE clause 1.7.79, the time of automatic power off in the TN system should not exceed 0.4 s at phase voltage 220V, but in circuits supplying distribution, group, floor and other switchboards and shields, the shutdown time should not exceed 5 s.

Thus, we have two points on the characteristic 0.4s and 5s. Depending on the installation location of the circuit breaker, we determine which point we need and find the tripping (shutdown) current of the circuit breaker at this point.

From the characteristics we received (green line) we can see that the machine will turn off in 0.4 s at seven times the rated current, and in 5 s at a current of 4.5 In.

I'll answer again frequently asked question: Why measure the resistance of the phase-zero loop?

Knowing the resistance of the phase-zero loop of a circuit (line), you can find the short circuit current that can develop in this line. And knowing this current, you can answer the question: will the circuit breaker installed in this line work and in what time?

That's all for today. If you have any questions, please ask.

Circuit breakers are commonly used to protect household electrical circuits. modular design. Compactness, ease of installation and replacement, if necessary, explains their wide distribution.

Externally, such a machine is a body made of heat-resistant plastic. On the front surface there is an on/off handle, on the back there is a latch for mounting on a DIN rail, and on the top and bottom there are screw terminals. In this article we will look at.

How does a circuit breaker work?

In normal operation mode, a current flows through the machine that is less than or equal to the rated value. The supply voltage from the external network is supplied to the upper terminal connected to the fixed contact. From the fixed contact, current flows to the movable contact closed with it, and from it, through a flexible copper conductor, to the solenoid coil. After the solenoid, the current is supplied to the thermal release and after it to the lower terminal, with the load network connected to it.

In emergency modes, the circuit breaker disconnects the protected circuit by triggering a free tripping mechanism driven by a thermal or electromagnetic release. The reason for this operation is an overload or a short circuit.

Thermal release is a bimetallic plate consisting of two layers of alloys with different coefficients of thermal expansion. When an electric current passes, the plate heats up and bends towards the layer with a lower coefficient of thermal expansion. When the specified current value is exceeded, the bending of the plate reaches a value sufficient to activate the release mechanism, and the circuit opens, cutting off the protected load.

Electromagnetic release consists of a solenoid with a movable steel core held by a spring. When the specified current value is exceeded, according to the law of electromagnetic induction, an electromagnetic field is induced in the coil, under the influence of which the core is drawn into the solenoid coil, overcoming the resistance of the spring, and triggers the release mechanism. In normal operation, a magnetic field is also induced in the coil, but its strength is not enough to overcome the resistance of the spring and retract the core.

How does the machine work in overload mode?

An overload mode occurs when the current in the circuit connected to the circuit breaker exceeds the rated value for which the circuit breaker is designed. In this case, the increased current passing through the thermal release causes an increase in the temperature of the bimetallic plate and, accordingly, an increase in its bending until the release mechanism is activated. The machine turns off and opens the circuit.

The thermal protection does not operate instantly, since it will take some time for the bimetallic strip to warm up. This time can vary depending on the magnitude of the excess current from a few seconds to an hour.

This delay allows you to avoid power outages during random and short-term increases in current in the circuit (for example, when turning on electric motors that have high starting currents).

The minimum current value at which the thermal release must operate is set using an adjusting screw at the manufacturer. Typically this value is 1.13-1.45 times higher than the denomination indicated on the machine’s labeling.

The magnitude of the current at which the thermal protection will operate is also affected by the ambient temperature. In a hot room, the bimetallic strip will warm up and bend until it triggers at a lower current. And in rooms with low temperatures the current at which the thermal release will operate may be higher than permissible.

The reason for network overload is the connection to it of consumers whose total power exceeds the calculated power of the protected network. Simultaneous inclusion of various types of powerful household appliances (air conditioner, electric stove, washing and Dishwasher, iron, electric kettle, etc.) - may well lead to the operation of the thermal release.

In this case, decide which consumers can be disabled. And don’t rush to turn on the machine again. You still won't be able to cock it in working position until it cools down and the bimetallic plate of the release returns to its original state. Now you know during overloads

How does a machine operate in short circuit mode?

In case of a short circuit it is different. During a short circuit, the current in the circuit increases sharply and many times to values ​​that can melt the wiring, or rather the insulation of the electrical wiring. In order to prevent such a development of events, it is necessary to immediately break the chain. This is exactly how an electromagnetic release works.

The electromagnetic release is a solenoid coil containing a steel core held in a fixed position by a spring.

A multiple increase in the current in the solenoid winding, which occurs during a short circuit in the circuit, leads to a proportional increase in the magnetic flux, under the influence of which the core is drawn into the solenoid coil, overcoming the resistance of the spring, and presses the release bar of the release mechanism. The power contacts of the machine open, interrupting the power supply to the emergency section of the circuit.

Thus, the operation of the electromagnetic release protects the electrical wiring, the closed electrical appliance and the machine itself from fire and destruction. Its response time is about 0.02 seconds, and the electrical wiring does not have time to warm up to dangerous temperatures.

At the moment the power contacts of the machine open, when a large current passes through them, an electric arc appears between them, the temperature of which can reach 3000 degrees.

To protect the contacts and other parts of the machine from the destructive effects of this arc, an arc-extinguishing chamber is provided in the design of the machine. The arc chamber is a grid of a set of metal plates that are insulated from each other.

An arc occurs at the point where the contact opens, and then one of its ends moves along with the movable contact, and the second slides first along the fixed contact, and then along the conductor connected to it, leading to back wall arc extinguishing chamber.

There it divides (splits) on the plates of the arc-extinguishing chamber, weakens and goes out. At the bottom of the machine there are special openings for the removal of gases formed during arc combustion.

If the machine turns off when the electromagnetic release is triggered, you will not be able to use electricity until you find and eliminate the cause of the short circuit. Most likely the cause is a malfunction of one of the consumers.

Disconnect all consumers and try to turn on the machine. If you succeed and the machine does not kick out, it means that one of the consumers is indeed to blame and you just have to find out which one. If the machine breaks down again even with the consumers disconnected, then everything is much more complicated, and we are dealing with a breakdown of the wiring insulation. We'll have to look for where this happened.

This is how it is in various emergency situations.

If tripping your circuit breaker has become an issue for you constant problem, do not try to solve it by installing a machine with a high rated current.

The machines are installed taking into account the cross-section of your wiring, and, therefore, higher current It's simply not allowed on your network. A solution to the problem can only be found after a complete inspection of your home’s electrical system by professionals.

Similar materials on the site:

Introduction

1. Circuit breakers

2. Circuit breakers with thermal releases

3. Automatic circuit breakers with combined releases

Bibliography

Introduction

Currently, to protect networks and electrical receivers from damage caused by current exceeding the permissible value, circuit breakers are increasingly used. They serve for conducting, switching and automatic opening electrical circuits during abnormal phenomena (for example, overload currents, short circuits, unacceptable voltage drops), as well as for infrequent switching on of circuits manually. Switches are produced with thermal, electromagnetic and combined (thermal and electromagnetic) releases with different numbers of poles - one, two and three. In single-phase circuits, one- and two-pole ones are used, and in three-phase circuits, three-pole ones are used.

1. Circuit breakers

Automatic circuit breakers with electromagnetic releases are used to protect the network and electrical receiver from damage caused by short-circuit current, even for a short time. The schematic diagram of such a switch is shown in Fig. 1, a.

The main circuit contact is closed by pressing a button or turning a handle. In this case, the force of the opening spring is overcome and the contact is held in the closed position by latch 3. As soon as the current in the protected circuit exceeds a certain value, core 6 will be drawn into coil 5 and through lever 4 will release latch 5. Under the action of spring 1, contact 2 will open. The diagram shows one contact of the main circuit, but practically there can be two or three of them, and there can be the same number of coils 5 with cores 6. When retracted, all cores act on the same latch 3. Increasing the current in any wire (coil) to a value exceeding the setting value of the operating current entails the opening of all main contacts.

An electromagnet with a tripping mechanism is called an electromagnetic release. The shutdown time of circuit breakers with electromagnetic releases is insignificant (fractions of a second), so they are classified as instantaneous maximum protection devices.

The advantage of circuit breakers over fuses is that they have multiple functions. After the fuse has tripped, the fuse link must be replaced. After eliminating the cause of the operation, the circuit breaker can be prepared for repeated operation by pressing a button or turning the handle.

Automatic switches are used not only to turn off receivers during short circuit currents, but also to turn them on and off manually infrequently during normal operation. The electric arc that occurs when the circuit is opened is extinguished in air or oil. Depending on this, circuit breakers are called air or oil circuit breakers. In circuits with voltages up to 500 V, air circuit breakers are mainly used.

2. Circuit breakers with thermal releases

Metals have different coefficients of linear expansion and therefore elongate differently when heated. If two metal plates with different expansion coefficients are placed on top of each other and firmly connected together, a bimetallic plate is obtained. When heated, it is deformed with a convexity towards the active metal layer. An active metal layer is one that has a high expansion coefficient. The other layer is called passive. The active layer is made of steel, and the passive layer is made of invar (an alloy consisting of 64% iron and 36% nickel). The coefficient of linear expansion of Invar is 12 times less than steel.

If one end of the bimetallic plate is fixed, then the other, when heated, will bend towards the passive layer. This property of the plate is used to release the circuit breaker latch. The degree of deformation of the plate depends on its heating temperature.

Two methods of heating the plate are used: direct and indirect. In the first, the current passes directly through the plate. In this case, the amount of heat that is released in it is proportional to the square of the current, its passage time and the resistance of the plate. In the second method, the current passes through a heating element (small spiral) made of nichrome or other alloy. The spiral is placed next to the plate or wound on it. The heat released in this spiral heats the bimetallic plate. Before winding the spiral, the bimetallic strip is coated with electrical insulation, such as mica.

Figure 1.6 shows a circuit diagram of a circuit breaker with a thermal release. Contact 2 of the main circuit is closed manually with a button or handle, and in the closed position it is held by latch 3. When a current passes through the network, the value of which is less than a certain value, the bimetallic plate 7 heats up slightly, and its upward bending is not enough to transfer force to latch 3 If a current passes through spiral 8, the magnitude of which exceeds this certain value, then after some time the right end of plate 7 will bend upward so much that through pusher 4 it will raise the latch lever 3. Under the action of spring 1, contact 2 will open. the contact will open, depending on the degree of network overload. Thermal releases cannot operate instantly, especially when the bimetallic strip is indirectly heated. Heating and deformation do not occur instantly even with a very large heat release in the spiral.

Automatic circuit breakers with thermal releases disconnect the network with a time delay in inverse proportion to the magnitude of the overload current. At higher overloads, shutdown occurs faster. The diagram shows one switch contact, but there may be two or three.