A circuit breaker is a device that, in the event of specified conditions, terminates the load supply circuit. The equipment is geographically included in the distribution panel. The goal is to turn off the load in an emergency, plus the possibility of de-energizing a part of the home network( for example, required for repair or maintenance work).
The history and design of automatic circuit breakers
The first references to switches that can function automatically are given by Thomas Edison in 1879.The task of the devices was to de-energize the circuits consisting of incandescent bulbs, in the event of a short circuit or abnormal situations. However, commercial versions of technical solutions were deprived of this innovation, and the first analogues of modern models were patented much later. Swiss firm Brown, Boveri &Sy did it in 1924.People today enjoy the company's products under the brand ABB.
Initially, the principle of operation of circuit breakers was based on the use of magnetothermal releases. From the first days spark ignition devices were introduced. Necessary step - typical contactors produced an arc when triggered. This created interference and led to the rapid failure of the circuit breakers themselves. To block the effect began to use compressed air and oil. Vacuum or rarefied gas is often used as the medium for producing the arc. In these conditions, the burning is not long.
switch As for the simpler models, the spark cameras help to solve the complexity. They consist of a variety of insulated copper plates and are located crossing the arc path. As a result, the discharge energy is lost on these improvised capacitors. Spark extinction methods are divided into categories:
- Deviation of the arc path, extension of the path.
- Splitting a discharge into several parts( for example, the camera discussed above).
- Break of contacts at the moment of alternating current zero crossing.
- The use of large capacitors for the storage of spark energy.
The magnetothermal release is considered to be the main component of most switches, simultaneously solving two tasks:
- The thermal part, based on a bimetallic relay, is responsible for switching off for a slow overheating for a long time. Suppose the instruction says that when the current exceeds the rated value by 45%, the switch will operate after 1 hour. This is the thermal( bimetallic) part of the device. Slowly and surely, a plate of two metals is heated to a temperature of operation.
- The electromagnetic part is activated when a strong overload occurs on the line. For example, short circuit. Then a large power passes through the switch and it is necessary to quickly open the contacts to block the occurrence of an electric arc( the faster the distance between the contacts increases, the weaker the negative effect).The control of the moving part is performed through an electromagnetic coil. If an emergency situation is threatened, it instantly clicks off the switch, an electric arc does not occur.
Note that in the first case there is no large current, and the bimetallic relay becomes a passive device that does not require external power. Similar technical solutions are applied everywhere. Directly in a similar form: as part of start-up relays of refrigerators, inside irons, heaters. Properties of bimetallic plates are used in electric kettles. This is a temperature sensor that responds to changes in environmental conditions. Try to heat the bimetallic plate with a match, and it will click off, as if the current is exceeding the allowable one by a given amount. Inertial mechanism, ideal for tracking slow changes.
The electromagnetic part consists of a solenoid, the winding of which is connected in series with the load. With a sharp increase in voltage, a powerful magnetic flux is formed between the turns, a jerk retractor rod with a contact at the end. The threshold is set by the class of the circuit breaker. Easier to demonstrate by example. In most of the brochures that advertise the properties of protective automats, there is a specific type of schedule. It is characterized by the presence of a vertical part, this is shown a segment of the action of the electromagnetic releaser.
Breaker class, time-current characteristics
Horizontally, the time-current characteristic of a circuit breaker postpones the ratio of current to nominal. Vertical time stamped complete break of the chain. The position of the vertical section of the graph gives reason to judge the class of the circuit breaker. For example, for B is the area from 3 to 5, for C - from 5 to 10, for D - from 10 to 20. It is easier to draw the illustration on the multi-colored graphics, and from the manual it is located slightly to the left, in black and white. If you look closely, it is clear that the example was borrowed from class D. From this characteristic, it is permissible to judge the purpose of the device. For example:
- Class B with a response threshold of 3 to 5 nominal values is suitable for resistive loads. Lighting, heaters.
- For inductive-capacitive load, a circuit breaker class C with a response threshold of up to 10 nominal current values is required. This includes all types of motors, including asynchronous and collector motors. Think of a class C, if the house has a vacuum cleaner, a washing machine, a construction tool.
- Class D is used for coarse circuits with high consumption: production areas of workshops with an abundance of engines of primarily asynchronous type.
- Class Z with a threshold of 2 - 3 is used mainly for electronics.
Other specific types are known. A, B, C and D are considered basic. In prices, letters refer to the type of instantaneous( electromagnetic) release, and then everyone chooses them according to their own needs. For a single rated current, the manufacturer presents a series of models at once( each has its own class).The response time is constant, only the threshold varies. The question is important and for some reason rarely discussed within the advertising campaigns of specific manufacturers. According to the modest conviction of the authors, the knowledge about classes is professional. It is believed that the person ordering the equipment is already in the know.
Periodically there are directories without indications of the class of circuit breakers. In this case, it is necessary to focus on the ratio of the rated and causing device tripping currents. They are listed in the tables, the manufacturer believes that the classiness becomes an extra parameter.
Varieties of automatic circuit breakers
The main distinction is made according to the number of phases. This is irrelevant for standard apartment models, gaining importance in industry. Often, if a single phase falls out, on the other hand, consumption increases. Formed skew leading to equipment failure. A three-phase circuit breaker breaks power to all outputs at once. It is irreplaceable with three ordinary ones for 220 V.
The tripping currents are selected according to the class of the release, but in individual devices it is possible to separately configure the option. For example, 3RV10 / 3RV11 circuit breakers( Siemens catalogs) are set to a tripping current 13 times the rated one. This deliberately overlaps the needs of starting most engines. If the consumer is dissatisfied with such characteristics, it is possible to change the parameters in the right direction.
Often among the parameters of the circuit breakers is found the maximum breaking capacity. Let us explain this figure with a simple example. Do not confuse it with the release current. The breaking capacity describes a terrible accident, when the current not only reached the threshold value, but repeatedly exceeded the limit. For example, a standard situation is considered when 10.5 A flows in a circuit. At the same time, the rated current is only 2.5 A. Therefore, the circuit breaker belongs to class B( 10.5 / 2.5 = 4.2).The breaking capacity may be, for example, 50 kA.
This is the current at which the device can still perform its duties. Does not melt, does not burn, does not short circuit tightly. If the short-circuit current exceeds the breaking capacity, the manufacturer withdraws the warranty. The task of the designer is to avoid this situation in principle. Make it simple - you need to take care that the resistance of the cables does not become too low. It becomes a current limiting factor. For example, tens of thousands of amps will never appear in a 220 V circuit. Otherwise, a reduction in the active resistance of the cables to 4.4 mΩ is required.
This is an extremely small value. For comparison, according to industry standards, the resistance of the ground circuit should not exceed 3 - 5 Ohms, which is three orders of magnitude higher than the specified figure. Manufacturers make devices with a giant stock. This also applies to the service life. A typical value is 10,000 switching cycles - 10,000 abnormal situations. It is clear that the figure is unattainable with reasonable operation of the home network. From the above, the main parameter of the circuit breaker is the rated current. But when exceeding the value of instantaneous shutdown does not occur.
The circuit breaker will continue to operate. And in order to follow the further course of events, you need to use the performance data. For example, based on the figure. Depending on the curvature, it is found that when the nominal current is exceeded by 13%, the circuit breaker will work for a couple of hours. Sometimes this information is placed in a table of characteristics in order to emphasize the specified point. This is discussed separately, the data directly affect the behavior of the circuit.
Erase when selecting the characteristics of circuit breakers:
- Limit operating temperature. It is clear that to place the frame is already, and the cost is lower than for use in outdoor conditions.
- Sometimes you need to know the degree of protection of the enclosure by IP class. This is explained by the prescriptions of the standards.
- Exterior performance is typical. More often the case under DIN-rail, allowing to put the device in a standard distribution box.
- Often the manufacturer cites the value of the internal resistance of the device. This parameter is indirectly linked to the breaking capacity and rated voltage( Ohm’s law).Resistance shows how much active power is released inside the case when current flows.
- Voltage frequency plays a much less frequent role. In industry, 400 Hz and other values are often used. Switches made to such requirements are not always suitable for an ordinary apartment.