What Is a Thermal Circuit Breaker?

A circuit breaker is a switching device capable of closing, carrying and breaking the current under normal circuit conditions and closing, carrying and breaking the current under abnormal circuit conditions within a specified time. Circuit breakers are divided into high-voltage circuit breakers and low-voltage circuit breakers according to their scope of use.

Circuit breakers are generally composed of contact systems, arc extinguishing systems, operating mechanisms,

The characteristics of the circuit breaker are: rated voltage Ue; rated current In; tripping current setting range of overload protection (Ir or Irth) and short circuit protection (Im); rated short circuit breaking current (industrial circuit breaker Icu; household circuit breaker Icn) )Wait.

Rated operating voltage (Ue): This is the voltage at which the circuit breaker operates under normal (uninterrupted) conditions.

Rated current (In): This is the maximum current that the circuit breaker equipped with a special overcurrent trip relay can withstand under the ambient temperature specified by the manufacturer, and will not exceed the temperature limit specified by the current bearing component.

Short-circuit relay trip current setting value (Im): The short-circuit trip relay (instantaneous or short delay) is used to quickly trip the circuit breaker when a high fault current value occurs, and its trip limit Im.

Rated short-circuit breaking capacity (Icu or Icn): The rated short-circuit breaking current of the circuit breaker is the highest (expected) current value that the circuit breaker can break without being damaged. The current value provided in the standard is the root mean square value of the AC component of the fault current. When calculating the standard value, the DC transient component (which always appears in the worst case short circuit) is assumed to be zero. Industrial circuit breaker ratings (Icu) and household circuit breaker ratings (Icn) are usually given in kA rms.

Short-circuit breaking capacity (Ics): The rated breaking capacity of the circuit breaker is divided into two types: rated limit short-circuit breaking capacity and rated operating short-circuit breaking capacity. The national standard "Low Voltage Switchgear and Control Equipment Low Voltage Circuit Breakers" (GB14048.2-94) explains the rated limit short-circuit breaking capacity and rated operating short-circuit breaking capacity of the circuit breaker as follows:

1. The rated short-circuit breaking capacity of the circuit breaker: According to the conditions specified in the prescribed experimental procedure, the breaking capacity of the circuit breaker to continue to carry its rated current capacity is not included;

2. The rated short-circuit breaking capacity of the circuit breaker: according to the conditions specified in the prescribed experimental procedures, including the breaking capacity of the circuit breaker to continue to carry its rated current capacity;

3. The test procedure for rated ultimate short-circuit breaking capacity is O-t-CO.

The specific test is: adjust the line current to the expected short-circuit current value (such as 380V, 50kA), and the test button is not closed, the circuit breaker under test is in the closed position, press the test button, the circuit breaker passes the 50kA short-circuit current, The circuit breaker opens immediately (open for short), the circuit breaker should be intact and can be closed again. t is the intermittent time, generally 3 minutes, at this time the line is still in the hot standby state, the circuit breaker is once again closed (C for short) and the next open (O), Electrical and thermal stability at current). This procedure is called CO. If the circuit breaker can be completely opened, its limit short-circuit breaking capacity is qualified.

4. The test procedure for the rated operating short-circuit breaking capacity (Icn) of the circuit breaker is O-t-CO-t-CO. It has one more CO than Icn's test procedure. After the test, the circuit breaker can completely break and extinguish the arc, and its rated short-circuit breaking capacity is deemed to be qualified.

Therefore, it can be seen that the rated limit short-circuit breaking capacity Icn refers to the low-voltage circuit breaker that can operate normally after breaking the maximum three-phase short-circuit current at the outlet end of the circuit breaker and break this short-circuit current once again. The circuit breaker is not guaranteed to make and break; the rated operating short-circuit breaking capacity Ics means that the circuit breaker can be normally opened multiple times when the maximum three-phase short-circuit current at its outgoing end occurs.

The IEC947-2 "Low-voltage switchgear and control equipment low-voltage circuit breakers" standard stipulates that the Ics of Class A circuit breakers (referring to circuit breakers with only overload and long delay and short-circuit transient) can be 25%, 50%, 75% and 100%. The Ics of Class B circuit breakers (three-stage protection circuit breakers with overload long delay, short circuit short delay, and short-circuit transient) can be 50%, 75%, and 100% of Ics. Therefore, it can be seen that the rated short-circuit breaking capacity is a breaking current value smaller than the rated limit short-circuit breaking current. ^[1]

Divided by the number of poles: unipolar, dipolar, tripolar, and quadrupole;

According to the installation method: there are plug-in, fixed and drawer.

Product introduction A9 / EC65 miniature circuit breaker: lighting distribution circuit, short circuit and overload protection.

A9LE / EPNLE earth leakage circuit breaker: short circuit overload and earth leakage protection in grounding system. Normal operation fault protection of the circuit breaker keeps the phase line and the neutral line in the disconnected state when the circuit breaker is in the disconnected position, to avoid being charged when the neutral line fails. When making and breaking operations, the neutral line is given priority and the breaking lags. With short circuit current limiting function, high rated short circuit breaking capacity. With overload protection short circuit leakage and voltage protection device, complete protection functions, convenient and reliable wiring. ^[2]

Circuit breakers are connected in front of the board, behind the board, plug-in type, and drawer type. If the user has no special requirements, they are supplied in front of the board. The wiring in front of the board is a common wiring method.

1. Wiring method behind the board: The biggest feature of the wiring behind the board is that the circuit breaker can be replaced or repaired without re-wiring, just disconnecting the front-end power supply. Due to the special structure, the product has been equipped with a special mounting plate and mounting screws and wiring screws according to the design requirements. It should be noted that the reliability of the large-capacity circuit breaker will directly affect the normal use of the circuit breaker. Attention must be paid to installation in strict accordance with the requirements of the manufacturer.

2. Plug-in wiring: On the mounting plate of the complete set of equipment, first install a mounting base for the circuit breaker, 6 plugs on the mounting base, and 6 sockets on the connection plate of the circuit breaker. There are connecting plates on the surface of the mounting base or bolts behind the mounting base. The mounting base is pre-connected with power and load wires. When in use, insert the circuit breaker directly into the mounting base. If the circuit breaker is broken, just pull out the bad one and replace it with a good one. Its replacement time is shorter than the wiring before and after the board, and it is convenient. Because plugging and unplugging requires a certain amount of manpower. Therefore, China's plug-in products are limited to a maximum frame current of 400A. This saves repair and replacement time. When installing a plug-in circuit breaker, check whether the plug of the circuit breaker is tight and securely tighten the circuit breaker to reduce contact resistance and improve reliability.

3. Drawer type wiring: The circuit breaker's in and out drawers are rotated clockwise or counterclockwise by the rocker, and both the main circuit and the secondary circuit are used.

The plug-in structure omits the isolator necessary for the fixed type, achieving one machine and two uses, improving the economical use, and bringing great convenience to operation and maintenance, and increasing safety and reliability. In particular, the main circuit contact tool holder of the drawer base can be used with the NT type fuse breaker contact tool holder. ^[1]

Ambient air temperature: upper limit of ambient air temperature + 40 ° C; lower limit of ambient air temperature -5 ° C; the average value of ambient air temperature for 24h does not exceed + 35 ° C

Altitude: The altitude of the installation site does not exceed 2000m.

Atmospheric conditions: the relative humidity of the atmosphere does not exceed 50% when the ambient air temperature is + 40 ° C; it can have a higher relative humidity at a lower temperature; the monthly maximum relative humidity of the wettest month is 90% The average monthly minimum temperature is + 25 ° C, taking into account the condensation that occurs on the product surface due to temperature changes.

Pollution level: Pollution level is 3.

Control loop:

1. It should be able to monitor the integrity of the control circuit protection device and its trip and closing circuits to ensure the normal operation of the circuit breaker;

2. It should be able to indicate the normal closing and opening position of the circuit breaker, and there should be obvious indication signals during automatic closing and automatic tripping;

3. After closing and tripping is completed, the command pulse should be released, that is, the power supply for closing or tripping can be cut off;

4. When there is no mechanical anti-jump device, an electric anti-jump device should be installed;

5. The accident trip signal circuit of the circuit breaker should be wired according to the "non-corresponding principle";

6. For equipment that may appear abnormal working conditions or failures, a notice signal should be installed;

7. The power supply of the spring operating mechanism and manual operating mechanism can be DC or AC. The power source of the electromagnetic operating mechanism requires DC.

The world's earliest circuit breaker was produced in 1885, it is a combination of knife opening and overcurrent release. In 1905, an air circuit breaker with a free trip device was born. Since 1930, with the advancement of science and technology, the discovery of the arc principle and the invention of various arc-extinguishing devices have gradually formed institutions. At the end of the 1950s, due to the rise of electronic components, electronic trip units were produced. Today, due to the popularity of single-chip microcomputers, intelligent circuit breakers have come out.

Common are low-voltage circuit breakers and vacuum interrupters:

Low-voltage circuit breakers are electrical appliances used for on-off, control or protection functions in circuits with AC voltage 1200V and DC voltage 1500V. Low-voltage circuit breakers are an important part of the electrical industry. They are basic supporting parts in the machinery industry. In the power distribution system, low-voltage circuit breakers are mainly composed of various low-voltage circuit breaker components. The functions and performance of low-voltage circuit breakers Equipment plays a vital role. More than 80% of the power generated by power generation equipment is distributed and used by low-voltage circuit breakers. For each additional 10,000 kW power generation equipment, about 20,000 pieces of various low-voltage circuit breakers are required to be matched with it. In the industrial automation system, various control panels, consoles, controllers and other products composed of low-voltage circuit breakers are also required. Since 1949, China's low-voltage circuit breaker industry has gradually developed into an industry capable of independent design and production on the basis of repairing and assembling simple electrical factories. By 1979, there were more than 600 manufacturing enterprises. After 1985-1986, 1990- Two development peaks in 1991, and in 1995 there were already about 1,500 manufacturers in the low-voltage circuit breaker industry.

China's low-voltage circuit breaker manufacturing enterprises are mainly concentrated in Beijing, Tianjin, Liaoning, Shanghai, Jiangsu, Zhejiang, Guangdong and other places. While promoting the development of the national economy, many problems have been exposed. There are two main points:

1. The scale of the enterprise is small and too many. Among the low-voltage circuit breaker manufacturers in China, there are only two to four large enterprises with annual sales revenue and total assets of more than 500 million yuan, and most of them are small and medium-sized enterprises, leading to a lack of economies of scale and competitiveness; and The number of device manufacturing enterprises has grown from the early days of the founding of the People s Republic of China to more than 1,500 today. The excessive number of enterprises has led to excessive dispersion of economic resources and lack of overall innovation momentum, leading to low production efficiency, economic benefits and market competitiveness.

2. Regional structures are converging, and repeated construction is serious. China's low-voltage electrical appliances industry has been blindly involved in projects and paving, and regional industrial convergence is serious. Low-level repeated construction has resulted in overproduction of products, low utilization of energy and raw materials, low economic benefits, regional protection, and vicious competition.

Advances in vacuum circuit breaker technology. Advances in vacuum circuit breaker technology are manifested in large capacity, low overvoltage, intelligence, and miniaturization. And this progress is the result of the development of vacuum technology, arc extinguishing chamber technology and the use of new processes, new materials and new operating technologies. According to the inventor, in addition to being a substitute for traditional motor technology, this technology will also expand the development and application space for DC motors. For example, the development of large-capacity DC motors to replace the AC synchronous generators and converter station equipment powered by the high-voltage DC transmission grid can not only save the construction cost of a large number of converter stations, but also greatly reduce the power loss. ^[3]