Selection and tuning of circuit breakers

The selection of circuit breakers should be based on the specific use conditions to select the use category, select the rated working voltage, rated current, tripper setting current and other parameters, refer to the protection characteristic curve provided by the product sample to select the protection characteristics, and need to verify the short-circuit characteristics and sensitivity coefficient.

Classification of circuit breakers

(1) Frame type circuit breaker (ACB)

Frame circuit breaker, all its parts are installed in an insulated metal frame, often open, can be installed with a variety of accessories, it is more convenient to replace contacts and components, and is mostly used in the main switch at the power end. There are several types of overcurrent trippers, such as electromagnetic, electronic and intelligent trippers. The circuit breaker has four stages of protection: long delay, short delay, instantaneous and ground fault, and the setting value of each protection is adjusted within a certain range according to its shell level.

The frame circuit breaker is suitable for AC 50Hz, rated voltage 380V, 660V, rated current of 200A-6300A in the distribution network, mainly used to distribute electric energy and protect lines and power equipment from overload, undervoltage, short circuit, single-phase grounding and other faults. Under normal conditions, it can be used for infrequent conversion of lines. Circuit breakers below 1250A can be used to protect the overload and short circuit of the motor in the network with AC 50Hz voltage and 380V.

Frame circuit breakers are also often used in transformer 400V side outlet main switches, bus contact switches, high-capacity feeder switches and large motor control switches.

(2) Molded Case Circuit Breaker (MCCB)

The molded case circuit breaker, also known as the device circuit breaker, is housed in a plastic shell with the outer contact of the grounding wire terminal, the arc extinguisher, the tripper and the operating mechanism. Auxiliary contacts, undervoltage trippers and shunt trippers are mostly modular, very compact in structure, generally do not consider maintenance, and are suitable for protection switches used as branches. Molded case circuit breakers usually contain a thermomagnetic trip unit, while larger molded case circuit breakers are equipped with a solid-state trip sensor.

There are two kinds of overcurrent trippers of molded case circuit breakers: electromagnetic and electronic, and the general electromagnetic molded case circuit breaker is a non-selective circuit breaker, with only two protection methods: long delay and instantaneous; The electronic molded case circuit breaker has four protection functions: long delay, short delay, instantaneous and ground fault. Some of the new products of electronic molded case circuit breakers also have a region-selective interlocking function.

Molded case circuit breakers are generally used for distribution feeder control and protection, low-voltage side outlet main switch of small distribution transformer, power distribution terminal control, and can also be used for power switch of various production machinery.

(3) Miniature Circuit Breaker (MCB)

Miniature circuit breaker is a kind of terminal protection appliance widely used in building electrical terminal power distribution devices. It is used for short-circuit, overload, overvoltage and other protection of single-phase and three-phase below 12, including single-pole 1P, two-pole 2P, three-pole 3P, and four-pole 4P.

The miniature circuit breaker is composed of an operating mechanism, a contact point, a protective device (various trippers), an arc extinguishing system, etc. The main contact is either manually operated or electrically closed. After the main contact is closed, the free trip mechanism locks the main contact in the closing position. The coil of the overcurrent tripper and the thermal element of the thermal tripper are connected in series with the main circuit, and the coil of the undervoltage tripper and the power supply are connected in parallel.

In the electrical design of civil buildings, miniature circuit breakers are mainly used for overload, short circuit, overcurrent, loss of voltage, undervoltage, grounding, leakage, automatic switching of dual power supplies and protection and operation of infrequent start of the motor.

Basic characteristics of circuit breaker:

(1) Rated working voltage Ue

The rated operating voltage refers to the nominal voltage of the circuit breaker, which can be operated continuously under the specified normal use and performance conditions.

China stipulates that at the voltage level of 220kV and below, 1.15 times of the rated voltage of the system is the high working voltage, and the voltage level of 330kV and above is 1.1 times of the rated voltage as the high working voltage. The circuit breaker can maintain insulation at the high operating voltage of the system, and can be closed and broken under specified conditions.

(2) Rated current In

The rated current refers to the current that can pass through the tripper for a long time at an ambient temperature of 40°C or less. In the case of circuit breakers with adjustable trippers, this is the current that can be passed through the trippers for a long time.

When used at an ambient temperature of more than 40°C, but not higher than 60°C, it is allowed to work with a reduced load for a long time.

(3) The current setting value of the overload tripper is Ir

If the current exceeds the current setting value of the tripper Ir, the circuit breaker will trip with a delay. It also represents the current that the circuit breaker can withstand if it does not trip. This value must be greater than the load current Ib, but less than the allowable current Iz of the line.

The thermal release relay Ir can usually be adjusted in the range of 0.7 - 1.0 In, but if electronic devices are used, the adjustment range is even larger, usually 0.4 - 1.0 In. For circuit breakers equipped with non-adjustable overcurrent off relays, Ir=In.

(4) The current setting value of the short-circuit tripper is Im

Short-circuit trip relays (transient or short-delayed) are used to trip the circuit breaker quickly in the presence of high fault current values with a trip threshold of Im.

(5) Rated short-term withstand current Icw

Refers to the value of the current that is allowed to pass through within an agreed time. This current value passes through the conductor within the agreed time and will not cause damage to the conductor due to overheating.

(6) Breaking capacity

The breaking capacity of the circuit breaker refers to the ability of the circuit breaker to safely cut off the fault current, which is not necessarily related to its rated current, and there are specifications such as 36KA and 50KA before. Generally, it is divided into limit short-circuit breaking capacity Icu and operating short-circuit breaking capacity Ics.

General principles for circuit breaker selection:

Firstly, the type and number of poles of the circuit breaker are selected according to the application; Select the rated current of the circuit breaker according to the high working current; Select the type of tripper, the type of accessory, and the size of the attachment as needed. The specific requirements are as follows.

1) The rated working voltage of the circuit breaker ≥ the rated voltage of the line.

2) The rated short-circuit on-off capacity of the circuit breaker calculates the load current ≥ the line.

3) The rated short-circuit on-off capacity of the circuit breaker ≥ the short-circuit current that may occur in the line (generally calculated according to the effective value).

4) The single-to-ground short-circuit current at the end of the line ≥ 1.25 times the instantaneous (or short-delay) tripping setting current of the circuit breaker.

5) The rated voltage of the circuit breaker undervoltage tripper is equal to the rated voltage of the line.

6) The rated voltage of the shunt tripper of the circuit breaker is equal to the control power supply voltage.

7) The rated working voltage of the electric transmission mechanism is equal to the control power supply voltage.

8) When the circuit breaker is used in the lighting circuit, the instantaneous setting current of the electromagnetic tripper is generally 6 times of the load current.

9) When the circuit breaker is used as the short-circuit protection of a single motor, the setting current of the instantaneous trip device is 1.35 times (DW series circuit breaker) or 1.7 times (DZ series circuit breaker) of the motor starting current.

10) When the circuit breaker is used as the short-circuit protection of multiple motors, the setting current of the instantaneous tripper is 1.3 times the starting current of one motor plus the working current of the rest of the motors.

11) When the circuit breaker is used as the main switch on the low-voltage side of the distribution transformer, its breaking capacity should be greater than the short-circuit current value of the low-voltage side of the transformer, the rated current of the tripper should not be less than the rated current of the transformer, and the setting current of short-circuit protection is generally 6-10 times of the rated current of the transformer; The setting current for overload protection is equal to the rated current of the transformer.

12) After the type and grade of the circuit breaker are preliminarily selected, it should be coordinated with the protection characteristics of the upper and lower switches to avoid skipping the level and expanding the scope of the accident.

Selectivity of the circuit breaker:

The circuit breakers used in the power distribution system can be divided into two categories: selective and non-selective according to their protection performance. Selective low-voltage circuit breaker with two-stage protection and three-stage protection. The instantaneous and short-delay characteristics are suitable for short-circuit operation, while the long-delay characteristics are suitable for overload protection. Non-selective circuit breakers are generally instantaneous and only used for short-circuit protection. Some of them are long-delay actions and are only used for load protection. In the power distribution system, if the upper circuit breaker adopts a selective circuit breaker, and the next circuit breaker adopts a non-selective circuit breaker or a selective circuit breaker, it is mainly to use the difference in the delay action or delay action time of the short delay trip device to obtain selectivity. When delaying the operation of the upper circuit breaker, please pay attention to the following issues.

1) Regardless of whether the next level is a selective circuit breaker or a non-selective circuit breaker, the setting current of the instantaneous overcurrent tripper of the upper circuit breaker shall generally not be less than 1.1 times of the large three-phase short-circuit current at the outlet end of the next level circuit breaker.

2) If the next stage is a non-selective circuit breaker, in order to prevent the short-circuit current from occurring in the circuit protected by the next level circuit breaker, the upper stage short-delay overcurrent tripper will act first because the instantaneous action sensitivity of this stage is not enough, so that it will lose its selectivity. Generally, the setting current of the short-delay overcurrent tripper of the upper circuit breaker is not less than 1.2 times that of the instantaneous overcurrent tripper of the next stage.

3) If the next stage is also a selective circuit breaker, in order to ensure selectivity, the short-delay action time of the higher-level circuit breaker is at least 0.1s longer than that of the next-level circuit breaker. Generally speaking, in order to ensure the selective action between the upper and lower low-voltage circuit breakers, the upper circuit breaker should choose an overcurrent tripper with a short delay, and its action current should be greater than the action current of the next level of overcurrent tripper, and at least the action current Iop.1 of the upper level is not less than 1.2 times of the action current Iop.2 of the next level, that is, Iop1≥1.2Iop.2.

Cascade protection of circuit breakers

In the design of the power distribution system, the selective coordination between the upper and lower stages of the circuit breaker must be "selective, fast and sensitive".

Selectivity is related to the coordination between the upper and lower circuit breakers, while rapidity and sensitivity are related to the characteristics of the protection appliance itself and the operation mode of the line, respectively.

If the upper and lower circuit breakers are properly coordinated, the fault circuit can be selectively removed to ensure that the other fault-free circuits of the power distribution system continue to work normally. On the contrary, it affects the reliability of the power distribution system.

Cascade protection is the specific application of the current limiting characteristics of the circuit breaker, and its main principle is to use the current limiting effect of the upper circuit breaker, and when selecting the lower circuit breaker, the circuit breaker with lower breaking capacity can be selected to achieve the purpose of reducing costs and saving costs. The upper current limiting circuit breaker QF1 can break the expected short-circuit current at its installation place, because the circuit breakers of the upper and lower levels in the power distribution system are installed in series, when a short circuit occurs at the outlet of the lower stage and the circuit breaker QF2, the actual value of the short-circuit current is much smaller than the expected short-circuit current at the upper level due to the current limiting effect of the upper circuit breaker QF1, that is to say, the breaking capacity of the lower circuit breaker QF2 is greatly enhanced with the help of the upper circuit breaker QF1, which exceeds its rated breaking capacity. There are also certain conditions for this cascade protection, such as the adjacent circuits must not have important loads (because once QF1 trips the QF3 circuit, there will also be a power outage), and the instantaneous setting value of QF1 and the instantaneous setting value of QF2 must also be properly matched. The cascade data can only be determined experimentally, and the mating selection of the upper and lower circuit breakers can only be determined by the circuit breaker manufacturer.

Selection and tuning of circuit breakers

Selection and tuning of circuit breakers

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