How to select circuit breakers?

In the selection of circuit breakers, the use category should be selected according to the specific use conditions, and the parameters such as rated working voltage, rated current and setting current of tripping device should be selected. The protection characteristics should be selected with reference to the protection characteristic curve provided by product samples, and the short-circuit characteristics and sensitivity coefficient should be verified. Classification of circuit breakers (1) Frame circuit breaker (ACB)

Frame circuit breaker, also known as universal circuit breaker, all its parts are installed in an insulated metal frame, which is usually open, and can be equipped with various accessories. It is convenient to replace contacts and parts, and it is often used in the main switch at the power supply end. Over-current release includes electromagnetic, electronic and intelligent release. The circuit breaker has four-stage protection for 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 frame grade.

     (2) molded case circuit breaker (MCCB)

The molded case circuit breaker is also called the device circuit breaker, and the outer contact of the grounding wire terminal, the arc extinguishing chamber, the release and the operating mechanism are all installed in a plastic case. Auxiliary contacts, undervoltage release and shunt release are mostly modularized, and the structure is very compact. Generally, maintenance is not considered, so they are suitable for branch protection switches. Molded case circuit breakers usually contain thermomagnetic tripping units, while large-sized molded case circuit breakers are equipped with solid-state tripping sensors.

There are two kinds of over-current tripping devices for molded case circuit breakers: electromagnetic and electronic. Generally, electromagnetic molded case circuit breakers are non-selective circuit breakers with only two protection modes: long delay and instantaneous. Electronic molded case circuit breaker has four protection functions: long delay, short delay, instantaneous and ground fault. Some new products of electronic molded case circuit breakers also have regional selective interlocking function.  

Molded case circuit breakers are generally used for the control and protection of distribution feeders, the main switch of low-voltage side outgoing lines of small distribution transformers, the control of power distribution terminals, and also for the power switches of various production machinery.

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

Frame-type circuit breakers are also often used in the main outlet switch, bus contact switch, large-capacity feeder switch and large-scale motor control switch on the 400V side of transformer.

(3) Miniature Circuit Breaker (MCB)

Miniature circuit breaker is one of the most widely used terminal protection appliances in building electrical terminal distribution equipment. It is used for short circuit, overload and overvoltage protection of single-phase and three-phase below 125A, including unipolar 1P, bipolar 2P, tripolar 3P and quadrupole 4P.

Miniature circuit breaker is composed of operating mechanism, contacts, protective devices (various tripping devices) and arc extinguishing system. Its main contact is manually operated or electrically closed. After the main contact is closed, the free tripping mechanism locks the main contact in the closing position. The coil of the overcurrent release and the thermal element of the thermal release are connected in series with the main circuit, and the coil of the undervoltage release is connected in parallel with the power supply.

In the electrical design of civil buildings, miniature circuit breakers are mainly used for overload, short circuit, overcurrent, voltage loss, undervoltage, grounding, leakage, automatic switching of dual power supplies and protection and operation of motors when they are not frequently started.

Basic characteristic parameters of circuit breaker

(1) rated working voltage Ue

Rated working voltage refers to the nominal voltage of circuit breaker, which can run continuously under the specified normal use and performance conditions.

In China, the maximum working voltage is 1.15 times of the rated voltage of the system at the voltage level of 220kV and below. The voltage level of 330kV and above is 1.1 times the rated voltage as the highest working voltage. The circuit breaker can keep insulation under the highest working voltage of the system, and can be closed and opened according to the specified conditions.

(2) Rated current In

Rated current refers to the current that the release can pass for a long time when the ambient

temperature is below 40℃. For a circuit breaker with an adjustable release, it is the maximum current that the release can pass for a long time.

When the ambient temperature exceeds 40℃ but not higher than 60℃, it is allowed to work for a long time with reduced load.

(3) Current setting value Ir of overload release

The current exceeds the current setting value Ir of the trip, and the circuit breaker trips in time. It also represents the maximum current that the circuit breaker can withstand without tripping. This value must be greater than the maximum load current Ib, but less than the maximum current Iz allowed by the line.

The Ir of thermal trip relay can usually be adjusted in the range of 0.7 ~ 1.0 in, but if electronic equipment is used, its adjustment range will be larger, usually 0.4 ~ 1.0 in. For circuit breakers equipped with non-adjustable overcurrent tripping relay, Ir=In.

(4) Short-circuit trip current setting value Im

Short-circuit trip relay (instantaneous or short delay) is used to make the circuit breaker trip quickly when high fault current value appears, and its trip threshold is Im.

(5) rated short-time withstand current Icw

Refers to the current value allowed to pass within the agreed time. The current value passes through the conductor within the agreed time, and the conductor will not be damaged 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. There are 36KA, 50KA and other specifications before. Generally, it is divided into limit short-circuit breaking capacity Icu and running short-circuit breaking capacity Ics.

General principles of circuit breaker selection

Firstly, the type and pole number of circuit breaker are selected according to the purpose; Select the rated current of the circuit breaker according to the maximum working current; Select the type of release and the types and specifications of accessories as required. The specific requirements are as follows.

1) The rated working voltage of the circuit breaker is greater than or equal to the rated voltage of the line.

2) The rated short-circuit on-off capacity of the circuit breaker is greater than or equal to the line calculation load current.

3) The rated short-circuit on-off capacity of the circuit breaker is greater than or equal to the maximum short-circuit current that may occur in the line (generally calculated according to the effective value).  

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

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

6) The rated voltage of the shunt release 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 release is generally 6 times of the load current.  

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

10) When the circuit breaker is used as the short-circuit protection of multiple motors, the setting current of the instantaneous release is 1.3 times the starting current of the largest motor plus the working current of the rest 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 release 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 of overload protection is equal to the rated current of transformer.

12) After the type and grade of the circuit breaker are preliminarily selected, it is necessary to cooperate with the protective characteristics of the upper and lower switches to avoid leapfrog tripping and expand the accident scope.

Selectivity of circuit breaker

Circuit breakers used in power distribution system can be divided into selective and non-selective types according to their protection performance. There are two kinds of selective low-voltage circuit breakers: two-stage protection and three-stage protection. Among them, instantaneous characteristics and short delay characteristics are suitable for short-circuit action, while 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, which are only used for load protection. In the power distribution system, if the upper level circuit breaker adopts selective circuit breaker and the lower level circuit breaker adopts non-selective circuit breaker or selective circuit breaker, it is mainly to use the difference of delay action or delay action time of short delay release to obtain selectivity. Please pay attention to the following problems when passing through the delay action of the circuit breaker at the next higher level.

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

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

3) If the next stage is also a selective circuit breaker, in order to ensure selectivity, the short delay action time of the circuit breaker at the next stage is at least 0.1s longer than that of the circuit breaker at the next stage. Generally speaking, to ensure the selective action between the upper and lower low-voltage circuit breakers, the upper-level circuit breaker should choose an overcurrent

release with short delay, and its action current should be more than one level higher than that of the lower-level overcurrent release, and at least the action current Iop.1 of the upper-level circuit breaker should not be less than 1.2 times that of the lower-level circuit breaker, that is, iop1 ≥ 1.2 iop.2.

Cascade protection of circuit breaker

In the design of power distribution system, the selective cooperation between the upper and lower stages of circuit breakers must have "selectivity, rapidity and sensitivity".

Selectivity is related to the cooperation between the upper and lower circuit breakers, while rapidity and sensitivity are related to the characteristics of protective appliances and line operation mode respectively.

If the upper and lower circuit breakers are properly matched, the fault circuit can be selectively removed to ensure that other fault-free circuits in the distribution system continue to work normally. Otherwise, it will affect the reliability of the distribution system.

Cascade protection is a concrete application of current limiting characteristics of circuit breakers. Its main principle is to use the current limiting function of superior circuit breakers to select the circuit breakers with lower breaking capacity in order to reduce costs and save expenses.

The superior current-limiting circuit breaker QF1 can break the maximum expected short-circuit current at its installation place. Because the superior and subordinate circuit breakers are installed in series in the power distribution system, when a short circuit occurs at the outlet of the lower-level circuit breaker QF2, the actual value of the short-circuit current is far less than the expected short-circuit current there due to the current-limiting effect of the superior circuit breaker QF1. That is to say, the breaking capacity of the lower-level circuit breaker QF2 is greatly enhanced with the help of the superior circuit breaker QF1, which exceeds its rated breaking capacity. This kind of cascade protection also has certain conditions, such as that the adjacent circuits cannot have important loads (because once QF1 trips, the QF3 circuit will also lose power), and at the same time, the instantaneous setting value of QF1 and QF2 should be properly matched. Cascade data can only be determined by experiments, and the matching selection of upper and lower circuit breakers can only be determined by the circuit breaker manufacturer.

Sensitivity of circuit breaker

In order to ensure that the instantaneous or short-delay overcurrent release of circuit breaker can operate reliably when the slightest short-circuit fault occurs within its protection range under the minimum operation mode of the system. The sensitivity of circuit breaker protection must meet the requirement of Code for Design of Low Voltage Distribution (CB50054-95) that its sensitivity should not be less than 1.3, that is, Sp=lk.min/Iop≥1.3. Where lop is the action current of instantaneous or short-delay overcurrent release, Ik.Min is the single-phase short-circuit current or two-phase short-circuit current at the line end protected by circuit breaker under the minimum operation mode of the system, and Sp is the sensitivity of circuit breaker.

When selecting a circuit breaker, attention should also be paid to the verification of its sensitivity. For a selective circuit breaker with both short delay and instantaneous overcurrent release, only the action sensitivity of short delay overcurrent release needs to be verified, and the action sensitivity of instantaneous overcurrent release does not need to be verified.

Selection and setting of circuit breaker release

(1) Setting of action current of instantaneous overcurrent release

Among the objects protected by the circuit breaker, there are some electrical equipment, which will produce high peak current several times its rated current in a short time during the starting process, thus making the circuit breaker bear a large peak current in a short time. The action current lop(o) of the instantaneous overcurrent release must avoid the peak current Ipk of the line,

that is, IOP (o) ≥ krel ipk. Where Krel is the reliability coefficient. When selecting a circuit breaker, attention should be paid to avoiding the peak current of the instantaneous overcurrent release of the circuit breaker, so as to avoid the misoperation of the circuit breaker.

(2) Setting of action current and action time of short delay overcurrent release.

The action current lop(s) of the short delay overcurrent release should also avoid the peak current Ipk of the line, that is, IOP (s) ≥ krel ipk. Where Krel is the reliability coefficient. The action time of short-delay overcurrent release is generally divided into three types: 0.2s, 0.4s and 0.6s. It is determined according to the protection selectivity of the front and rear protection devices, and the action time of the former protection should be longer than that of the latter protection by a time difference.

(3) Setting of action current and action time of long-delay overcurrent release.

The long-delay overcurrent release is mainly used to protect overload, so its action current Iop(l) only needs to avoid the maximum load current of the line, that is, the calculated current I30, that is, IOP (L) ≥ Krel I30. Where Krel is the reliability coefficient. The action time of long-delay overcurrent release should avoid the duration of short-time overload, so as not to cause misoperation of low-voltage circuit breaker.

(4) The matching requirements between the action current of the overcurrent release and the protected line.

In order to prevent the insulated cable from overheating, damage or even fire caused by overload or short circuit, and the circuit breaker does not trip, the action current lop of the overcurrent release of the circuit breaker should meet the requirements of the formula, and lop≤Kol·Ial. Where Ial is the allowable current carrying capacity of insulated cable; Kol is the allowable short-time overload coefficient of insulated cable, and it is generally 4.5 for instantaneous and short-delay overcurrent release; For long-delay overcurrent release, take 1.1 for short-circuit protection and 1 for load protection only. If the above matching requirements are not met, the operating current of the release should be re-elected, or the cross-sectional area of the conductor or cable should be appropriately enlarged.