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Molded Case Circuit Breaker knowlege

Date:2019-05-22   Click:2829  

The circuit breaker is a mechanical opening-closing device, which is used for closing, breaking,

separating circuit and carrying current of that circuit under ordinary conditions and for automatically breaking the circuit under extraordinary conditions like short circuit and over


current

Operating Principle of the Circuit Breaker:

The most important function of the circuit breaker, in addition to opening-closing the circuit, is to protect the circuit under extraordinary conditions. 

There are some units inside the device to let the breaker fulfill its protection functions. Opening units of LV circuit breakers are described as release mechanism in TS EN 60947-2 standard

 

Releases:

Over current releases (Over current opening unit)

Under voltage releases (Low voltage opening unit)

Shunt Trip releases (Remote release unit)

 

All the circuit breakers are equipped with over current releases. However, under voltage and shunt trip-release coil is not a standard accessory and added to the circuit breaker as per requirement

 

Over Current Option:

All the values exceeding rated current value are called over current

 

Formation of Over Current:

Over currents in electrical circuits result from increase of power expended or a short circuit. Both over currents are very dangerous for electrical devices. Over currents lead to thermal and dynamic forcing in electrical circuit.

Although over currents, which are a result of increase in power expended, are not usually too high, they can go up to (2-3) time more than the rated current

Currents resulting from short circuit depend on characteristic of the electrical circuit. For example, they can go up to 3,2 kA in a transformer of 100 kVA; or 60kA In a transformer of 2500 kVA Electrical devices such as transformer, generator, motor, cable etc. have a thermal forcing value 12 to resist without damage due to the heat caused by over current. As it can be seen in the formula, both current value and current delay time is very important. In order to keep I².t value under a particular value, flow duration of the current should decrease as the current increases. LV circuit breakers open the circuit below I².t value of the protected device to provide safe protection (Figure-1)

 

Over Current Release are divided into two:

1. Releases opened under over load conditions,

2. Releases opened under short circuit Conditions

 

Releases opened under over load conditions: These are the releases that

operate when the current expended in the circuit exceeds the rated current value

of the breaker. They operate on reverse time delay basis. As current value increases, opening duration decreases.

Releases opened under short circuit conditions:

These are the releases that open the circuit in a very short time when the short

circuit current exceeds the adjustment value of the release

Under voltage Releases:

Voltage going below a particular value in electrical circuits or failure of any phase in tri-phase circuits may result in failure of devices. For example, failure of any phase in tri-phase motor shall overload other phases and result in failure of the motor. When required, low voltage coil can be assembled to the breaker to prevent occurrence of such failures. As under voltage coil Is usually supplied by two phases, control of other phase is performed by a contactor  (Figure-2)

Shunt Trip Releases

They are used for remote-release of the circuit breaker. When a voltage is applied to a shunt trip release, opening should be made up to 70% and 110% of the supply voltage

TYPES OF CIRCUIT BREAKERS:

LV circuit breakers are manufactured in two different types depending on the release type. These are thermal-magnetic and electronic circuit breakers

Thermal-Magnetic Circuit Breakers:

Thermal protection function, (1,1-3) x In:(For protection under over load conditions) 

Bimetal,  which provides thermal protection, consists of combination of two metals with different extension coefficients under heat. When bimetal is heated, it bends towards the metal with less extension. In this way, a notch that assists opening of the breaker mechanism is released to disable the breaker. Bending speed of bimetal is in direct proportion with size of the current passing through the breaker. Because, increase of current means increase of heat. In this way, over current protection function of the breaker is fulfilled by bimetal at load currents higher than the rated current

 

 

 

 

 

 

 

 

Magnetic protection function, >3 x In (For protection under short circuit

conditions)

Another function of the breaker is to protect the connected circuit against short circuits. Short circuit may occur as a result of contact of phases with each other or contact of phase-ground. Since a very high current shall pass through the cables in case of short circuit, system energy should be broken in a shorter time due to thermal protection. Breaker should perform instant opening to protect load it is connected to. The part fulfilling this function is a mechanical opening

mechanism that operates with magnetization caused by the magnetic

area formed by the short circuit current

Circuit Breakers with Electronic Over Current Release:

The feature discriminating electronic circuit breakers from thermal-magnetic

breakers is to control the over current releases with electronic is circuit. Electronic is

control is performed via microprocessor.During design of the electronic circuit,

worst possibilities to encounter in operation have been taken into consideration. In high circuit currents, direct opening has been ensured without operating electronic circuit. In this way, possibility of failure in the electronic circuit has been eliminated. DADA electronic circuit breakers can be connected to computer through RS-232 protocol upon request. In this way;

-Computer memory can be used Instead of current recording devices

-Maximum, minimum, average etc. Values of the drawn current at various time intervals (day-night) can be taken

-Statistical information can be accessed any time

-Opening period of the breaker can be adjusted in case of over current formation

-Rated current and instant opening current of the breaker can be changed on computer.

External opening control can be provided

Rated and Instant opening current adjustment areas of electronic circuit breakers are quite wide. This feature allows wide use opportunity to the breaker Furthermore, electronic circuit breakers are not affected from ambient temperatures.

Operating principle of limiter circuit breaker:

While breaker is opened and closed with lever, moving contact should be in ON

position in pos-1, in OFF position in pos-3 (Figure3). The short circuit current that comes into existence when there is a short circuit in a breaking current without limiter opens the breaker by enabling the breaking mechanism via releases and takes breaking lever to trip position

Opening duration varies between 10-20ms. In DADA limiter breakers, reverse magnetic area where short circuit occurs takes moving contact from pos-1 to pos-2 and contact remains In this position. That is, contact does not come to ON position again. Opening of the moving contact starts with the first millisecond of the short circuit. The contact arrives pos-2 in the first two milliseconds and complete cut-off of the arc lasts in 3-5 milliseconds maximum. Magnetic releases, which get into operation with start of the short circuit, take the breaking mechanism to  OFF position; the mechanism takes the moving contact in pos-2 to pos-3 and the breaking lever

remains in trip position. The current, which takes the moving contact from pos-1 to pos-2, is a lower current than the expected short circuit current. Limited current is at one-eighth and even one-tenth of the expected current  (Figure- 4), The expected short circuit current would flow

In a shorter time than the current if there was no limiter circuit breaker

Advantages of limiter circuit breaker:

-They protect transformers, cables and other devices in circuit by limiting the

current up to 90% depending on the breaker type.

-As explosions and arcs remain at a very low level, critical safety is guaranteed in order not to give damage to other devices in the panel

PARTS OF CIRCUIT  BREAKERS

Body and Cover: Fiber-glass polyester resin has been used as the body and cover material in accordance with EN 60512-20-2 standard. This material, which is called BMC (Bulk Molding Compound) in the literature, is preferred due to high electrical and mechanical values and can

resist to a temperature of 160℃continuously. BMC material does not burn when in contact with wire at a temperature of 960℃ in accordance with IEC 695-2-1

Bimetal: Bimetal is a material consisting of combination of two plate metals with different extension coefficients against heat. The current passing through the breaker heats up bimetal. Due to effect of this heat. bimetal bends towards the less-extending plate. Since heat increases as the current passing through the breaker increases, bimetal is heated more and bends more. In this way, it controls the opening mechanism to open the breaker.

Contacts: Contact alloy is determined for breakers by considering broken and carried current values and construction Usually silver, graphite, nickel, wolfram alloy contacts are used in breakers Contacts, which are made of silver-graphite alloys with a smoothers structure, are used in fixed (bottom) contacts, silver wolfram contacts, which are harder, are used in moving (top) contacts. A swaged structure has been ensured in moving contacts. In this way, swaged and hard alloy contacts have a place on soft fixed contacts in each opening-closing. In this way, the lowest resistance is ensured.Moving contact should touch the fixed contact very well in order to have low contact resistance. However, excessive contact pressure force results in damage of contacts in a shorter period than normal.Contact alloys are very important for a healthy opening-closing

Arc Extinguisher Cell (Separator):

Separators are used to extinct the arc which is formed during operation of the breaker operating under energy. While moving contact is separated from fixed contact, current continues to flow between contacts for a while and this is called arc.This arc should be extinct in a very short time

Extinction of Arc:

Arc is pushed towards separators due to magnetic field formed around the arc. In this way, arc is extended and becomes slim and broken off between separator plates (Figure-5). Due to characteristic of the material used on side walls of the separators, a gas comes out due to high temperature caused by the arc. This gas has an important effect on extinction of the arc.

 Utilization Type of the Circuit Breaker:

There are 3 positions indicating position of the breaker. These positions are shown in Figure-6

ON/I Position: It indicates that contacts of the breaker are closed. In this position, the breaker lever is in the top position

TRIP Position: It indicates that the breaker is opened due to any failure (over load or short circuit). In this case, breaker lever is in the middle position between ON and OFF positions. In order to take the breaker, which is in trip position, to ON position; push the breaker lever downwards as shown by the OFF sign

Breaker shall be set with "click" sound.After that, pull the lever as shown by ON sign to close the breaker 

OFF/0 Position: It indicates that contacts of the breaker are open. In this way, the breaker lever is in the bottom position.

Assembly: Important considerations during assembly are listed below.

The place to assemble the breaker should be free of dust and moisture.

—Breaker should be assembled in a way not to be subject to gas and vapor.If the environment is dusty and moist, the breaker must be assembled in a housing with appropriate protection degree.

While the breaker is in operation, it should not be exposed to vibration and sudden impacts

Minimum distances between two breakers assembled one on another should be as shown  in  Figure -7.

Minimum distances between grounded or insulated wall and the breaker should be as shown in Figure -8

Assembly method of the connectors (for F31 and F51 type switches) vary according to connection at the front or at the back. Connector may be demounted, reversed and mounted again if required.

Cable connections of measurement devices should be made through busbars, no connection should be made through terminals of the breaker (Please request extension busbars from factor for connection to be made with cable shoes.)

End insert should be used in connections of multi -wire cables to breaker connector and no brazing should be made at cable ends

一in connection is made to the breaker via copper busbars, busbars should be painted and feather edges should be rounded to minimize the risk of jumping.

Phase curtains must be placed in the conduit between two busbars in the breaker body

  Grounding should be made in accordance with the regulations.

 

 

 

 

 

 

 

 

EARTH - LEAKAGE CIRCUIT BREAKER (ELCB)

Against earth leakage currents resulting from the low voltage circuits, with combination of fault current sensor relay and toroidal transformer are detected and the protection can be achieved by controlling of shunt trip coil or under voltage release coil on the mounted circuit breakers, such as protection can be achieved by residual current protected type circuit breakers

Toroidal transformer, sensors relay and shunt trip are placed into circuit breakers .Without the need any external accessory connector can be Installed only by connecting the input and output terminals .For leakage current protection selectivity, the leakage current threshold and leakage.current time delay can be set by user. There is test button for leakage current protection function as separately from trip test button. In this way, the earth leakage protection function can also be tested.Earth-leakage circuit breakers have also high thermal-magnetic protection like as

our other compact type circuit breakers.

.Led indicating neutral is connected

.Thermal and magnetic protection on 4th pole(neutral). (optional)

.Available for auxiliary contact connection

Protection System Against Earth Leakage Protection Relay

Even small values (>30mA) of ground fault currents to occur in electrical circuits are quite dangerous in terms of safety of life and fire. As normal breakers cannot detect such small earth leakage protection relay, no additional protection is provided against ground leakages. Earth leakage protection relay can be added to electronic breakers without an additional mechanism. With this system, protection at (0,11)x ln sensitivity can be provided Protection against earth leakage in non-electronic breakers and electronic breakers needing protection against leakage currents lower than the value mentioned above is provided with combination of toroidal transformer and leakage current detection relay .However, in this system, in order to let the circuit breaker open in ground leakage currents. remote-release cull or low voltage coil should have already been mounted to the  breaker (Figure-9) Fault current of the fault current detection relay should be adjusted according to protection type and appropriate values to ensure selectivity among other detection relays. According to the standards, this values has been determined as 30mA for life protection and (300-500)mA for fire protection

Assembly:

All the phases and neutral cable, if any, shall pass through the toroidal transformer .Ground cable should not pass through the toroidal. Secondary cables of toroidal shall be connected to ground leakage detection relay (Z1-Z2) terminals and appropriate voltage written on the relay is supplied to energy Input terminals of the relay. Remote-release coil or low voltage coil must have been connected to the breaker in order to open the circuit breaker due to ground fault. If remote - release coil has been connected to the circuit breaker, energy supplied to opening coil should be supplied through open contact of the ground leakage detection coil normally (Figure-10). If low voltage coil has been connected to the circuit breaker, energy supplied to the low voltage coil should be supplied through upper part of the energy breaker and normally closed contact of the ground leakage detection relay (Figure-11)

 

 

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