Mould Case Circuit Breaker (MCCB) is a mechanical device, capable of making, carrying

and breaking currents under normal circuit conditions and also making, carrying for a

specified time and breaking currents under specified abnomal circuit conditions such as

short-circuit and over-current.

Operating Principle of Mould Case Circuit Breaker (MCCB):

Most important function of the circuit breaker is the protection of the circuit at the abnormal

operating conditions besides making, carrying and breaking the currents. MCCBs are

equipped with some units to carry out protection function. Opening units of low voltage

circuit breakers are defined as a release according to EN/IEC60947-2 standards and CE

certificate.

Releases:

A release which permits a MCCB to open, without any intentional time delay.

• Overcurrent releases
• Undervoltage releases
• Shunt releases

All of the breakers are equipped with over current releases. Undervoltage and shunt

releases are not standard accessories, but they can be fixed on the MCCB by the request

of customer.

Overcurrent Releases:

General:

A release which causes a mechanical switching device to open with or without time-delay

when the current in the release exceeds a predetermined value.

How overcurrent occurs...?

Values of all currents which exceeds to the rated currents are defined as overcurrent.

Overcurrent occurs when there is an increase in the power, which is being absorbed from

the circuit or when a short circuit occurs. Both of the overcurrents are dangerous for

electrical circuits. Overcurrents cause thermal and dynamic forces in electrical circuits.

Overcurrents which occur with increasing the power can be 2 or 3 fold from rated current.

Due to short-circuit current, abnormally high current occurs according to feature of electrical

circuit. For example this can be 3.2kA at 100kVA transformer and 60kA at the 2500kVA

transformer.

Electrical devices such as transformers, generators, motors, capacitors, cables...etc. Have

a value of withstand against the thermal and dynamic forces due to when a short-circuit

occurs, the overcurrent passing through the circuit causes a temperature rise. The value of

current is so important as well as the period of time of current. When current builds up, it is

necessary to decrease the time of flow current in order to hold the I2t value less than

determineted value. Low voltage circuit breakers provide confident protection by opening

the circuit under the I2t value for device.

Short-circuit over current releases:

They are manufactured in two categories:

• With time delay current releases (Category A)
• Time-delay overcurrent releases (Category B)

Without time-delay overcurrent releases (Category A):

When the current passing through the circuit increases the value of determined current,

breaker rapidly opens the circuit. These kind of releases especially effect at high

overcurrent which occurs as a result of short-circuit. When a current builds-up, it is

necessary to decrease the breaking time of current around ms in order to hold I2t value low.

Time-delay overcurrent releases (Category B):

An over current release intended to operate at the end of time delay. The current passing

through the circuit reaches the value of adjusted short-circuit current, breaker doesn't open

rapidly. (Only after some time delay. In other terms they permitted the short-circuit current if

the time is not dangerous for equipment and cable.) This process is to be realized by

giving delayed signal the opening coil by means of electronic circuit at the electronic

breakers. Delay overcurrent releases are usually connected to main circuit at the beginning

of line for selectivity.

Undervoltage Releases:

A release which permits a mechanical switching device to open or close, with or without

time-delay, when the voltage across the terminals of the release falls below a

predetermined value. Whenever the voltage falls below a predetermined level or breaks

one of phases, undervoltage release mechanism (it trips) operates. When this condition

occurs, the circuit breaker trips and can not be energized until the power has been restored

to 85% of the rated voltage. The dropping or breaking of voltage causes many kind of

break-down. For example, if there is a disturbance of any one of the phases separately,

this will cause the other phases to overload and burn the motor. This kind of break-down is

prevented by installing undervoltage relay (customer supplied when required) in breaker.

Control of other phases have been done by using contactor so that as a general

undervoltage relay feeds from two phase.

Shunt Trip Releases:

As a definition, a release energized by a source of voltage. A shunt trip provides remote

controlled tripping of a circuit breaker. Remote device can be a pushbutton, cut off switch

or an other pilot device. Shunt releases trip consist of an intermittent rated selonoid tripping

device installed in the breaker on site or our factory (customer supplied when required) The

tripping device is energized by AC or DC voltage. When voltage is applied to shunt trip

release, it should trip between 70% and 110% of the feeding voltage. (According to

EN/IEC60947).

Types of MCCB:

Low voltage circuit breakers are manufactured as two types according to type of overcurrent releases:

• Thermal-Magnetic type MCCB
• Electronic type MCCB

Thermal-Magnetic MCCB

Thermal Protection:

For the protection between (1.1-3) x In overcurrent.

A bimetal consists of two strips of metal bonded together. Each strip has a different

thermal rate of heat expansion. Heat due to excessive current causes the bimetal to bend,

this causes the operating mechanism to trop, pushing the trip bar and unlatch the contact.

Bending of bimetal is direct proportional to the current which passes through the contacts

of the breaker. Building up current means increasing heat. By using this method thermal trip

action is achieved through the use of a bimetal heated by over load current which bigger

than perdetermined rated current. Thermal elements (bimetal) are calibrated and supplied

for each current rating by us.

Magnetic Protection:

For the protection over 3 x In short circuit.

Another function of the circuit breaker is to protect circuit against short-circuits. Short-circuit

occurs when different phase or phase-neutral is in series. When a short-circuit occurs, the

current passing through the circuit causes a magnetic field and magnetization due to

overcurrent. In that case the energy of system should be interrupted rapidly in comparision

with thermal protection. So that breaker instantly should be trip to protect the load.

Magnetic trip action is achieved by mechanical opening mechanism which operates by

magnetic field and magnetization. Magnetic elecments can be either fixed or adjustable by

us, depending upon the type of MCCB.

MCCB Material:

• Base, frame and cover:

Frame and cover are molded from glass-polyester material which is mentioned as BMC

(Bulk Moulding Compound) in literature that combines ruggedness, high dielectric strength

and high mechanical resistance in a compact design. Molded case circuit breaker (MCCB)

is designed and tested in compliance with IEC60947-2 Standard and can withstand up to

160℃ continuously.

• Bimetal:

A bimetal consists of two strips of metal bonded together. Each strip has a different

thermal rate of heat expension. Heat due to excessive current causes the bimetal to bend.

The bimetals having the greater rate of expansion will be on the outside (Longer boundary)

of the bend curve. To trips breaker, the bimetal must bend far enough to physically the trip

bar and unlatch the contacts.

• Contacts:

Contact alloy is determined by taking into consideration the value of current which breaking

and making in breaker. Generally, silver, carbon, nickel, wolfram alloys are used at breaker.

Silver, carbon alloys which constructed soft constraction are used on the constants (bottom)

contacts. Silver-wolfram contacts which constructed more stiff construction are used

moving (upper) contacts. Moving contacts are constructed as a convex construction. So

that convex and stiff alloy contacts set properly on the soft constant contacts at the every

opening and closing of breaker. So this provides the lowest passing withstand. Moving

contact touches properly on the constant contacts for low touching withstand. Excessive

contact press force causes the destruction of contacts at a short time. For this reason

contact alloy is so important for the suitable making and breaking.

• Arc Extinguisher:

The function of the arc extinguisher (separator) is the confine, divide and extinguish an arc.

During the moving contact separates from the constant contact, the current flows though air.

This is defined as an arc. These are should be extinguished simultaneously. How arc is

extinguished? When an interruption occurs and the contacts separate, the arc flows through

the ionized region between the contacts induces a magnetic field around the arc and arc

extinguisher. As the lines of magnetic flux, force drive the arc into the metal plates,

deionizing the gas dividing and cooling the arc.