برق. قدرت. کنترل. الکترونیک. مخابرات. تاسیسات.

دایره المعارف تاسیسات برق (اطلاعات عمومی برق)

Introduction to MCB

The miniature circuit breaker (MCB) plays an important role in providing overcurrent protection and a disconnect means in electrical networks. Recent advancements in circuit breaker technology has increased breaker performance and protection.

A breaker is a device designed to isolate a circuit during an overcurrent event without the use of a fusible element. A breaker is a resettable protective device that protects against two types of overcurrent situations; Overload and Short Circuit.

MCB Construction Details

MCB - Miniature circuit breaker construction details

MCB - Miniature circuit breaker construction details

Thermal / Magnetic trip units

ABB Current Limiting Breakers use an electromechanical (Thermal /Magnetic) trip unit to open the breaker contacts during a overcurrent event. The thermal trip unit is temperature sensitive and the magnetic trip unit is current sensitive.

Both units act independently and mechanically with the breaker’s trip mechanism to open the breaker’s contacts.

Current Flow

MCB - Miniature circuit breaker current flow during operation

MCB - Current flow during operation (All highlighted components are energized during operation)

Overload protection

The thermal trip unit protects against a continuous overload. The thermal unit is comprised of a bimetal element located behind the circuit breaker trip bar and is part of the breaker’s current carrying path.

When there is an overload, the increased current flow heats the bimetal causing it to bend. As the bimetal bends it pulls the trip bar which opens the breaker’s contacts. The time required for the bimetal to bend and trip the breaker varies inversely with the current. Because of this, the tripping time becomes quicker as current increases in magnitude.Overload protection is applicable to any installation, conductor, or component which can be subjected to low-magnitude but longtime over-currents.

Low-magnitude, long-time over-currents can be dangerous because they reduce the life of the electrical installation, conductor, and components and if left unchecked could result in fire.

MCB - Miniature circuit breaker overload protection

MCB Overload protection

Magnetic trip units (short circuit protection)

The Magnetic trip unit protects against a short circuit. The magnetic trip unit is comprised of an electromagnet and an armature.

MCB - Miniature curcuit breaker magnetic trip units

MCB Magnetic trip units

Components of a magnetic trip unit

When there is a short circuit, a high magnitude of current passes through the coils creating a magnetic field that attracts the movable armature towards the fixed armature. The hammer trip is pushed against the movable contact and the contacts are opened. The opening of the breakers contacts during a short circuit is complete in .5 milli-seconds.

MCB - Miniature curcuit breaker components of a magnetic trip unit

MCB Components of a magnetic trip unit

Arc runners / Arc chutes

The arc runner and arc chute limit and dissipate the arc energy during the interruption of an overload or short circuit event.

During an overload or short circuit event, the contacts of the breaker separate and an electrical arc is formed between the contacts through air. The arc is moved into the arch chute by “running” the arc down the interior of the breaker along the arc runner. When the arc reaches the arc chute it is broken into small segmented arcs. The segmented arcs split the overall energy level into segments less than 25V.

Each 25V segment does not have a high enough energy level to maintain an arc and all energy is naturally dissipated.

MCB - Miniature curcuit breaker Arc runners / Arc chutes

MCB Arc runners / Arc chutes

Breaker curves

Thermal Trip Unit (region one)

The first sloping region of the breaker curve is a graphical representation of the tripping characteristics of the thermal trip unit.

This portion of the curve is sloped due to the nature of the thermal trip unit. The trip unit bends to trip the breaker’s trip bar in conjunction with a rise in amperage (temperature) over time. As the current on the circuit increases, the temperature rises, the faster the thermal element will trip.

Example using the curve below:

If you had a 10A breaker and the circuit was producing 30 amps of current, the breaker would trip between 2 seconds and 1 minute. In this example you would find the circuit current on the bottom of the graph (Multiples of rated current). The first line is 10 amps (10 amp breaker x a multiple of one), the second line is 20 amps (10 amp breaker x multiple of 2), and the third line is 30 amps (10amp breaker x multiple of 3). Next you would trace the vertical 30A line up until it intersects the red portion of the breaker thermal curve.

If you follow the horizontal lines, on both sides of the red curve, to the left you will see that the breaker can trip as fast as 2 seconds and no slower than 1 minute.


Magnetic Trip Unit (region two)

This region of the breaker curve is the instantaneous trip unit. ABB’s MCB – miniture circuit breaker’s instantaneous trip unit interrupts a short circuit in 2.3 to 2.5 milliseconds. Because of this the curve has no slope and is graphically represented as a vertical straight line.

Example using the curve above:

If you had a 10 amp breaker the magnetic trip element would interrupt a short circuit between 10 and 30 amps (10 amp breaker x multiple of 2 and 3) in 2.3 to 2.5 milliseconds.


Breaker Contacts (region three)

This region of the curve is the time required for the contacts of the breaker to begin to separate. The contacts will open in less than .5 milliseconds and is graphically represented by the bottom vertical portion of the curve.

MCB - Miniature curcuit breaker curves



http://electrical-engineering-portal.com/mcb-miniature-circuit-breaker-construction



MCB - Miniature curcuit breaker curves



Thermal-Magnetic Trip Unit

Thermal-Magnetic Trip Unit

In addition to providing a means to open and close its contacts manually, a circuit breaker must automatically open its contacts when an overcurrent condition is sensed.

The trip unit is the part of the circuit breaker that determines when the contacts will open automatically.

In a thermal-magnetic circuit breaker, the trip unit includes elements designed to sense the heat resulting from an overload condition and the high current resulting from a short circuit. In addition, some thermal magnetic circuit breakers incorporate a “PUSH TO TRIP” button.


Trip Mechanism

The trip unit includes a trip mechanism that is held in place by the tripper bar. As long as the tripper bar holds the trip mechanism, the mechanism remains firmly locked in place.

Trip Unit with Trip Mechanism

Trip Unit with Trip Mechanism


The operating mechanism is held in the “ON” position by the trip mechanism. When a trip is activated, the trip mechanism releases the operating mechanism, which opens the contacts.

Note: the drawings in this section show an AC power source; however, a DC source could also be used.
The operating mechanism is held in the “ON” position by the trip mechanism.

The operating mechanism is held in the “ON” position by the trip mechanism.

Manual Trip

Some molded case circuit breakers, especially larger breakers, can be manually tripped by pressing the “PUSH TO TRIP” button on the face of the circuit breaker. When the button is pressed the tripper bar rotates up and to the right. This allows the trip mechanism to “unlock” releasing the operating mechanism.

The operating mechanism opens the contacts.

The “PUSH TO TRIP” button also serves as a safety device by preventing access to the circuit breaker interior in the “ON” position. If an attempt is made to remove the circuit breaker cover while the contacts are in the closed (“ON”) position, a spring located under the pushbutton causes the button to lift up and the breaker to trip.

Manual trip mechanism

Manual trip mechanism

Overload Trip

Thermal-magnetic circuit breakers employ a bi-metalic strip to sense overload conditions. When sufficient overcurrent flows through the circuit breaker’s current path, heat build up causes the bi-metalic strip to bend. After bending a predetermined distance, the bi-metalic strip makes contact with the tripper bar activating the trip mechanism.

Thermal-magnetic circuit breakers employ a bi-metalic strip to sense overload conditions.

Thermal-magnetic circuit breakers employ a bi-metalic strip to sense overload conditions.


Circuit breaker contacts

Circuit breaker contacts

A bi-metalic strip is made of two dissimilar metals bonded together. The two metals have different thermal expansion characteristics, so the bi-metalic strip bends when heated. As current rises, heat also rises.

The hotter the bi-metalic becomes the more it bends. After the source of heat is removed, as when the circuit breaker contacts open, the bi-metalic strip cools and returns to its original condition. This allows a circuit breaker to be manually reset once the overload condition has been corrected.


Short Circuit Trip

As previously described, current flow through a circuit breaker’s blow-apart contacts creates opposing magnetic fields. Under normal operating conditions, these opposing forces are not sufficient to separate the contacts. When a short circuit occurs, however, these opposing forces increase significantly.

The current that flows through the contacts also flows through a conductor that passes close to the circuit breaker’s trip unit. At fault current levels, the magnetic field surrounding this conductor provides sufficient force to unlatch the trip unit and trip the breaker.

Short Circuit Trip

Short Circuit Trip


The combined actions of magnetic fields forcing contacts apart while simultaneously tripping the circuit breaker result in rapid interruption of the fault current. In addition, because the magnetic forces are proportional to the current, the greater the fault current, the shorter the time it takes to interrupt the current.

http://electrical-engineering-portal.com/how-circuit-breaker-trip-unit-works

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