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MCB - Miniature Circuit breaker Construction (ABB)

http://electrical-engineering-portal.com/mcb-miniature-circuit-breaker-construction January 6, 2013

MCB Miniature Circuit Breaker Construction

Introduction to MCB

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

A breaker is a device designed to isolate a circuit during an overcurrent event without the useof a fusible element. A breaker is a resettable protective device that protects against t wo t ypesof overcurrent situations; Overload and Short Circuit.

MCB Construction Details

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MCB - Miniature circuit breaker construction details

MCB - Current f low during operation (Allhighlighted components are energized during

operation)

ABB Current Limit ing Breakers use anelectromechanical (Thermal/Magnet ic) trip unit to open thebreaker contacts during aovercurrent event. The thermal tripunit is temperature sensit ive and themagnet ic trip unit is current sensit ive.

Both units act independentlyand mechanically with the breakerstrip mechanism to open thebreakers contacts.

Current Flow

Overload protection

The thermal t rip unit protects againsta cont inuous overload. The thermalunit is comprised of a bimetalelement located behind the circuitbreakertrip bar and is part of thebreakers current carrying path.

When there is an overload, theincreased current f low heats thebimetal causing it t o bend. As thebimetal bends it pulls the t rip bar

which opens the breakerscontacts. The time required for the bimetal tobend and t rip the breaker varies inversely withthe current. Because of this, the tripping timebecomes quicker as current increases inmagnitude.Overload protection is applicableto any installat ion, conductor, or componentwhich can be subjected to low-magnitude butlongtime over-currents.

Low-magnitude, long-t ime over-currents can

be dangerous because they reduce the life ofthe electrical installat ion, conductor, andcomponents and if left unchecked couldresult in fire.

Magnetic trip units (short circuitprotection)

The Magnet ic t rip unit prot ects against ashort circuit . The magnetic trip unit is

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MCB Overload protection

MCB Magnetic t rip units

comprsed o an electromagnet and anarmature.

Components of a magnetic trip unit

When there is a short circuit, a high magnitudeof current passes through the coils creating amagnetic f ield that att racts the

movable armature towards the f ixed armature.The hammer trip is pushed against the movablecontact and the contacts are opened.The opening of the breakers contacts during ashort circuit is complete in .5 milli-seconds.

Arc runners / Arc chutes

The arc runner and arc chute limit and dissipatethe arc energy during the interruption of anoverload or short circuit event.

During an overload or short circuit event, thecontacts of the breaker separate and anelectrical arc is formed between the contactsthrough air. The arc is moved into the arch chuteby running the arc down the interior of thebreaker along the arc runner. When the arcreaches the arc chute it is broken into smallsegmented arcs. The segmented arcs split theoverall energy level into segments less than 25V.

Each 25V segment does not have a high enoughenergy level to maintain an arc and all energy isnaturally dissipated.

Breaker curves

Thermal Trip Unit (region one)

The f irst sloping region o f the breaker curve is a

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MCB Components o f a magnetic tr ip unit

MCB Arc runners / Arc chutes

graphical representat ion of the trippingcharacterist ics of the thermal t rip unit .

This port ion of the curve is sloped dueto the nature of the thermal t rip unit.The trip unit bends to trip the breakerstrip bar in conjunction with a rise inamperage (temperature) over time. As

the current on the circuit increases, thetemperature rises, the faster thethermal element will trip.

Example using the curve below:

If you had a 10A breaker and thecircuit was producing 30 amps ofcurrent, the breaker would tripbetween 2 seconds and 1 minute. Inthis example you would find the

circuit current on the bot tom of thegraph (Multiples of rated current). The f irst line is10 amps (10 amp breaker x a multiple of one), thesecond line is 20 amps (10 amp breaker x multipleof 2), and the t hird line is 30 amps (10amp breakerx mult iple of 3). Next you would trace the vert ical30A line up until it intersects the red port ion o f thebreaker thermal curve.

If you follow the horizontal lines, on both sides ofthe red curve, to the left you will see that the

breaker can trip as fast as 2 seconds andno slower than 1 minute.

Magnetic Trip Unit (region two)

This region of the breaker curve is the instantaneous t rip unit. ABBs MCB miniture circuitbreakers instantaneous trip unit interrupts a short circuit in 2.3 to 2.5 milliseconds. Because ofthis 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 between10 and 30 amps (10 amp breaker x mult iple 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 toseparate. The contacts will open in less than .5 milliseconds and is graphically represented bythe bot tom vert ical portion of the curve.

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MCB - Miniature curcuit breaker curves

Reference: ABB Application guide |Miniature circuit breakers