ABB Miniature circuit breaker Application Information
Application guide—Miniature circuit breaker
Introduction
The circuit breaker 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.
Overload
An overload is a slow and small overcurrent situation that causes
the ampacity and temperature of the circuit to gradually increase
over time. This type of event is characterized by a slight increase
in the load (ampacity) on the circuit and is interrupted by the
thermal trip unit of the breaker.
Thermal Example
10 A
15 A
Breaker
Light
Breaker definition
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.
ABB current limiting breaker
Electromagnetic protection
Tripping lever
Operating
mechanism
Operator
Upper terminal
Thermal
protection
(bimetal)
Arc chamber
The light draws more than 10 amps for
an extended period of time creating a
thermal overload.
Short circuit
A short circuit is a rapid and intense overcurrent situation that
causes the ampacity of the circuit to increase. This type of event is
characterized by a dramatic increase in the load (ampacity) on the
circuit and is interrupted by the magnetic trip unit of the breaker.
Magnetic Example
10 A
10 A
Breaker
Light
The wire connected between the light and
breaker is cut and shorted to ground
creating a short circuit.
Space for
identification marker
Fixed
contact
Moving
contact
DIN rail holder
Lower terminal
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Trip bar
Current Flow During Operation
All highlighted components are energized during operation
Current flow during operation
Circuit breaker construction
Thermal/Magnetic trip units definition
ABB Current Limiting Breakers use an electromechanical
(Thermal/Magnetic) trip unit to open the breaker contacts during
an 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 fl ow during operation
Magnetic Trip Unit
Magnetic trip unit
All highlighted components are energized during operation
Thermal Trip Unit
Thermal trip unit
Thermal trip unit
.
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 that opens the breaker’s contacts.
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.
Armature and plunger
Armature and plunger
Movable contact
Movable contact
Magnetic trip unit
Magnet trip unit
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 breaker’s contacts during a short
circuit is complete in .5 milli-seconds.
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 that can be subjected to low-magnitude but
long-time overcurrents. Low-magnitude, long-time overcurrents
can be dangerous because they reduce the life of the electrical
installation, conductor, and components. If left unchecked, fire
could result.
Trip bar
Operating mechanism
Operating mechanism
Thermal trip unit
Thermal trip unit
US Catalog | Miniature Circuit Breakers 43
Circuit breaker construction
Arc runners/arc chutes
The arc runner guides the electric arc away from the open contacts
into the arc chute where it is extinguished.
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 arc 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 25 V. Each 25 V segment
does not have a high enough energy level to maintain an arc
and all energy is naturally dissipated.
Arc runner
Arc runner Arc chute
Arc chute
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 10 A breaker and
the circuit was producing 30 amps of current, the breaker would
trip between two seconds and one 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 two), and the third line is 30 amps (10 amp
breaker x multiple of three). Next, you would trace the vertical 30
A 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
two seconds and no slower than one minute.
Magnetic trip unit (region two)
This region of the breaker curve is the instantaneous trip unit.
ABB’s miniature 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.
44 Miniature Circuit Breakers | US Catalog
Thermal Trip Unit
(Region One)
Magnetic Trip Unit
(Region Two)
Moving Contacts
(Region Three)
See curve example. 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 two and three) 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.
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