ABB MCB Catalog
ABB MCB Catalog
ABB MCB Catalog
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Notes
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Introduction
The circuit breaker plays an important role in providing over-current protection and a disconnect means in electrical networks. Recent advancements in circuit breaker technology has increased breaker performance and protection.
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.
Overload
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
10A 15A
Operating mechanism
Breaker
Light
Operator
The light draws more than 10 amps for an extended period of time creating a thermal overload.
Short circuit
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.
Arc chamber
Magnetic Example
10A
10A
Breaker
Light
The wire connected between the light and breaker is cut and shorted to ground creating a short circuit.
Lower terminal
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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 breakers 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 breakers 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.
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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 breakers 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. ABBs miniature circuit breakers 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.
Arc Runner
Arc Chute
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Current limiting definitions
All ABB Miniature Circuit Breakers are UL tested and certified as current limiting protective devices. Current limiting circuit breakers provide a higher level of circuit protection than a typical zero point external breakers.
UL AC 60Hz cycle
UL defines an AC cycle as the potential energy of the wave form traveling from Zero-to-Positive amplitude, Positive-to-Zero amplitude, Zero-to-Negative amplitude, Negative-to-Zero amplitude 60 times in one second. One cycle is completed every 16.6 milliseconds.
IsqT
The true destructive nature of a short circuit is measured by the time it is available combined with the peak value of the short circuit. The IsqT (Amps Squared over Time) value represents the amount of energy available on a network during a short circuit and is represented by the shaded area on the graph below. During a short circuit both magnetic forces and thermal energy combine to damage devices on the electrical network. The level of thermal energy and magnetic forces are directly proportional to the square of the current. The magnetic forces vary as a square of the peak current available and the thermal energy varies as a square of the RMS (root mean square) current available. ABBs current limiting breakers will limit the let-through energy to a fraction (1/100th) of the value which is available from the network. By comparison, a Zero Crossing breaker would let-through approximately 100 times as much destructive energy as the current limiting circuit breaker [ (100,000A / 10,000A) squared 100X]. ABBs current limiting breakers limit the short circuit current to a relatively small magnitude in a extremely short time, which dramatically limits a short circuits destructive energy.
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400
300
200
100
I-L2 2 k/div
-200
-4
-300
-6
-400
-8
-500
500 V 10 kA
-10
400
300
200
100
I-L2 2 k/div
-100
-2
-200
-4
-300
-6
-400
-8
-500
Time A Time B
3.9994 4.9993 5.9991 6.999 kSample
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Main breaker
CB1
CB2
CB3
CB4
Short circuit
Problems in coordination occur when the branch breaker allows the "I Square T" value of the short circuit to rise to a level that is in the operating range of the up-stream main breaker. Proper breaker coordination is easier to achieve with the use of current limiting breakers at the branch level.
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Series ratings
Series ratings are different from coordination ratings. Unlike coordination ratings where the branch opens and the main remains closed, a series rated combination is one where both the branch and main breakers open and work together to isolate the fault. The series rating combination of two breakers is equal to the stand alone interrupting value of the main breaker. This is a result of the main breaker let-through value being lower than the stand alone interrupting value of the branch breaker. During a short circuit the main breaker will limit the energy to a level that is below the stand alone value of the branch breaker. Example: 65kA rated main breaker 10kA rated branch breaker Series combination rating between the two breakers up to 65kA There can be a short circuit on the branch breaker up to 65kA where the branch will open and the main breaker will open. Although the branch breaker (CB1) has a 10kA stand alone rating the main breaker has a let-through value below 10kA. If there is a fault up to 65kA on the network the main breaker will limit the energy to a value less than the rating of the branch breaker (CB1). Both breakers will trip (no coordination) but the network can safely withstand a fault of 65kA.
Main breaker 65 kA
CB1 10 kA
CB2 10 kA
CB3 10 kA
CB4 10 kA
35 kA or 65 kA short circuit
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Tripping lever
Operator
Arc chamber
Fixed contact
Moving contact
Lower terminal
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Notes
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Notes
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