The traditional molded-case circuit breaker uses electromechanical (thermal magnetic) trip units that may be fixed or interchangeable. . A MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device. . ID: 753930
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Presentations text content in Low and Medium Voltage Circuit Breakers
Slide1
Low and Medium Voltage Circuit Breakers
Slide2
The traditional molded-case circuit breaker uses electromechanical (thermal magnetic) trip units that may be fixed or interchangeable. A MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device.
Both these devices act mechanically on the trip mechanism.
Circuit Breaker Basics
Slide3
Depending upon the application and required protection, a MCCB will use one or a combination of different trip elements that protect against the following conditions:
Thermal overloads;
Short circuits; and Ground faults.
Circuit Breaker Basics (cont.)
Slide4
Thermal overload: In an overload condition, there's a temperature buildup between the insulation and conductor.
If left unchecked, the insulation's life will drastically reduce, ultimately resulting in a short circuit.
Thermal Overload
Slide5
Usually, a short circuit occurs when abnormally high currents flow as a result of the failure of an insulation system. This high current flow, termed short-circuit current, is limited only by the capabilities of the distribution system.
To stop this current flow quickly so that major damage can be prevented, the short circuit or instantaneous element of an MCCB is used.
Short Circuit
Slide6
A ground fault actually is a type of short circuit, only it's phase-to-ground, which probably is the most common type of fault on low-voltage systems (600V or less).Usually, arcing ground-fault currents are not large enough to be detected by the standard MCCB protective device. But, if left undetected, they can increase sufficiently to trip the standard protective device.
Ground Fault
Slide7
Prior to the introduction of electronic CBs, separate ground fault protection devices were used to provide this additional level of protection. Today's modern electronic CB has the ground fault protection as an integral part of the trip unit.
Ground Fault (cont.)
Slide8
Overload, or thermal trip action uses a piece of bimetal heated by the load current. This bimetal is actually two strips of metal bonded together, with each having a different thermal rate of heat expansion. They are factory-calibrated and not field-adjustable.
To trip the CB, this bimetal must deflect enough to physically push the trip bar and unlatch the contacts.
Overload Trip Action
Slide9
Uses an electromagnet having a winding that's in series with the load current. When a short circuit occurs, the current flowing through the circuit conductor causes the magnetic field strength of the electromagnet to increase rapidly and attract the armature.
When this happens, the armature rotates the trip bar, causing the CB to trip.
Short Circuit Trip Action
Slide10
The only time delay factor involves the time it takes for the contacts to physically open and extinguish the arc; this usually is less than one cycle.
Magnetic elements are either fixed or adjustable, depending upon the type of CB and frame size. For example, most thermal magnetic breakers above the 150A frame size have adjustable magnetic trips.
Short Circuit Trip Action
Slide11
Circuit Breaker Ratings
Slide12
The automatic operation of a circuit breaker occurs when there is an abnormal condition such as an overcurrent or short circuit. Overcurrent is electrical current in excess of the equipment limit, total amperage load of a circuit, or conductor or equipment rating.
A
short circuit
is an unintentional connection of two ungrounded conductors that have a potential difference between them.
DC circuit breakers work with the same principle of thermal protection and magnetic protection which is found in AC circuit breakers.Thermal protection trips: This protection mechanism is based on a bimetallic contact that heats, expands and trips the circuit breaker.
Magnetic protection trips: This protection in a
DC circuit breaker
protects against short circuits and faults, which are drastically larger than an overload.
DC Circuit Breaker
Slide21
A very important difference when interrupting alternating current and direct current is that the arc extinguishing point is higher for a DC circuit breaker
.
In direct current where voltage is continuous, the electric arc is constant and more resistant to interruption.
For this reason, DC circuit breakers must include additional arc extinguishing measures: they typically have a mechanism to elongate and dissipate the electric arc in order to simplify interruption.
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DownloadNote - The PPT/PDF document "Low and Medium Voltage Circuit Breakers" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Presentations text content in Low and Medium Voltage Circuit Breakers
Low and Medium Voltage Circuit Breakers
Slide2The traditional molded-case circuit breaker uses electromechanical (thermal magnetic) trip units that may be fixed or interchangeable. A MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device.
Both these devices act mechanically on the trip mechanism.
Circuit Breaker Basics
Slide3Depending upon the application and required protection, a MCCB will use one or a combination of different trip elements that protect against the following conditions:
Thermal overloads;
Short circuits; and Ground faults.
Circuit Breaker Basics (cont.)
Slide4Thermal overload: In an overload condition, there's a temperature buildup between the insulation and conductor.
If left unchecked, the insulation's life will drastically reduce, ultimately resulting in a short circuit.
Thermal Overload
Slide5Usually, a short circuit occurs when abnormally high currents flow as a result of the failure of an insulation system. This high current flow, termed short-circuit current, is limited only by the capabilities of the distribution system.
To stop this current flow quickly so that major damage can be prevented, the short circuit or instantaneous element of an MCCB is used.
Short Circuit
Slide6A ground fault actually is a type of short circuit, only it's phase-to-ground, which probably is the most common type of fault on low-voltage systems (600V or less).Usually, arcing ground-fault currents are not large enough to be detected by the standard MCCB protective device. But, if left undetected, they can increase sufficiently to trip the standard protective device.
Ground Fault
Slide7Prior to the introduction of electronic CBs, separate ground fault protection devices were used to provide this additional level of protection. Today's modern electronic CB has the ground fault protection as an integral part of the trip unit.
Ground Fault (cont.)
Slide8Overload, or thermal trip action uses a piece of bimetal heated by the load current. This bimetal is actually two strips of metal bonded together, with each having a different thermal rate of heat expansion. They are factory-calibrated and not field-adjustable.
To trip the CB, this bimetal must deflect enough to physically push the trip bar and unlatch the contacts.
Overload Trip Action
Slide9Uses an electromagnet having a winding that's in series with the load current. When a short circuit occurs, the current flowing through the circuit conductor causes the magnetic field strength of the electromagnet to increase rapidly and attract the armature.
When this happens, the armature rotates the trip bar, causing the CB to trip.
Short Circuit Trip Action
Slide10The only time delay factor involves the time it takes for the contacts to physically open and extinguish the arc; this usually is less than one cycle.
Magnetic elements are either fixed or adjustable, depending upon the type of CB and frame size. For example, most thermal magnetic breakers above the 150A frame size have adjustable magnetic trips.
Short Circuit Trip Action
Slide11Circuit Breaker Ratings
Slide12The automatic operation of a circuit breaker occurs when there is an abnormal condition such as an overcurrent or short circuit. Overcurrent is electrical current in excess of the equipment limit, total amperage load of a circuit, or conductor or equipment rating.
A
short circuit
is an unintentional connection of two ungrounded conductors that have a potential difference between them.
Arc Conditions
Slide13Arc Interruption
Slide14Arc Interruption
Slide15Racking LVPCB
Slide16Racking LVPCB
Slide17Racking LVPCB
Slide18http://www.youtube.com/watch?v=4bBvmPRqfmo
Slide19https://www.youtube.com/watch?v=I4-rwcDbPcQhttps://vimeo.com/156623494
https://www.youtube.com/watch?v=8sc8QDdnwbA
https://www.youtube.com/watch?v=bVn8sl3AxRs
Remote Racking
Slide20DC circuit breakers work with the same principle of thermal protection and magnetic protection which is found in AC circuit breakers.Thermal protection trips: This protection mechanism is based on a bimetallic contact that heats, expands and trips the circuit breaker.
Magnetic protection trips: This protection in a
DC circuit breaker
protects against short circuits and faults, which are drastically larger than an overload.
DC Circuit Breaker
Slide21A very important difference when interrupting alternating current and direct current is that the arc extinguishing point is higher for a DC circuit breaker
.
In direct current where voltage is continuous, the electric arc is constant and more resistant to interruption.
For this reason, DC circuit breakers must include additional arc extinguishing measures: they typically have a mechanism to elongate and dissipate the electric arc in order to simplify interruption.
DC Circuit Breaker
Slide22DC Circuit Breaker
Slide23DC Circuit Breaker