Low and Medium Voltage Circuit Breakers PowerPoint Presentation

Low and Medium Voltage  Circuit Breakers PowerPoint Presentation

2019-02-26 4K 4 0 0

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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.

Arc Conditions

Slide13

Arc Interruption

Slide14

Arc Interruption

Slide15

Racking LVPCB

Slide16

Racking LVPCB

Slide17

Racking LVPCB

Slide18

http://www.youtube.com/watch?v=4bBvmPRqfmo

Slide19

https://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

Slide20

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.

DC Circuit Breaker

Slide22

DC Circuit Breaker

Slide23

DC Circuit Breaker


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