INTRODUCTION
TO CIRCUIT BREAKER
The
circuit breaker is an absolutely essential device in the modern world, and one
of the most important safety mechanisms in places where electricity is being
used. Whenever electrical wiring in a building or a power supply system has too
much current flowing through it, these simple machines cut the power until
somebody fixes the problem. Without circuit breakers (or the alternative,
fuses), flow of electricity would be impractical because of the potential for
fires and other problems resulting from short circuit problems and equipment
failures. Electric breakers play a great role in power generation, transmission
and distribution system.
A
circuit breaker is an automatically operated electrical switch designed to
protect an electrical circuit from damage caused by overload or short circuit.
Its basic function is to detect a fault condition and interrupt current flow.
Electrical circuit breaker is a switching device that can be operated manually
as well as automatically for controlling and protection of electrical power
system respectively.
An early form of circuit breaker was described
by Thomas Edison in an 1879 patent application, although his commercial power
distribution system used fuses. Its purpose was to protect lighting circuit
wiring from accidental short circuits and overloads. A modern miniature circuit
breaker similar to the ones now in use was patented by Brown, Boveri & Cie
in 1924. Hugo Stotz, an engineer who had sold his company to BBC, was credited
as the inventor on DRP (Deutsches Reichspatent). Stotz's invention was the
forerunner of the modern thermal-magnetic breaker commonly used in household
load centers to this day.
TYPES
OF CIRCUIT BREAKERS
According
different criteria there are different types of circuit breaker.
According to their arc quenching media the circuit breaker can be divided as- According to their arc quenching media the circuit breaker can be
divided as-
1. Oil
circuit breaker.
2. Air
circuit breaker.
3. SF6
circuit breaker.
4. Vacuum
circuit breaker.
According to their
services the circuit breaker can be divided as-
1. Outdoor
circuit breaker
2. Indoor
breaker.
According to the
operating mechanism of circuit breaker they can be divided as-
1. Spring
operated circuit breaker.
2. Pneumatic
circuit breaker.
3. Hydraulic
circuit breaker.
According to the voltage
level of installation types of circuit breaker
are referred as-
1. High
voltage circuit breaker.
2. Medium
voltage circuit breaker.
3. Low
voltage circuit breaker.
1
LOW VOLTAGE CIRCUIT
BREAKERS
Low-voltage
(less than 1,000 VAC) types are common in domestic, commercial and
industrial application, and they include:
- Miniature circuit breaker (MCB) rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation.
- Molded Case Circuit Breaker (MCCB) rated current up to 2,500 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.
Low-voltage
power circuit breakers can be mounted in multi-tiers in low-voltage
switchboards or switchgear cabinets. These circuit breakers are often installed
in draw-out enclosures that allow removal and interchange without dismantling the
switchgear and large low-voltage molded case and power circuit breakers may
have electric motor operators so they can open and close under remote control
system.
2 MEDIUM-VOLTAGE
CIRCUIT BREAKERS
Medium-voltage
circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed
switchgear line ups for indoor use, or may be individual components installed
outdoors in a substation. Air-break circuit breakers replaced oil-filled units
for indoor applications, but are now themselves being replaced by vacuum
circuit breakers (up to about 40.5 kV). Medium-voltage circuit breakers can be
classified by the medium used to extinguish the arc:
- Vacuum circuit breakers > with rated current up to 6,300 A, and higher for generator circuit breakers. These breakers interrupt the current by creating and extinguishing the arc in a vacuum container - aka "bottle". Long life bellows are designed to travel the 6–10 mm the contacts must part. These are generally applied for voltages up to about 40,500V which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.
- Air circuit breakers > Rated current up to 6,300A and higher for generator circuit breakers. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.
- SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas
3 HIGH-VOLTAGE CIRCUIT BREAKERS
Electrical
power transmission
networks are protected and controlled by high-voltage breakers. The definition
of high voltage varies but in power transmission work is usually thought
to be 72.5 kV or higher, according to a recent definition by the International
Electro technical Commission
(IEC). High-voltage breakers are nearly always solenoid-operated, with current sensing protective
relays operated through current transformers. In substations
the protective relay scheme can be complex, protecting equipment and buses from
various types of overload or ground/earth fault.
High-voltage
breakers are broadly classified by the medium used to extinguish the arc:
- Bulk oil
- Minimum oil
- Air blast
- Vacuum
- SF6
- CO2
Due
to environmental and cost concerns over insulating oil spills, most new
breakers use SF6 gas to quench the arc.
Circuit
breakers can be classified as live tank, where the enclosure that
contains the breaking mechanism is at line potential, or dead tank with
the enclosure at earth potential. High-voltage AC circuit breakers are
routinely available with ratings up to 765 kV. 1,200 kV breakers were
launched by Siemens in November 2011.
WORKING PRINCIPLE OF
CIRCUIT BREAKER
The
circuit breaker mainly consists of fixed contacts and moving contacts. In
normal condition of circuit breaker, these two contacts are physically
connected to each other due to applied mechanical pressure on the moving
contacts. There is an arrangement stored potential energy in the operating
mechanism of circuit breaker which
is released if switching signal is given to the breaker. The potential energy
can be stored in the circuit breaker by different ways like by deforming metal
spring, by compressed air, or by hydraulic pressure. But whatever the source of
potential energy, it must be released during operation. Release of potential
energy makes sliding of the moving contact at extremely fast manner.
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