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. Unlike a fuse, which operates once and then must be
replaced, a circuit breaker can be reset (either manually or automatically) to
resume normal operation. Circuit breakers are made in varying sizes, from small
devices that protect an individual household appliance up to large switchgear
designed to protect high voltage circuits feeding an entire city.
Origins
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) 458392. Stotz's invention was the
forerunner of the modern thermal-magnetic breaker commonly used in household
load centers to this day.
Interconnection of multiple generator sources into an
electrical grid required development of circuit breakers with increasing
voltage ratings and increased ability to safely interrupt the increasing short
circuit currents produced by networks. Simple air-break manual switches
produced hazardous arcs when interrupting high currents; these gave way to
oil-enclosed contacts, and various forms using directed flow of pressurized
air, or of pressurized oil, to cool and interrupt the arc. By 1935, the
specially constructed circuit breakers used at the Boulder Dam project use
eight series breaks and pressurized oil flow to interrupt faults of up to 2,500
MVA, in three cycles of the AC power frequency.
Operation
All circuit breaker systems have common features in their
operation, although details vary substantially depending on the voltage class,
current rating and type of the circuit breaker.
The circuit breaker must detect a fault condition; in low
voltage circuit breakers this is usually done within the breaker enclosure.
Circuit breakers for large currents or high voltages are usually arranged with
protective relay pilot devices to sense a fault condition and to operate the
trip opening mechanism. The trip solenoid that releases the latch is usually
energized by a separate battery, although some high-voltage circuit breakers
are self-contained with current transformers, protective relays and an internal
control power source.
Once a fault is detected, contacts within the circuit
breaker must open to interrupt the circuit; some mechanically-stored energy (using
something such as springs or compressed air) contained within the breaker is
used to separate the contacts, although some of the energy required may be
obtained from the fault current itself. Small circuit breakers may be manually
operated, larger units have solenoids to trip the mechanism, and electric
motors to restore energy to the springs.
The circuit breaker contacts must carry the load current
without excessive heating, and must also withstand the heat of the arc produced
when interrupting (opening) the circuit. Contacts are made of copper or copper
alloys, silver alloys and other highly conductive materials. Service life of
the contacts is limited by the erosion of contact material due to arcing while
interrupting the current. Miniature and molded-case circuit breakers are
usually discarded when the contacts have worn, but power circuit breakers and
high-voltage circuit breakers have replaceable contacts.
When a current is interrupted, an arc is generated. This arc
must be contained, cooled and extinguished in a controlled way, so that the gap
between the contacts can again withstand the voltage in the circuit. Different
circuit breakers use vacuum, air, insulating gas or oil as the medium the arc
forms in. Different techniques are used to extinguish the arc including:
Lengthening /
deflection of the arc
Intensive cooling
(in jet chambers)
Division into
partial arcs
Zero point
quenching (Contacts open at the zero current time crossing of the AC waveform,
effectively breaking no load current at the time of opening. The zero crossing
occurs at twice the line frequency, i.e. 100 times per second for 50 Hz and 120
times per second for 60 Hz AC)
Connecting
capacitors in parallel with contacts in DC circuits.
Finally, once the fault condition has been cleared, the
contacts must again be closed to restore power to the interrupted circuit.
Arc interruption
Low-voltage MCB uses air alone to extinguish the arc. Larger
ratings have metal plates or non-metallic arc chutes to divide and cool the
arc. Magnetic blowout coils or permanent magnets deflect the arc into the arc
chute.
In larger ratings, oil circuit breakers rely upon
vaporization of some of the oil to blast a jet of oil through the arc.
Gas (usually sulfur hexafluoride) circuit breakers sometimes
stretch the arc using a magnetic field, and then rely upon the dielectric
strength of the sulfur hexafluoride (SF6) to quench the stretched arc.
Vacuum circuit breakers have minimal arcing (as there is
nothing to ionize other than the contact material), so the arc quenches when it
is stretched a very small amount (less than 2–3 mm (0.079–0.118 in)). Vacuum
circuit breakers are frequently used in modern medium-voltage switchgear to
38,000 volts.
Air circuit breakers may use compressed air to blow out the
arc, or alternatively, the contacts are rapidly swung into a small sealed
chamber, the escaping of the displaced air thus blowing out the arc.
Circuit breakers are usually able to terminate all current
very quickly: typically the arc is extinguished between 30 ms and 150 ms after
the mechanism has been tripped, depending upon age and construction of the
device.
Short-circuit current
Circuit breakers are rated both by the normal current that
they are expected to carry, and the maximum short-circuit current that they can
safely interrupt.
Under short-circuit conditions, a current many times greater
than normal can exist (see maximum prospective short circuit current). When
electrical contacts open to interrupt a large current, there is a tendency for
an arc to form between the opened contacts, which would allow the current to
continue. This condition can create conductive ionized gases and molten or
vaporized metal, which can cause further continuation of the arc, or creation
of additional short circuits, potentially resulting in the explosion of the
circuit breaker and the equipment that it is installed in. Therefore, circuit
breakers must incorporate various features to divide and extinguish the arc.
In air-insulated and miniature breakers an arc chute
structure consisting (often) of metal plates or ceramic ridges cools the arc,
and magnetic blowout coils deflect the arc into the arc chute. Larger circuit
breakers such as those used in electrical power distribution may use vacuum, an
inert gas such as sulfur hexafluoride or have contacts immersed in oil to
suppress the arc.
The maximum short-circuit current that a breaker can
interrupt is determined by testing. Application of a breaker in a circuit with
a prospective short-circuit current higher than the breaker's interrupting
capacity rating may result in failure of the breaker to safely interrupt a
fault. In a worst-case scenario the breaker may successfully interrupt the
fault, only to explode when reset.
MCB used to protect control circuits or small appliances may
not have sufficient interrupting capacity to use at a panel board; these
circuit breakers are called "supplemental circuit protectors" to
distinguish them from distribution-type circuit breakers.
Standard current ratings
Circuit breakers are manufactured in standard sizes, using a
system of preferred numbers to cover a range of ratings. Miniature circuit
breakers have a fixed trip setting; changing the operating current value
requires changing the whole circuit breaker. Larger circuit breakers can have
adjustable trip settings, allowing standardized elements to be applied but with
a setting intended to improve protection. For example, a circuit breaker with a
400 ampere "frame size" might have its overcurrent detection set to
operate at only 300 amperes, to protect a feeder cable.
International Standard--- IEC 60898-1 and European Standard
EN 60898-1 define the rated current In of a circuit breaker for low voltage
distribution applications as the maximum current that the breaker is designed
to carry continuously (at an ambient air temperature of 30 °C). The
commonly-available preferred values for the rated current are 6 A, 10 A, 13 A,
16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A, 100 A, and 125 A (Renard
series, slightly modified to include current limit of British BS 1363 sockets).
The circuit breaker is labeled with the rated current in amperes, but without
the unit symbol "A". Instead, the ampere figure is preceded by a
letter "B", "C" or "D", which indicates the instantaneous
tripping current — that is, the minimum value of current that causes the
circuit breaker to trip without intentional time delay (i.e., in less than 100
ms), expressed in terms of In:
Type Instantaneous
tripping current
B above 3
In up to and including 5 In
C above 5
In up to and including 10 In
D above 10
In up to and including 20 In
K above 8
In up to and including 12 In
For the protection of loads that cause frequent short
duration (approximately 400 ms to 2 s) current peaks in normal operation.
Z above 2
In up to and including 3 In for periods in the order of tens of seconds.
For the protection of loads such as semiconductor devices or
measuring circuits using current transformers.
Circuit breakers are also rated by the maximum fault current
that they can interrupt; this allows use of more economical devices on systems
unlikely to develop the high short-circuit current found on, for example, a
large commercial building distribution system.
In the United States, Underwriters Laboratories (UL)
certifies equipment ratings, called Series Ratings (or “integrated equipment
ratings”) for circuit breaker equipment used for buildings. Power circuit
breakers and medium- and high-voltage circuit breakers used for industrial or
electric power systems are designed and tested to ANSI/IEEE standards in the
C37 series.
Types of circuit breakers
Front panel of a 1250 A air circuit breaker manufactured by
ABB. This low voltage power circuit breaker can be withdrawn from its housing
for servicing. Trip characteristics are configurable via DIP switches on the
front panel.
Many different classifications of circuit breakers can be
made, based on their features such as voltage class, construction type,
interrupting type, and structural features.
Low-voltage circuit breakers
Low-voltage (less than 1,000 VAC) types are common in
domestic, commercial and industrial application, and include:
MCB (Miniature
Circuit Breaker)—rated current not more than 100 A. Trip characteristics
normally not adjustable. Thermal or thermal-magnetic operation. Breakers
illustrated above are in this category.
There are three main types of MCBs: 1. Type B - trips
between 3 and 5 times full load current; 2. Type C - trips between 5 and 10
times full load current; 3. Type D - trips between 10 and 20 times full load
current. In the UK all MCBs must be selected in accordance with BS 7671.
MCCB (Molded Case
Circuit Breaker)—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.
The characteristics of low-voltage circuit breakers are
given by international standards such as IEC 947. These circuit breakers are
often installed in draw-out enclosures that allow removal and interchange
without dismantling the switchgear.
Large low-voltage molded case and power circuit breakers may
have electric motor operators so they can open and close under remote control.
These may form part of an automatic transfer switch system for standby power.
Low-voltage circuit breakers are also made for
direct-current (DC) applications, such as DC for subway lines. Direct current
requires special breakers because the arc is continuous—unlike an AC arc, which
tends to go out on each half cycle. A direct current circuit breaker has
blow-out coils that generate a magnetic field that rapidly stretches the arc.
Small circuit breakers are either installed directly in equipment, or are
arranged in a breaker panel.
Inside of a circuit breaker
The DIN rail-mounted thermal-magnetic miniature circuit
breaker is the most common style in modern domestic consumer units and
commercial electrical distribution boards throughout Europe. The design
includes the following components:
Actuator lever -
used to manually trip and reset the circuit breaker. Also indicates the status
of the circuit breaker (On or Off/tripped). Most breakers are designed so they
can still trip even if the lever is held or locked in the "on"
position. This is sometimes referred to as "free trip" or
"positive trip" operation.
Actuator mechanism
- forces the contacts together or apart.
Contacts - Allow
current when touching and break the current when moved apart.
Terminals
Bimetallic strip.
Calibration screw
- allows the manufacturer to precisely adjust the trip current of the device
after assembly.
Solenoid
Arc
divider/extinguisher
Magnetic circuit breakers
Magnetic circuit breakers use a solenoid (electromagnet)
whose pulling force increases with the current. Certain designs utilize
electromagnetic forces in addition to those of the solenoid. The circuit
breaker contacts are held closed by a latch. As the current in the solenoid
increases beyond the rating of the circuit breaker, the solenoid's pull
releases the latch, which lets the contacts open by spring action. Some
magnetic breakers incorporate a hydraulic time delay feature using a viscous
fluid. A spring restrains the core until the current exceeds the breaker
rating. During an overload, the speed of the solenoid motion is restricted by
the fluid. The delay permits brief current surges beyond normal running current
for motor starting, energizing equipment, etc. Short circuit currents provide
sufficient solenoid force to release the latch regardless of core position thus
bypassing the delay feature. Ambient temperature affects the time delay but
does not affect the current rating of a magnetic breaker
Thermal magnetic circuit breakers
Thermal magnetic circuit breakers, which are the type found
in most distribution boards, incorporate both techniques with the electromagnet
responding instantaneously to large surges in current (short circuits) and the
bimetallic strip responding to less extreme but longer-term over-current
conditions. The thermal portion of the circuit breaker provides an
"inverse time" response feature, which trips the circuit breaker
sooner for larger overcurrents.[6]
Common trip breakers
Three-pole common trip breaker for supplying a three-phase
device. This breaker has a 2 A rating
When supplying a branch circuit with more than one live
conductor, each live conductor must be protected by a breaker pole. To ensure
that all live conductors are interrupted when any pole trips, a "common
trip" breaker must be used. These may either contain two or three tripping
mechanisms within one case, or for small breakers, may externally tie the poles
together via their operating handles. Two-pole common trip breakers are common
on 120/240-volt systems where 240 volt loads (including major appliances or
further distribution boards) span the two live wires. Three-pole common trip
breakers are typically used to supply three-phase electric power to large
motors or further distribution boards.
Two- and four-pole breakers are used when there is a need to
disconnect multiple phase AC, or to disconnect the neutral wire to ensure that
no current flows through the neutral wire from other loads connected to the
same network when workers may touch the wires during maintenance. Separate
circuit breakers must never be used for live and neutral, because if the
neutral is disconnected while the live conductor stays connected, a dangerous
condition arises: the circuit appears de-energized (appliances don't work), but
wires remain live and some RCDs may not trip if someone touches the live wire
(because some RCDs need power to trip). This is why only common trip breakers
must be used when neutral wire switching is needed.
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).
Like the high voltage circuit breakers described below, these are also operated
by current sensing protective relays operated through current transformers. The
characteristics of MV breakers are given by international standards such as IEC
62271. Medium-voltage circuit breakers nearly always use separate current
sensors and protective relays, instead of relying on built-in thermal or
magnetic overcurrent sensors.
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 to 10 mm the contacts must part. These are generally
applied for voltages up to about 40,500 V,[7] 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 6,300 A 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.
Medium-voltage circuit breakers may be connected into the
circuit by bolted connections to bus bars or wires, especially in outdoor
switchyards. Medium-voltage circuit breakers in switchgear line-ups are often
built with draw-out construction, allowing breaker removal without disturbing
power circuit connections, using a motor-operated or hand-cranked mechanism to
separate the breaker from its enclosure. Some important manufacturer of VCB
from 3.3 kV to 38 kV are Eaton, ABB, Siemens, HHI(Hyundai Heavy Industry),
S&C Electric Company, Jyoti and BHEL.
High-voltage circuit breakers
Three single phase Russian 110 kV oil circuit breakers
400 kV SF6 live tank circuit breakers
72.5 kV Hybrid Switchgear Module
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 Electrotechnical
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
Mineral oil
Air blast
Vacuum
SF6
CO2
Some of the manufacturers are ABB, Alstom, General Electric,
Hitachi, Hyundai Heavy Industry(HHI), Mitsubishi Electric, Pennsylvania
Breaker, Siemens, Toshiba, Končar HVS, BHEL, CGL, and Becker/SMC (SMC
Electrical Products).
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, followed by ABB in April the following
year.
High-voltage circuit breakers used on transmission systems
may be arranged to allow a single pole of a three-phase line to trip, instead
of tripping all three poles; for some classes of faults this improves the
system stability and availability.
A high-voltage direct current circuit breaker uses DC
transmission lines rather than the AC transmission lines that dominate as of
2013. An HVDC circuit breaker can be used to connect DC transmission lines into
a DC transmission grid, thereby making it possible to link renewable energy
sources and even out local variations in wind and solar power.
Sulfur hexafluoride (SF6) high-voltage circuit breakers
A sulfur hexafluoride circuit breaker uses contacts
surrounded by sulfur hexafluoride gas to quench the arc. They are most often
used for transmission-level voltages and may be incorporated into compact
gas-insulated switchgear. In cold climates, supplemental heating or de-rating
of the circuit breakers may be required due to liquefaction of the SF6 gas.
Disconnecting circuit breaker (DCB)
72.5 kV carbon dioxide high-voltage circuit breaker
The disconnecting circuit breaker (DCB) was introduced in
2000 and is a high-voltage circuit breaker modeled after the SF6-breaker. It
presents a technical solution where the disconnecting function is integrated in
the breaking chamber, eliminating the need for separate disconnectors. This
increases the availability, since open-air disconnecting switch main contacts
need maintenance every 2–6 years, while modern circuit breakers have
maintenance intervals of 15 years. Implementing a DCB solution also reduces the
space requirements within the substation, and increases the reliability, due to
the lack of separate disconnectors.
In order to further reduce the required space of substation,
as well as simplifying the design and engineering of the substation, a fiber
optic current sensor (FOCS) can be integrated with the DCB. A 420 kV DCB with
integrated FOCS can reduce a substation’s footprint with over 50 % compared to
a conventional solution of live tank breakers with disconnectors and current
transformers, due to reduced material and no additional insulation medium.
Carbon dioxide (CO2) high-voltage circuit breakers
In 2012 ABB presented a 75 kV high-voltage breaker that uses
carbon dioxide as the medium to extinguish the arc. The carbon dioxide breaker
works on the same principles as an SF6 breaker and can also be produced as a
disconnecting circuit breaker. By switching from SF6 to CO2 it is possible to
reduce the CO2 emissions by 10 tons during the product’s life cycle.
Other breakers
Residual current circuit breaker with overload protection
The following types are described in separate articles.
Breakers for
protections against earth faults too small to trip an over-current device:
Residual-current device (RCD, formerly known as a residual current
circuit breaker) — detects current imbalance, but does not provide over-current
protection.
Residual
current breaker with over-current protection (RCBO) — combines the functions of
an RCD and an MCB in one package. In the United States and Canada,
panel-mounted devices that combine ground (earth) fault detection and
over-current protection are called Ground Fault Interrupter (GFI) breakers; a
wall mounted outlet device or separately enclosed plug-in device providing
ground fault detection and interruption only (no overload protection) is called
a Ground Fault Circuit Interrupter (GFCI).
Earth leakage
circuit breaker (ELCB)—This detects earth current directly rather than
detecting imbalance. They are no longer seen in new installations for various
reasons.
Recloser—A type of
circuit breaker that closes automatically after a delay. These are used on
overhead electric power distribution systems, to prevent short duration faults
from causing sustained outages.
Polyswitch
(polyfuse)—A small device commonly described as an automatically resetting fuse
rather than a circuit breaker.
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