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Rating Definitions Applied to Low Voltage Molded-Case
CircuitBreaker (MCCB)
Rating Definitions Applied to Low Voltage Molded-Case Circuit
Breaker (MCCB)
For system protection 600V and belowThe molded-case circuit
breaker is the workhorse for system protection 600V and below . A
circuit breaker isa device designed to open and close by
nonautomatic means and to open the circuit automatically ona
predetermined overcurrent without damage to itself when properly
applied within its rating.
The following terms apply to molded-case circuit breakers:
Voltage - Circuit breakers are designed and marked with the
maximum voltage at which they can be applied. Circuitbreaker
voltage ratings distinguish between delta-connected, 3-wire systems
and wye-connected, 4-wire systems.
As stated in NEC article 240.85, a circuit breaker with a
straight voltage rating, such as 240 or 480 V can be used ina
circuit in which the nominal voltage between any two conductors
does not exceed the circuit breakers voltagerating.
Breakers with slash ratings, such as 120/240 V or 480 Y/277 V,
can be applied in a solidly-grounded circuit where thenominal
voltage of any conductor to ground does not exceed the lower of the
two values of the circuit breakers
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voltage rating and the nominal voltage between any two
conductors does not exceed the higher value of the circuitbreakers
voltage rating.
Frequency - Molded-case circuit breakers are normally suitable
for 50Hz or 60Hz. Some have DC ratings as well.
Continuous current or Rated current - This is the maximum
current a circuit breaker can carry continuously at agiven ambient
temperature rating without tripping (typically 40C).
In accordance with NEC article 210.20 a circuit breaker (or any
branch circuit overcurrent device) should not beloaded to over 80%
of its continuous current unless the assembly, including the
circuit breaker and enclosure, is listedfor operation at 100% of
its rating.
Poles - The number of poles is the number of ganged circuit
breaker elements in a single housing. Circuit breakersare available
with one, two, or three poles, and also four poles for certain
applications.
Per NEC article 240.85 a two-pole circuit breaker cannot be used
for protecting a 3-phase, corner-grounded deltacircuit unless the
circuit breaker is marked 1 3 to indicate such suitability.
Control voltage - The control voltage rating is the AC or DC
voltage designated to be applied to control devicesintended to open
or close a circuit breaker. In most cases this only applies to
accessories that are custom-ordered,such as motor operators.
Interrupting rating - This is the highest current at rated
voltage that the circuit breaker is intended to interrupt
understandard test conditions.
Short-time or Withstand Rating - This characterizes the
circuit-breakers ability to withstand the effects of short-circuit
current flow for a stated period. Molded-case circuit breakers
typically do not have a withstandrating, although some newer-design
breakers do.
Instantaneous override - A function of an electronic trip
circuit breaker that causes the instantaneous functionto operate
above a given level of current if the instantaneous function
characteristic has been disabled.
Current Limiting Circuit Breaker - This is a circuit breaker
which does not employ a fusible element and,when operating in its
current-limiting range, limits the let-through I2t to a value less
than the I 2t of a _-cycle waveof the symmetrical prospective
current.
HID - This is a marking that indicates that a circuit breaker
has passed additional endurance and temperaturerise tests to assess
its ability to be used as the regular switching device for high
intensity discharge lighting. PerNEC 240.80 (D) a circuit breaker
which is used as a switch in an HID lighting circuit must be marked
as HID.
HID circuit breakers can also be used as switches in fluorescent
lighting circuits.
SWD - This is a marking that indicates that a circuit breaker
has passed additional endurance and temperature risetests to assess
its ability to be used as the regular switching device fluorescent
lighting.
Per NEC 240.80 (D) a circuit breaker which is used as a switch
in an HID lighting circuit must bemarked as SWD or HID.
Frame - The term Frame is applied to a group of circuit breakers
of similar configuration. Frame size isexpressed in amperes and
corresponds to the largest ampere rating available in that
group.
Thermal-magnetic circuit breaker - This type of circuit breaker
contains a thermal element to trip the circuit breaker
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for overloads and a faster magnetic instantaneous element to
trip the circuit breaker for short circuits.
On many larger thermal-magnetic circuit breakers the
instantaneous element is adjustable.
Electronic trip circuit breaker - An electronic circuit breaker
contains a solid-state adjustable trip unit. These circuitbreakers
are extremely flexible in coordination with other devices.
Sensor - An electronic-trip circuit breakers sensor is usually
an air-core current transformer (CT)designed specifically to work
with that circuit breakers trip unit.
The sensor size, in conjunction with the rating plug, determines
the electronic-trip circuit breakers continuouscurrent rating.
Rating plug - An electronic trip circuit breakers rating plug
can vary the circuit breakers continuous current rating asa
function of its sensor size.
Typical molded-case circuit breakers are shown in Figure 1,
where on the left is a thermal-magnetic circuit breaker,and on the
right is an electronic-trip circuit breaker. The thermal-magnetic
circuit breaker is designed for cableconnections and the electronic
circuit breaker is designed for bus connections, but neither type
is inherently suited forone connection type over another.
Circuit breakers may be mounted in stand-alone enclosures, in
switchboards, or in panelboards.
Figure 1 Molded-Case circuit breakers
Thermal-magnetic circuit breaker time-current characteristic
A typical thermal-magnetic circuit breaker time-current
characteristic is shown in figure 2.
Note the two distinct parts of the characteristic curve: The
thermal or long-time characteristic is used for overloadprotection
and the magnetic or instantaneous characteristic is used for
short-circuit protection.
Note also that there is a band of operating times for a given
fault current. The lower boundary represents the lowestpossible
trip time and the upper boundary represents the highest possible
trip time for a given current.
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Figure 2 Thermal magnetic circuit breaker time-current
characteristic
Electronic-trip circuit breaker time-current characteristic
The time-current characteristic for an electronic-trip circuit
breaker is shown in figure 3. The characteristic for anelectronic
trip circuit breaker consists of the long time pickup, long-time
delay, short-time pickup, short time delay, andinstantaneous pickup
parameters, all of which are adjustable over a given range.
This adjustability makes the electronic-trip circuit breaker
very flexible when coordinating with otherdevices. The adjustable
parameters for an electronic trip circuit breaker are features of
the trip unit.
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In many cases the trip unit is also available without the
short-time function.
In catalog data the long-time characteristic is listed as L, the
short-time is listed as S, and the instantaneous as I.Therefore an
LSI trip unit has long-time, short-time, and instantaneous
characteristics, whereas an LI trip unit has onlythe long-time and
instantaneous characteristics.
For circuit breakers that have a short-time rating, the
instantaneous feature may be disabled, enhancing coordinationwith
downstream devices.
Figure 3 Electronic-trip circuit breaker time-current
characteristic
If the instantaneous feature has been disabled one must still be
cognizant of any instantaneous override feature thebreaker has,
which will engage the instantaneous function above a given level of
current even if it has been disabledin order to protect the circuit
breaker from damage.
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Coordination
Typical coordination between an electronic and a thermal
magnetic circuit breaker is shown in figure 4below. Because the
time bands do not overlap, these two devices are considered to be
coordinated.
Figure 4 Typical molded-case circuit breaker coordination
A further reduction in the let-through energy for a fault in the
region between two electronic-trip circuit breakers can
beaccomplished through zone-selective interlocking. This consists
of wiring the two trip units such that if the downstreamcircuit
breaker senses the fault (typically this will be based upon the
short-time pickup) it sends a restraining signaltothe upstream
circuit breaker.
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The upstream circuit breaker will then continue to time out as
specified on its characteristic curve, tripping if thedownstream
device does not clear the fault.
However, if the downstream device does not sense the fault and
the upstream devices does, the upstream device willnot have the
restraining signal from the downstream device and will trip with no
intentional delay.
Example
For example, if zone selective interlocking were present in the
system of figure 4 and fault occurs on bus C circuitbreaker B will
sense the fault and send a restraining signal to circuit breaker A.
Circuit breaker A is coordinated withcircuit breaker B, so circuit
breaker B will trip first.
If circuit breaker B fails to clear the fault, circuit breaker A
will time out on its time-current characteristic per figure 4and
trip. If the fault occurs at bus B, circuit breaker B will not
detect the fault and thus will not send the restrainingsignal to
circuit breaker A. Circuit breaker A will sense the fault and will
trip with no intentional delay, which is fasterthan dictated by its
time-current characteristic per figure 4.
Care must be used when applying zone-selective interlocking
where there are multiple sources of power and faultcurrents can
flow in either direction through a circuit breaker.
Table 1 shows typical characteristics of molded-case circuit
breakers for commercial and industrial applications. Thistable is
for reference only; when specifying circuit breakers manufacturers
actual catalog data should be used.
Frame Size (A) Number ofPoles
Interrupting Rating at AC voltage (kA, RMS symmetrical)120 V 240
V 277 V 480 V 600 V
100 1 10 141 65 65
100, 150 2, 3 18 14 142, 3 65 25 182, 3 100 65 25
225, 250 2, 3 25 22 222, 3 65 25 222, 3 100 65 25
400, 600 2, 3 42 30 222, 3 65 65 252, 3 100 35
800, 1000 3 42 30 2265 50 25200 100 65
1200 3 42 30 22
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3 65 50 253 200 100 65
1600, 2000 3 65 50 423 125 100 65
3000, 4000 3 100 100 853 200 150 100
Note that the continuous current rating is set by the sensor and
rating plug sizes for a given electronic-tripcircuit breaker. This
can be smaller than the frame size. As can be seen from table 1,
more than one interruptingrating can be available for a given frame
size.
Current-limiting circuit breakers are also available.
Coordination between two current-limiting circuit breakerswhen they
are both operating in the current limiting range is typically
determined by test.
By definition, low voltage molded case circuit breakers are not
maintainable devices. Failure of a componentgenerally requires
replacement of the entire circuit breaker unless the circuit
breaker has been specifically designedfor maintainability.
Magnetic-only circuit breaker swhich have only magnetic tripping
capability are available. These are often used asshort-circuit
protection for motor circuits. For this reason these are often
referred to as motor circuit protectors.
Reference: System Protection Bill Brown, P.E., Square D
Engineering Services
Rating Definitions Applied to Low Voltage Molded-Case Circuit
Breaker (MCCB)For system protection 600V and belowThermal-magnetic
circuit breaker time-current characteristicElectronic-trip circuit
breaker time-current characteristicCoordinationExample