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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

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

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.

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.

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.

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.

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

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