The selection of circuit breakers should be based on specific usage conditions and select the category of use, rated working voltage, rated current, and the set current of the trip unit, etc. Refer to the protection characteristics curves provided by the product samples for selecting the protection characteristics, and it is necessary to verify the short-circuit characteristics and sensitivity coefficient.

Circuit Breaker Classification:

(1) Frame Circuit Breaker (ACB)

ACB

Also known as the universal circuit breaker, all its parts are installed in an insulated metal frame, often of an open type. It can be equipped with various accessories, and the replacement of contacts and components is relatively convenient. It is often used at the power supply end’s main switch. The overcurrent trip unit includes electromagnetic, electronic, and intelligent trip units. The circuit breaker has four protection levels: long delay, short delay, instantaneous, and ground fault. Each type of protection setting value can be adjusted within a certain range according to its frame level.

The frame circuit breaker is suitable for an AC 50Hz, rated voltage of 380V, 660V, and rated current of 200A-6300A distribution network, mainly used to distribute electric energy and protect lines and power supply equipment from overload, undervoltage, short circuit, single-phase grounding, and other faults. This circuit breaker has various intelligent protection functions and can achieve selective protection. Under normal conditions, it can be used for infrequent switching of the line. Circuit breakers below 1250A can be used to protect motors from overload and short circuit in a 380V AC 50Hz network.

Frame circuit breakers are also often used for transformer 400V side outgoing line main switches, bus tie switches, high capacity feeder switches, and large motor control switches.

 

(2) Molded Case Circuit Breaker (MCCB)
MCCB
MCCB

The molded case circuit breaker, also known as the device circuit breaker, has its ground terminal external contacts, arc extinguishing chamber, trip unit, and operating mechanism all housed within a plastic case. Auxiliary contacts, undervoltage trip units, and shunt trip units are often modular, with a very compact structure. They generally don’t require maintenance and are suitable for branch circuit protection switches. MCCBs usually contain thermal-magnetic trip units, and larger models are equipped with solid-state trip sensors.

The overcurrent trip units of molded case circuit breakers come in two types: electromagnetic and electronic. Generally, electromagnetic MCCBs are non-selective circuit breakers with only long-delay and instantaneous protection modes. Electronic MCCBs have long delay, short delay, instantaneous, and ground fault protection functions. Some new products of electronic MCCBs also come with a zone selective interlocking function.

Molded case circuit breakers are commonly used for distribution feeder control and protection, as the main switch for the low voltage side outgoing line of small distribution transformers, power distribution terminal control, and can also be used as power switches for various production machinery.

 

(3) Miniature Circuit Breaker (MCB)

AC MCB

The miniature circuit breaker is the most widely used terminal protection appliance in building electrical terminal distribution devices. It is used for protection against short circuit, overload, and overvoltage in single-phase and three-phase systems below 125A, including single-pole 1P, double-pole 2P, triple-pole 3P, and four-pole 4P.

The miniature circuit breaker consists of an operating mechanism, contacts, protection devices (various trip units), and an arc extinguishing system. The main contacts are closed by manual operation or electrical closing. After the main contacts are closed, the free trip mechanism locks the main contacts in the closed position. The coil of the overcurrent trip unit and the thermal element of the thermal trip unit are connected in series with the main circuit, and the coil of the undervoltage trip unit is connected in parallel with the power supply.

In the design of electrical systems for residential buildings, miniature circuit breakers are mainly used for line overload, short circuit, overcurrent, voltage loss, undervoltage, grounding, leakage, automatic switching of dual power sources, and protection and operation when the motor is infrequently started.

Basic Characteristics Parameters of the Circuit Breaker (1) Rated Operating Voltage Ue

Characteristic Curve

The rated operating voltage refers to the nominal voltage of the circuit breaker, which is the voltage it can continuously operate under specified normal usage and performance conditions.

In China, the regulation stipulates that 1.15 times the system rated voltage is the maximum operating voltage for voltage levels up to and including 220kV. For voltage levels of 330kV and above, the maximum operating voltage is 1.1 times the rated voltage. The circuit breaker can maintain insulation at the system’s maximum operating voltage and perform close and open operations under the specified conditions.

(2) Rated Current In

The rated current is the current that the trip unit can pass for a long time at an ambient temperature below 40℃. For circuit breakers with adjustable trip units, it is the maximum current that the trip unit can pass for a long time.

When used in ambient temperatures exceeding 40℃ but not exceeding 60℃, it is permissible to reduce the long-term operation of the load.

(3) Overload Trip Unit Current Setting Value Ir

If the current exceeds the trip unit current setting value Ir, the circuit breaker trips with delay. It also represents the maximum current the circuit breaker can withstand without tripping. This value must be greater than the maximum load current Ib but less than the maximum current Iz allowed by the line.

The thermal trip relay Ir can usually be adjusted within the range of 0.7 to 1.0In. However, if electronic devices are used, its adjustment range will be larger, usually from 0.4 to 1.0In. For circuit breakers equipped with non-adjustable overcurrent trip relays, Ir=In.

(4) Short Circuit Trip Unit Current Setting Value Im

The short-circuit trip relay (instantaneous or short-delay) is used to quickly trip the circuit breaker when a high fault current value occurs, with its tripping threshold as Im.

(5) Rated Short-Time Withstand Current Icw

This refers to the current value that is allowed to pass through for a specified time. The current value passing through the conductor within the specified time will not cause damage to the conductor due to overheating.

(6) Breaking Capacity

The breaking capacity of the circuit breaker refers to its ability to safely cut off the fault current, which has no necessary connection with its rated current. Currently, there are specifications such as 36KA and 50KA. It is generally divided into ultimate short-circuit breaking capacity Icu and operational short-circuit breaking capacity Ics.

DC-MCCB

General Principles for Circuit Breaker Selection

Firstly, choose the type and pole number of the circuit breaker according to its use; select the rated current of the circuit breaker based on the maximum working current; choose the type of trip unit, the kind, and specifications of accessories as needed. The specific requirements are as follows:

  1. The rated operating voltage of the circuit breaker ≥ the rated voltage of the line.
  2. The rated short-circuit breaking capacity of the circuit breaker ≥ the calculated load current of the line.
  3. The rated short-circuit breaking capacity of the circuit breaker ≥ the maximum short-circuit current that may occur in the line (usually calculated by effective value).
  4. Single-phase to ground short-circuit current at the end of the line ≥ 1.25 times the instantaneous (or short-delay) trip setting current of the circuit breaker.
  5. The rated voltage of the undervoltage trip unit of the circuit breaker equals the rated voltage of the line.
  6. The rated voltage of the circuit breaker’s shunt trip unit equals the control power supply voltage.
  7. The rated operating voltage of the electric drive mechanism equals the control power supply voltage.
  8. When the circuit breaker is used for lighting circuits, the instantaneous setting current of the electromagnetic trip unit is generally taken as 6 times the load current.
  9. When using a circuit breaker as short-circuit protection for a single motor, the set current of the instantaneous trip unit is 1.35 times the motor’s starting current (for DW series circuit breakers) or 1.7 times (for DZ series circuit breakers).
  10. When using a circuit breaker as short-circuit protection for multiple motors, the set current of the instantaneous trip unit is 1.3 times the starting current of the largest motor plus the working current of the remaining motors.
  11. When using a circuit breaker as the main switch on the low-voltage side of the distribution transformer, its breaking capacity should be greater than the short-circuit current value on the low-voltage side of the transformer. The rated current of the trip unit should not be less than the transformer’s rated current. The set current for short-circuit protection is generally 6-10 times the transformer’s rated current; the set current for overload protection equals the transformer’s rated current.
  12. After preliminarily selecting the type and level of the circuit breaker, it should be coordinated with the protection characteristics of the upstream and downstream switches to avoid overstepping tripping and enlarging the accident range.

Selectivity of Circuit Breakers

In the distribution system, the circuit breakers used can be divided into two categories based on their protection performance: selective and non-selective. Selective low-voltage circuit breakers have two types: two-stage protection and three-stage protection. The instantaneous and short-delay characteristics are suitable for short-circuit actions, while the long-delay characteristics are applicable for overload protection. Non-selective circuit breakers generally perform instantaneous actions, only for short-circuit protection. Some perform long-delay actions for overload protection only.

In the distribution system, if the upper-level circuit breaker adopts a selective circuit breaker, and the next-level circuit breaker uses a non-selective circuit breaker or a selective circuit breaker, the main purpose is to use the delayed action of the short-delay trip unit or the difference in delay time to achieve selectivity. When the upper-level circuit breaker is delayed, please note the following points.

  1. Regardless of whether the next level is a selective circuit breaker or a non-selective circuit breaker, the set current of the upper-level circuit breaker’s instantaneous overcurrent trip unit generally should not be less than 1.1 times the maximum three-phase short-circuit current at the next-level circuit breaker’s outgoing line.
  2. If the next level is a non-selective circuit breaker, to prevent the upper-level short-delay overcurrent trip unit from acting first when a short-circuit current occurs in the circuit protected by the next-level circuit breaker (because this level’s instantaneous action sensitivity is not enough), causing it to lose selectivity. Generally, the set current of the short-delay overcurrent trip unit of the upper-level circuit breaker is not less than 1.2 times the next-level instantaneous overcurrent trip unit.
  3. If the next level is also a selective circuit breaker, to ensure selectivity, the short delay action time of the upper-level circuit breaker is at least 0.1s longer than the short delay action time of the next-level circuit breaker. In general, to ensure the selective action between the two levels of low-voltage circuit breakers, the upper-level circuit breaker should choose an overcurrent trip unit with a short delay. Also, its action current should be greater than the action current of the next-level overcurrent trip unit by at least one level. At least the action current Iop.1 of the upper level is not less than 1.2 times the action current Iop.2 of the next level, that is, Iop1 ≥ 1.2Iop.2.

Overload Current Protection Characteristics

Cascading Protection of Circuit Breakers

In the design of distribution systems, the selective coordination between the upper and lower levels of circuit breakers must possess “selectivity, speed, and sensitivity”.

Selectivity relates to the coordination between the upper and lower levels of circuit breakers, while speed and sensitivity are associated with the characteristics of the protective device itself and the operation mode of the line.

If the upper and lower circuit breakers coordinate properly, they can selectively cut off the faulty circuit, ensuring that other non-faulty circuits of the distribution system continue to work normally. Conversely, it affects the reliability of the distribution system.

Cascading protection is a specific application of the current limiting characteristics of circuit breakers. Its main principle is to use the current limiting function of the upper-level circuit breaker. When selecting the lower-level circuit breaker, you can choose a circuit breaker with a lower breaking capacity to reduce costs and save expenses. The upper-level current-limiting circuit breaker QF1 can break the maximum expected short-circuit current at its installation point. Since the upper and lower circuit breakers in the distribution system are installed in series, when a short circuit occurs at the outlet of the lower-level circuit breaker QF2, the short-circuit current is far less than the expected short-circuit current at that point due to the current limiting action of the upper-level circuit breaker QF1. In other words, the breaking capacity of the lower-level circuit breaker QF2 greatly increases with the help of the upper-level circuit breaker QF1, exceeding its rated breaking capacity.

This kind of cascading protection also has certain conditions, such as the nearby circuit cannot have important loads (because once QF1 trips, the QF3 circuit also loses power), and the instantaneous setting values of QF1 and QF2 must also match properly, etc. Cascading data can only be determined by experiment, and the selection of the upper and lower level circuit breakers can only be provided by the circuit breaker manufacturer.

 

Sensitivity of Circuit Breakers

To ensure that the instantaneous or short-delay overcurrent trip of a circuit breaker can reliably operate during the slightest short-circuit fault within its protection range under the system’s minimal operational mode, the sensitivity of circuit breaker protection must satisfy the requirements set forth in the “Low-Voltage Power Distribution Design Specification” (CB50054-95). The regulation stipulates that its sensitivity should not be less than 1.3, i.e., Sp=Ik.min/Iop≥1.3. In the formula, Iop is the action current of the instantaneous or short-delay overcurrent trip, Ik.Min is the single-phase or two-phase short-circuit current at the end of the line protected by the circuit breaker under the system’s minimal operational mode, and Sp is the sensitivity of the circuit breaker.

When selecting a circuit breaker, attention should also be paid to verifying its sensitivity. For selective circuit breakers that simultaneously have a short-delay and instantaneous overcurrent trip, it’s only necessary to verify the action sensitivity of the short-delay overcurrent trip. There’s no need to verify the sensitivity of the instantaneous overcurrent trip action.

Selection and Adjustment of Circuit Breaker Trips

(1) Setting the action current of the instantaneous overcurrent trip

Some electrical equipment protected by the circuit breaker will produce peak currents several times their rated current during the startup process, causing the circuit breaker to bear a larger peak current in a short period of time. The action current Iop(o) of the instantaneous overcurrent trip must avoid the peak current Ipk of the line, i.e., Iop(o) ≥ Krel·Ipk. Here, Krel is the reliability factor. When selecting a circuit breaker, attention should be paid to ensure that the set current of the instantaneous overcurrent trip of the circuit breaker avoids the peak current to prevent misoperation of the circuit breaker.

(2) Setting the action current and action time of the short-delay overcurrent trip

The action current Iop(s) of the short-delay overcurrent trip should also avoid the peak current Ipk of the line, i.e., Iop(s) ≥ Krel·Ipk. Here, Krel is the reliability factor. The action time of the short-delay overcurrent trip is generally divided into 0.2s, 0.4s, and 0.6s. It is determined according to the protective selectivity of the previous and next protection devices, and the action time of the previous protection should be longer than that of the next protection by one time difference.

(3) Setting the action current and action time of the long-delay overcurrent trip

The long-delay overcurrent trip is mainly used to protect overload, so its action current Iop(l) only needs to avoid the maximum load current or calculated current I30 of the line, i.e., Iop(l) ≥ Krel·I30. Here, Krel is the reliability factor. The action time of the long-delay overcurrent trip should avoid the allowable short-term overload duration to prevent misoperation of the low-voltage circuit breaker.

(4) Coordination requirements of the action current of the overcurrent trip and the protected line

To prevent the occurrence of incidents where the line is not tripped by its circuit breaker due to overload or short-circuit causing overheating and damage or even fire to the insulation cable, the action current Iop of the circuit breaker’s overcurrent trip should meet the requirements of the formula, Iop ≤ Kol·Ial. Here, Ial is the allowable carrying capacity of the insulated cable; Kol is the allowable short-term overload factor of the insulated cable, generally taking 4.5 for instantaneous and short-delay overcurrent trips, taking 1.1 when providing short-circuit protection for long-delay overcurrent trips, and 1 when providing only overload protection. If the above coordination requirements are not met, the action current of the trip should be re-selected, or the cross-sectional area of the conductor or cable should be appropriately increased.