Selection and Setting Points of Circuit Breakers
Mar 19, 2026| The selection of circuit breakers shall be based on actual operating conditions, including utilization category, rated operational voltage, rated current, rated setting current of the release, and other parameters. The protection characteristics shall be selected according to the protection characteristic curves provided in the product catalog, and the short-circuit characteristics and sensitivity coefficient shall be verified.
Classification of Circuit Breakers
(1) Air Circuit Breaker (ACB)
ACB is also known as universal circuit breaker. All components are mounted in an insulated metal frame, usually open type. It can be equipped with various accessories, and the replacement of contacts and parts is convenient. It is mostly used as the main switch at the power supply side.
Over-current releases include electromagnetic, electronic and intelligent types. The circuit breaker provides four-stage protection:long-time delay, short-time delay, instantaneous and earth fault protection.The setting value of each protection can be adjusted within a certain range according to its frame rating.
ACB is applicable to AC 50 Hz, rated voltage 380 V, 660 V, and distribution networks with rated current from 200 A to 6300 A.It is mainly used for power distribution and protecting lines and power supply equipment against overload, undervoltage, short-circuit, single-phase earth fault and other faults.
With various intelligent protection functions, it can realize selective protection.Under normal conditions, it can be used for infrequent switching of circuits.Breakers below 1250 A can be used to protect motors against overload and short circuit in 380 V AC 50 Hz networks.
ACB is also often used as the main outgoing switch on the 400 V side of transformers, bus tie switches, large-capacity feeder switches and large motor control switches.
(2) Moulded Case Circuit Breaker (MCCB)
MCCB is also known as modular circuit breaker. Its grounding terminal, external contacts, arc extinguishing chamber, release and operating mechanism are enclosed in a plastic moulded case.
Auxiliary contacts, undervoltage release, shunt release and other components are mostly modularized with a very compact structure.Generally, maintenance is not considered, and it is suitable for branch circuit protection switches.
MCCB usually contains a thermal-magnetic trip unit, while large-size MCCBs are equipped with solid-state trip sensors.
Over-current releases for MCCB include electromagnetic and electronic types.Generally, electromagnetic MCCBs are non-selective circuit breakers with only long-time delay and instantaneous protection.Electronic MCCBs provide four protection functions: long-time delay, short-time delay, instantaneous and earth fault protection.
Some newly launched electronic MCCBs are also equipped with zone selective interlocking (ZSI) function.
MCCB is generally used for distribution feeder control and protection, main low-voltage outgoing switch of small distribution transformers, power distribution terminal control, and can also be used as power switch for various production machinery.
(3) Miniature Circuit Breaker (MCB)
MCB is the most widely used terminal protection device in building electrical terminal distribution systems.It is used for short-circuit, overload, overvoltage and other protections for single-phase and three-phase circuits below 125 A, including 1P, 2P, 3P and 4P poles.
MCB consists of operating mechanism, contacts, protection devices (various releases), arc extinguishing system, etc.Its main contacts are closed manually or electrically.After closing, the free trip mechanism locks the main contacts in the closed position.
The coils of over-current releases and thermal elements of thermal releases are connected in series with the main circuit.The coils of undervoltage releases are connected in parallel with the power supply.
In civil building electrical design, MCB is mainly used for overload, short-circuit, over-current, loss of voltage, undervoltage, earthing, earth leakage, automatic transfer of dual power supplies, and protection and control of motors during infrequent starting.
Basic Characteristic Parameters of Circuit Breakers
(1) Rated Operational Voltage Ue
Rated operational voltage is the nominal voltage of the circuit breaker, at which it can operate continuously under specified normal service and performance conditions.
In China, for voltage levels of 220 kV and below, the maximum operating voltage is 1.15 times the system rated voltage.For voltage levels of 330 kV and above, the maximum operating voltage is 1.1 times the rated voltage.
The circuit breaker shall maintain insulation at the system maximum operating voltage and is capable of making and breaking operations under specified conditions.
(2) Rated Current In
Rated current is the current that the release can carry continuously at an ambient temperature below 40 ℃.For circuit breakers with adjustable releases, it is the maximum current that the release can carry continuously.
When used at an ambient temperature exceeding 40 ℃ but not higher than 60 ℃, the load shall be derated for continuous operation.
(3) Overload Release Current Setting Ir
When the current exceeds the overload release setting Ir, the circuit breaker trips with a time delay.This value also represents the maximum current that the circuit breaker can carry without tripping.
This value must be greater than the maximum load current Ib, but less than the maximum permissible current Iz of the circuit.
For thermal-magnetic releases, Ir is usually adjustable in the range of 0.7 ~ 1.0 In.For electronic releases, the adjustment range is wider, typically 0.4 ~ 1.0 In.For circuit breakers equipped with non-adjustable over-current releases, Ir = In.
(4) Short-Circuit Release Current Setting Im
Short-circuit release relays (instantaneous or short-time delay) are used to trip the circuit breaker rapidly when high fault current occurs. The tripping threshold is Im.
(5) Rated Short-Time Withstand Current Icw
This is the current value allowed to flow during a specified time, which will not cause damage to conductors due to overheating within the specified duration.
(6) Breaking Capacity
The breaking capacity of a circuit breaker refers to its ability to safely interrupt fault currents, which is not necessarily related to its rated current.
Common ratings include 36 kA, 50 kA, etc.It is generally divided into:
Ultimate Short-Circuit Breaking Capacity Icu
Service Short-Circuit Breaking Capacity Ics
General Principles for Circuit Breaker Selection
First, select the type and number of poles of the circuit breaker according to the application;
select the rated current according to the maximum operating current;
select the type of release, types and specifications of accessories as required.
The specific requirements are as follows:
⑴The rated operational voltage (Ue) of the circuit breaker ≥ the rated voltage of the circuit.
⑵The rated short-circuit making and breaking capacity of the circuit breaker ≥ the calculated load current of the circuit.
⑶The rated short-circuit making and breaking capacity of the circuit breaker ≥ the maximum short-circuit current that may occur in the circuit (generally calculated as RMS value).
⑷The single-phase earth fault current at the circuit end ≥ 1.25 times the instantaneous (or short-time delay) tripping setting current of the circuit breaker.
⑸The rated voltage of the undervoltage release of the circuit breaker is equal to the rated voltage of the circuit.
⑹The rated voltage of the shunt release of the circuit breaker is equal to the control power supply voltage.
⑺The rated operational voltage of the motorized operating mechanism is equal to the control power supply voltage.
⑻When the circuit breaker is used for lighting circuits, the instantaneous setting current of the electromagnetic release is generally 6 times the load current.
⑼When a circuit breaker is used for short-circuit protection of a single motor:
Instantaneous tripping setting current =1.35 times the motor starting current (for DW series)or 1.7 times the motor starting current (for DZ series).
⑽When a circuit breaker is used for short-circuit protection of multiple motors:
Instantaneous tripping setting current =1.3 times the starting current of the largest motorplus the operating current of the remaining motors.
⑾When a circuit breaker is used as the main switch on the low-voltage side of a distribution transformer:
●Its breaking capacity shall be greater than the short-circuit current on the low-voltage side of the transformer.
●The rated current of the release shall not be less than the rated current of the transformer.
●Short-circuit protection setting current: generally 6–10 times the rated current of the transformer.
●Overload protection setting current: equal to the rated current of the transformer.
⑿After preliminarily selecting the type and rating of the circuit breaker,coordinate with the protection characteristics of upstream and downstream circuit breakersto prevent cascading tripping and expanding the fault scope.
Selectivity of Circuit Breakers
According to their protection performance, circuit breakers used in power distribution systems can be classified into two categories: selective and non‑selective.
Selective low‑voltage circuit breakers include two‑stage protection and three‑stage protection types.Among them, the instantaneous characteristic and short‑time delay characteristic are used for short‑circuit tripping, while the long‑time delay characteristic is used for overload protection.
Non‑selective circuit breakers usually operate instantaneously and are only used for short‑circuit protection.Some provide long‑time delay operation and are only used for overload protection.
In a power distribution system, if the upstream circuit breaker is selective and the downstream circuit breaker is non‑selective or selective, selectivity is mainly achieved by using the time delay of the short‑time delay release or different time delays.
When using the time delay of the upstream circuit breaker, attention shall be paid to the following points:
⑴Regardless of whether the downstream breaker is selective or non‑selective,the setting current of the instantaneous over‑current release of the upstream breaker shall generally not be less than 1.1 times the maximum three‑phase short‑circuit current at the outgoing terminal of the downstream breaker.
⑵If the downstream breaker is non‑selective,to prevent the upstream short‑time delay over‑current release from operating first due to insufficient instantaneous tripping sensitivity of the downstream breaker during a short circuit in its protected circuit (which would lose selectivity),the setting current of the upstream short‑time delay over‑current release shall generally not be less than 1.2 times that of the downstream instantaneous over‑current release.
⑶If the downstream breaker is also selective,to ensure selectivity, the short‑time delay operating time of the upstream breaker shall be at least 0.1 s longer than that of the downstream breaker.
Generally speaking, to ensure selective operation between two levels of low‑voltage breakers,the upstream breaker should be equipped with a short‑time delay over‑current release,and its operating current should be at least one level higher than that of the downstream breaker.At least the operating current of the upstream breaker Iop.1 shall not be less than 1.2 times the operating current of the downstream breaker Iop.2, i.e.:Iop.1≥1.2Iop.2
Cascade Protection of Circuit Breakers
In the design of power distribution systems, the selective coordination between upstream and downstream circuit breakers must meet the requirements of selectivity, speed and sensitivity.
Selectivity is related to the coordination between upstream and downstream circuit breakers, while speed and sensitivity are related to the characteristics of the protective device itself and the operation mode of the circuit respectively.
Proper coordination between upstream and downstream circuit breakers can selectively disconnect the faulty circuit, ensuring that other healthy circuits in the distribution system continue to operate normally.On the contrary, it will affect the reliability of the distribution system.
Cascade protection is a specific application of the current-limiting characteristic of circuit breakers.Its main principle is to utilize the current-limiting effect of the upstream circuit breaker, so that breakers with lower breaking capacity can be selected for downstream circuits, so as to reduce cost and save expenses.
The upstream current-limiting circuit breaker QF1 is capable of breaking the maximum prospective short-circuit current at its installation location.Since the upstream and downstream breakers in the distribution system are installed in series, when a short circuit occurs at the outlet of the downstream breaker QF2, the actual short-circuit current is much lower than the prospective short-circuit current at that location due to the current-limiting effect of QF1.
In other words, the breaking capacity of the downstream breaker QF2 is greatly enhanced with the assistance of QF1, exceeding its rated breaking capacity.
This kind of cascade protection also has certain conditions.For example, adjacent circuits shall not carry important loads (because once QF1 trips, the QF3 circuit will also lose power).At the same time, the instantaneous setting of QF1 must match properly with that of QF2.
Cascade data can only be determined by tests, and the coordination and selection of upstream and downstream circuit breakers can only be confirmed and provided by the circuit breaker manufacturer.
Sensitivity of Circuit Breakers
To ensure that the instantaneous or short-time delay over-current release of the circuit breaker can operate reliably under the minimum operating mode of the system when the slightest short-circuit fault occurs within its protection range, the protection sensitivity of the circuit breaker must meet the requirements specified in Code for Design of Low-Voltage Power Distribution (GB 50054-95).
The sensitivity shall be not less than 1.3, that is:Sp=Ik.min/Iop≥1.3
Where:
Iop = Operating current of the instantaneous or short-time delay over-current release
Ik.min = Single-phase or two-phase short-circuit current at the end of the protected line under the minimum operating mode of the system
Sp = Sensitivity of the circuit breaker
Attention shall also be paid to sensitivity verification during circuit breaker selection.For selective circuit breakers equipped with both short-time delay and instantaneous over-current releases, only the operating sensitivity of the short-time delay over-current release needs to be verified; verification of the instantaneous over-current release is not required.
Selection and Setting of Circuit Breaker Releases
(1) Setting of Operating Current for Instantaneous Over‑current Release
Among the equipment protected by the circuit breaker, some electrical apparatus will generate peak current several times their rated current in a short time during starting, which makes the circuit breaker bear large peak current in a short period.
The operating current Iop(o) of the instantaneous over‑current release must exceed the peak current Ipk of the circuit, namely:Iop(o)≥Krel⋅IpkWhere Krel is the reliability coefficient.
During circuit breaker selection, ensure that the setting current of the instantaneous over‑current release exceeds the peak current to avoid unwanted tripping.
(2) Setting of Operating Current and Time for Short‑time Delay Over‑current Release
The operating current Iop(s) of the short‑time delay over‑current release shall also exceed the peak current Ipk of the circuit, namely:Iop(s)≥Krel⋅IpkWhere Krel is the reliability coefficient.
The operating time of short‑time delay releases is usually graded as 0.2 s, 0.4 s and 0.6 s.It shall be determined according to the selectivity coordination of upstream and downstream protective devices.The operating time of the upstream protection shall be one time interval longer than that of the downstream protection.
(3) Setting of Operating Current and Time for Long‑time Delay Over‑current Release
The long‑time delay over‑current release is mainly used for overload protection.Therefore, its operating current Iop(l) only needs to exceed the maximum load current (calculated current I30) of the circuit:Iop(l)≥Krel⋅I30Where Krel is the reliability coefficient.
The operating time of the long‑time delay release shall exceed the duration of allowable short‑time overload to avoid unwanted tripping of the circuit breaker.
(4) Coordination Between Operating Current of Over‑current Release and Protected Cable
To prevent insulation overheating, damage or even fire caused by overload or short circuit without tripping, the operating current Iop of the over‑current release shall meet:Iop≤Kol⋅IalWhere:
Ial = permissible current‑carrying capacity of insulated cable
Kol = permissible short‑time overload factor of insulated cable
The value of Kol:
For instantaneous and short‑time delay releases: 4.5
For long‑time delay release used as short‑circuit protection: 1.1
For long‑time delay release used only as overload protection: 1
If the above coordination requirements are not met, adjust the release operating current,or increase the cross‑sectional area of the conductor or cable accordingly.