What is a Circuit Breaker

What is a Circuit Breaker

A circuit breaker is a device that cuts off the flow of electricity, just like a faucet. It controls the flow of water, while the circuit breaker controls the flow of electricity.

Circuit breakers are mainly used to protect circuits from short-circuit currents. When a short circuit occurs in a circuit, the circuit is immediately disconnected to protect the safety of the load. The rated breaking capacity of a circuit breaker is divided into two types: rated ultimate short-circuit breaking capacity and rated operating short-circuit breaking capacity.

The national standard "Low-voltage switchgear and control equipment low-voltage circuit breakers" (GB14048.2-94) explains the rated ultimate short-circuit breaking capacity and rated operating short-circuit breaking capacity of circuit breakers as follows:

(1) Rated ultimate short-circuit breaking capacity of circuit breaker (Icn): the breaking capacity of the circuit breaker under the conditions specified in the prescribed test procedure, excluding the breaking capacity of the circuit breaker to continue to carry its rated current capacity;

(2) Rated operating short-circuit breaking capacity of circuit breaker (Icn): the breaking capacity of the circuit breaker under the conditions specified in the prescribed test procedure, including the breaking capacity of the circuit breaker to continue to carry its rated current capacity;

(3) The test procedure for rated ultimate short-circuit breaking capacity (Icn) is O-t-CO. The specific test is: adjust the line current to the expected short-circuit current value (e.g. 380V, 50kA), while the test button is not closed, the circuit breaker under test is in the closed position, press the test button, the circuit breaker passes a 50kA short-circuit current, and the circuit breaker immediately breaks (open, referred to as O). The circuit breaker should be intact and can be closed again. t is the interval time, generally 3 minutes. At this time, the line is still in hot standby state. The circuit breaker is connected again (closed, referred to as C) and then disconnected (O) (the connection test is to assess the electrical and thermal stability of the circuit breaker under peak current). This procedure is CO. If the circuit breaker can completely break, its ultimate short-circuit breaking capacity is qualified.

(4) The test procedure for the rated operating short-circuit breaking capacity (Icn) of the circuit breaker is O—t—CO—t—CO. It has one more CO than the test procedure of Icn. After the test, if the circuit breaker can completely break and extinguish the arc, it is considered that its rated operating short-circuit breaking capacity is qualified. Therefore, it can be seen that the rated ultimate short-circuit breaking capacity Icn refers to the fact that the low-voltage circuit breaker can operate normally and break the short-circuit current once again after breaking the maximum three-phase short-circuit current at the outlet of the circuit breaker. As for whether it can be connected and disconnected normally in the future, the circuit breaker does not guarantee it; while the rated operating short-circuit breaking capacity Ics refers to the fact that the circuit breaker can break normally multiple times when the maximum three-phase short-circuit current at its outlet occurs.

IEC947-2 "Low-voltage switchgear and control equipment low-voltage circuit breakers" standard stipulates: Class A circuit breakers (circuit breakers with only overload long delay and short-circuit instantaneous) can have an Ics of 25%, 50%, 75% and 100% of Ics. Class B circuit breakers (circuit breakers with three-stage protection of overload long delay, short circuit short delay and short circuit instantaneous) can have an Ics of 50%, 75% and 100% of Ics. Therefore, it can be seen that the rated operating short-circuit breaking capacity is a breaking current value that is smaller than the rated limit short-circuit breaking current, and Ics is a percentage of Icu. Generally speaking, circuit breakers with three-stage protection functions of overload long delay, short circuit short delay and short circuit instantaneous can achieve selective protection, and most trunk lines (including the outlet end of the transformer) use it as the main protection switch. Circuit breakers that do not have the short-circuit short-delay function (only have two-stage protection of overload long delay and short-circuit instantaneous action) cannot be used for selective protection and can only be used in branches.

IEC92 "Marine Electrical" points out that the circuit breaker with three-stage protection focuses on its operating short-circuit breaking capacity value, while the circuit breaker used in the branch line should ensure that it has sufficient ultimate short-circuit breaking capacity value. Regardless of the type of circuit breaker, although it has the two important technical indicators of Icu and Ics. However, as a circuit breaker used on the branch line, it can only meet the rated ultimate short-circuit breaking capacity. The more common bias now is to take a large rather than just the right one, and think that taking a large insurance. But taking too large will cause unnecessary waste (for the same type of circuit breaker, its H type-high breaking type is 1.3 times to 1.8 times more expensive than the S type-ordinary type). Therefore, there is no need for the circuit breaker on the branch line to blindly pursue its operating short-circuit breaking capacity index. For the circuit breaker used on the trunk line, it must not only meet the requirements of the rated ultimate short-circuit breaking capacity, but also meet the requirements of the rated operating short-circuit breaking capacity. If the rated ultimate short-circuit breaking capacity Icu is used to measure whether its breaking capacity is qualified or not, it will bring unsafe hidden dangers to users.

Definition of Circuit Breaker Terms

1. Rated current parameters of circuit breaker

The national standard "Low-voltage switchgear and control equipment low-voltage circuit breakers" GBl4048.2-94 (equivalent to IEC947-2) uses two concepts for the rated current of the circuit breaker, the rated current 1n of the circuit breaker and the rated current 1nm of the circuit breaker frame grade, and gives the following definitions:

——The rated current 1n of the circuit breaker refers to the current that the release can pass for a long time, that is, the rated current of the release. For a circuit breaker with an adjustable release, it is the maximum current that the release can pass for a long time.

——The rated current of the circuit breaker frame grade is lnm, expressed as the maximum rated current of the trip unit installed in a frame or plastic casing with the same basic geometric dimensions and similar structure.

The definition of circuit breaker rated current in the national standard GBl4048.2-94 is somewhat different from the concept we usually talk about. When we mention the concept of "circuit breaker rated current", we usually refer to "circuit breaker frame grade rated current" rather than "trip unit rated current". For example, when we choose a DZ20Y-100/3300-80A circuit breaker, we usually simply say that its rated current is 100A and the rated current of the trip unit is 80A. In the product information provided by most low-voltage circuit breaker suppliers, the complex term "circuit breaker frame grade rated current" is generally not mentioned, but only the parameter "circuit breaker rated current" is given. In fact, "circuit breaker rated current" is a short name for "circuit breaker frame grade rated current", which seems to be more appropriate. Perhaps the inconsistency between the definition of rated current in the standard and the usual use is one of the reasons for the confusion.

"Circuit breaker frame grade rated current" is a parameter that indicates the current carrying capacity of the circuit breaker frame, which is mainly determined by the current carrying capacity of the main contacts. It also determines the maximum rated current value of the release that can be installed. This parameter is indispensable when selecting a circuit breaker.

2. Current parameters of overcurrent release

The types of circuit breaker releases include overcurrent releases, undervoltage releases, shunt releases, etc. Overcurrent releases can also be divided into overload releases and short-circuit (electromagnetic) releases, and can be divided into long delay, short delay, and instantaneous. Overcurrent releases are the most commonly used.

The operating current setting value of the overcurrent release can be fixed or adjustable, usually using a knob or lever. Electromagnetic overcurrent releases can be either fixed or adjustable, while electronic overcurrent releases are usually always adjustable.

Overcurrent releases can be divided into fixed installation or modular installation according to the installation method. Fixed installation releases and circuit breaker housings are processed as one piece. Once shipped, the rated current of the release cannot be adjusted, such as DZ20 type; while modular installation releases are a mounting module of the circuit breaker and can be replaced at any time, which is very flexible, such as MerlinGerin's NS type.

The current of the overcurrent release is indicated by the following parameters:

——The rated current 1n of the release refers to the maximum current that the release can pass for a long time.

——The setting value of the action current of the long-time delay overload release is Ir. For fixed releases, 1r=In; for adjustable releases, Ir is a multiple of the rated current 1n of the release, such as 1r=0.4~1×1n.

——The short-time delay electromagnetic release action current setting value Im is a multiple of the overload release action current setting value Ir. The multiple is fixed or adjustable, such as Im=2～10×Ir. For non-adjustable types, an appropriate setting value can be selected.

——The rated value of the instantaneous electromagnetic release action current Im' is a multiple of the release rated current In, and the multiple is fixed or adjustable, such as Im'=1.5~11×In. For non-adjustable types, an appropriate setting value can be selected.

3. Short-circuit characteristic current parameters of circuit breaker

3.1 Rated short-circuit breaking capacity Icn

The rated short-circuit breaking capacity Icn of the circuit breaker should be expressed in Icu and Ics and determined in the specific product standards.

3.2 Rated ultimate short-circuit breaking capacity Icu

The rated ultimate short-circuit breaking capacity Icu is the value of the ultimate short-circuit breaking current under the specified test voltage and other specified conditions of the circuit breaker. It can be expressed as the expected short-circuit current. After the specified test procedure o-t-co is operated, the circuit breaker is not considered to continue to carry its rated current.

o—indicates breaking operation;

co—indicates that the disconnection operation is followed by the connection operation;

t—represents the time interval between two consecutive operations, generally not less than 3 minutes.

3.3 Rated operating short-circuit breaking capacity Ics

Rated operating short-circuit breaking capacity Ics refers to a breaking current value smaller than the rated limit short-circuit breaking current of the circuit breaker under the specified test voltage and other specified conditions. Ics is a percentage of Icu. After operating according to the specified test procedure o-t-co-t-co, the circuit breaker should have the ability to continue to carry its rated current.

For small circuit breakers with a rated short-circuit breaking capacity greater than 1500A, the national standard "Circuit Breakers for Household and Similar Places" GBl0963 (equivalent to IECB98) stipulates that the rated ultimate short-circuit breaking capacity Icu and the rated operating short-circuit breaking capacity Ics test should be carried out. When Icu≤6000A, Icu=Ics, so only the Ics test is required. Therefore, for small circuit breakers with a short-circuit breaking capacity of 4500A or 6000A, Icu=Ics=Icn, so generally only the rated short-circuit breaking capacity Icn value is mentioned.

3.4 Rated short-time withstand current Icw

The rated short-time withstand current Icw refers to the current value that the circuit breaker withstands for a short time under the specified test conditions. For AC, this current value is the effective value of the periodic component of the expected short-circuit current, and the time related to the rated short-time withstand current is at least 0.05s.

4. Calibrate the current parameters of the circuit breaker

The short-circuit current parameters Icu, Ics, and Icw of the circuit breaker need to be considered when selecting the circuit breaker. The circuit breaker model and frame grade rated current Inm are determined after selection, so there is no need to indicate them separately; the rated current parameters of the circuit breaker and the current parameters of the selected release need to be clearly indicated according to the actual situation.

How to choose a circuit breaker

1. Select the breaking capacity of the circuit breaker according to the expected short-circuit current of the line. The accurate calculation of the expected short-circuit current of the line is an extremely tedious task. Therefore, there are some simple calculation methods with small errors and acceptable in engineering:

(1) For transformers with a voltage level of 10/0.4KV, the short-circuit capacity on the high-voltage side can be considered to be infinite (the short-circuit capacity on the 10KV side is generally 200-400MVA or even larger, so if it is considered infinite, the error is less than 10%).

(2) Article 2.1.2 of GB50054-95 "Low Voltage Power Distribution Design Code" stipulates: "When the sum of the rated currents of the motors connected near the short-circuit point exceeds 1% of the short-circuit current, the influence of the motor feedback current should be taken into account." If the short-circuit current is 30KA, 1% of it should be 300A. The total power of the motor is about 150KW, and when it is started and used at the same time, the feedback current taken into account at this time should be 6.5∑In.

(3) The impedance voltage UK of the transformer indicates that the secondary side of the transformer is short-circuited. When the secondary side reaches its rated current, the primary side voltage is a percentage of its rated voltage. Therefore, when the primary side voltage is the rated voltage, the secondary side current is its expected short-circuit current.

(4) The rated secondary current of the transformer is Ite=Ste/1.732U, where Ste is the capacity of the transformer (KVA), and Ue is the rated secondary voltage (no-load voltage). At 10/0.4KV, Ue=0.4KV. Therefore, the secondary rated current of the transformer should be simply calculated as the transformer capacity x1.44~1.50.
(5) According to the definition of Uk in (3), the short-circuit current (three-phase short circuit) of the secondary is I(3)=Ite/Uk, which is the AC effective value.

(6) Under the same transformer capacity, if there is a short circuit between two phases, then I (2) = 1.732I (3) / 2 = 0.866I (3)

(7) The above calculations are all current values ​​when the transformer outlet is short-circuited, which is the most serious short-circuit accident. If the short-circuit point is a certain distance away from the transformer, the line impedance needs to be considered, so the short-circuit current will be reduced. For example, the SL7 series transformer (the distribution conductor is a three-core aluminum wire cable) has a capacity of 200KVA. When the transformer outlet is short-circuited, the three-phase short-circuit current I (3) is 7210A. When the short-circuit point is 100m away from the transformer, the short-circuit current I (3) drops to 4740A; when the transformer capacity is 100KVA, the short-circuit current at its outlet is 3616A. When the short-circuit occurs at a distance of 100m from the transformer, the short-circuit current is 2440A. When the short-circuit occurs at a distance of 100m, the short-circuit current is 65.74% and 67.47% of that at 0m respectively. Therefore, when designing, users should calculate the rated current of the installation location (line) and the maximum short-circuit current that may occur at that location. And select the circuit breaker according to the following principles: the rated current In of the circuit breaker ≥ the rated current IL of the line; the rated short-circuit breaking capacity of the circuit breaker ≥ the expected short-circuit current of the line. Therefore, when selecting a circuit breaker, there is no need to give too large a margin to avoid waste.

The Ics value determined by the circuit breaker manufacturer, any Icu percentage value that meets the above standards is a valid and qualified product. Most (not all specifications) of the universal (frame type) circuit breakers have three-stage protection functions of overload long delay, short circuit short delay and short circuit instantaneous, which can realize selective protection. Therefore, most trunk lines (including the output end of the transformer) use it as the main (protection) switch, while molded case circuit breakers generally do not have the short circuit short delay function (only overload long delay and short circuit instantaneous two-stage protection), cannot be used for selective protection, and can only be used in branches. Due to different usage (applicability) situations, IEC92 "Marine Electrical" recommends: universal circuit breakers with three-stage protection focus on their operating short-circuit breaking capacity value, while molded case circuit breakers used in large quantities on branch lines ensure that they have sufficient ultimate short-circuit capacity value. Our understanding of this is that it is necessary to be cautious when replacing the circuit breaker after the main line has cut off the fault current. A power outage on the main line will affect a large number of users, so two COs are required when a short-circuit fault occurs, and they are required to continue to carry the rated current for a period of time. On the branch line, after the breaking of the extreme short-circuit current and the closing and opening again, it has completed its mission and no longer carries the rated current, so it can be replaced with a new one (the impact of the power outage is relatively small).

However, both universal and molded case circuit breakers must have two important technical indicators: Icu and Ics. Only the Ics value is slightly different on the two types of circuit breakers. The minimum allowable Ics of the molded case circuit breaker can be 25%Icu, and the minimum allowable Ics of the universal circuit breaker is 50%. There are very few circuit breakers with Ics=Icu, and even the universal type rarely has Ics=100% [There is a molded case circuit breaker with rotating double breaking (point) technology abroad. It has excellent current limiting performance and a large margin of breaking capacity, which can achieve Ics=Icu, but the price is very high. The Ics of my country's DW45 intelligent universal circuit breaker is 62.5%~65%Icu. Internationally, ABB's F series and Schneider's M series are only about 70%, while the Ics of various new models of molded case circuit breakers in China are generally between 50% and 75%Icu.

The electrical clearance and creepage distance of the circuit breaker determine the electrical clearance of electrical products, which must be based on the insulation coordination of the low-voltage system. The insulation coordination is based on the fact that the transient overvoltage is limited to the specified impulse withstand voltage, and the transient overvoltage generated by the electrical appliances or equipment in the system must also be lower than the impulse voltage specified by the power system. Therefore:

(1) The rated insulation voltage of the appliance should be ≥ the rated voltage of the power supply system

(2) The rated impulse withstand voltage of the electrical appliance should be ≥ the rated impulse withstand voltage of the power supply system

(3) The transient overvoltage generated by the electrical equipment should be ≤ the rated impulse withstand voltage of the power supply system.

Application of four-pole circuit breakers Regarding the application of four-pole circuit breakers, there are currently no national standards or regulations in China that provide rigid requirements for their use. Although the design specifications for regional four-pole electrical appliances (circuit breakers) have been issued, the debate over whether to install or not to install four-pole electrical appliances is still ongoing. In recent years, there has been a trend of widespread use in some areas, and various circuit breaker manufacturers have also designed and manufactured various types of four-pole circuit breakers and put them on the market. The author agrees with one opinion, that is, whether to use or not should be based on whether the reliability and safety of power supply can be ensured, so generally speaking:

(1) TN-C system. In the TN-C system, the N line and the protective line PE are combined into one (PEN line). For safety reasons, the PEN line is not allowed to be disconnected at any time, so four-pole circuit breakers are absolutely prohibited;

(2) TT system, TN-CS system and TN-S system can use four-pole circuit breakers to ensure the safety of maintenance personnel during maintenance. However, in TN-CS and TN-S systems, the N pole of the circuit breaker can only be connected to the N wire, not the PEN or PE wire;

(3) In places where dual power switching is installed, since all neutral lines (N lines) in the system are interconnected, a four-pole circuit breaker must be used to ensure the safety of maintenance of the switched power switch (circuit breaker);

(4) For the single-phase main switch entering the residence, it is advisable to use a two-pole circuit breaker with an N pole (used as an isolator during maintenance)

(5) For residual current protectors (leakage circuit breakers) used in 380/220V systems, the neutral line must pass through the zero-sequence current transformer (iron core) of the protector to prevent the passage of no neutral line, which would cause the 220V load to have leakage current and malfunction. In this case, a four-pole or two-pole residual current protector with a neutral line should be selected.