The characteristics of the circuit breaker are mainly: rated voltage Ue; rated current In; tripping current setting range of overload protection (Ir or Irth) and short-circuit protection (Im); rated short-circuit breaking current (industrial circuit breaker Icu; household circuit breaker Icn), etc.
Rated working voltage (Ue): This is the voltage at which the circuit breaker works under normal (uninterrupted) conditions.
Rated current (In): This is the maximum current value that a circuit breaker equipped with a special overcurrent tripping relay can withstand indefinitely at the ambient temperature specified by the manufacturer, and will not exceed the temperature limit specified by the current-bearing component.
Short-circuit relay tripping current setting value (Im): Short-circuit tripping relays (instantaneous or short-delay) are used to quickly trip the circuit breaker when high fault current values occur, and its tripping limit is Im.
Rated short-circuit breaking capacity (Icu or Icn): The rated short-circuit breaking current of the circuit breaker is the highest (expected) current value that the circuit breaker can break without being damaged. The current value provided in the standard is the RMS value of the AC component of the fault current. The DC transient component (which always appears under the worst-case short circuit) is assumed to be zero when calculating the standard value. Industrial circuit breaker ratings (Icu) and household circuit breaker ratings (Icn) are usually given in the form of kA RMS values.
Short-circuit breaking capacity (Ics): The rated breaking capacity of the circuit breaker is divided into 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 breaker" (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 breakers: under the conditions specified in the specified test procedures, 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 breakers: under the conditions specified in the specified test procedures, 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 is O-t-CO.
The specific test is: adjust the current of the line to the expected short-circuit current value (for example, 380V, 50kA), and 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 is immediately disconnected (open is 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 (closed for short 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 disconnect, 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 disconnect 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 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 broken 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 breaker" standard stipulates: Class A circuit breakers (referring to circuit breakers with only overload long delay and short-circuit instantaneous) can have an Ics of 25%, 50%, 75% and 100%. 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 ultimate short-circuit breaking current. [1]
Generally speaking, circuit breakers with three-stage protection functions, i.e., long overload delay, short short circuit delay and short circuit instantaneous action, can realize selective protection. Most trunk lines (including transformer outlets) use them as main protection switches. Circuit breakers without short circuit delay function (only two-stage protection functions, i.e., long overload delay and short circuit instantaneous action) cannot be used for selective protection. They can only be used in branch lines. IEC92 "Marine Electrical" points out that circuit breakers with three-stage protection are biased towards their operating short circuit breaking capacity, while circuit breakers used in branch lines should ensure that they have sufficient ultimate short circuit breaking capacity.
Regardless of the type of circuit breaker, although they all have the two important technical indicators of Icu and Ics, as a circuit breaker used on a branch line, it is sufficient to only meet the rated ultimate short circuit breaking capacity. The more common bias is to prefer a larger value to a more appropriate value, and to believe that a larger value is safer. However, if the value is too high, it will cause unnecessary waste (for the same type of circuit breaker, the H type - high breaking type is 1.3 to 1.8 times more expensive than the S type - ordinary type). Therefore, it is not necessary for the circuit breaker on the branch line to pursue its operating short-circuit breaking capacity index. For the circuit breaker used on the main line, it should 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.
The circuit breaker is a basic low-voltage electrical appliance. The circuit breaker has overload, short circuit and undervoltage protection functions, and has the ability to protect lines and power supplies.
The main technical indicators are rated voltage and rated current. Circuit breakers have different functions according to different applications, and there are many varieties and specifications, as well as many specific technical indicators.
Free tripping of circuit breaker: At any time during the closing process of the circuit breaker, if the protection action connects the tripping circuit, the circuit breaker can be completely and reliably disconnected, which is called free tripping. A circuit breaker with free tripping can ensure that it can be quickly disconnected when a short-circuit fault occurs, thereby avoiding the expansion of the scope of the accident.