Main characteristics of circuit breakers
Release time:
2023-02-11
The main characteristics of circuit breakers include: 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 operating voltage (Ue): This is the voltage at which the circuit breaker operates under normal (uninterrupted) conditions
The main characteristics of circuit breakers include: 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 operating voltage (Ue): This is the voltage at which the circuit breaker operates under normal (uninterrupted) conditions.
Rated current (In): This is the maximum current value that a circuit breaker equipped with a dedicated overcurrent tripping relay can withstand indefinitely at the ambient temperature specified by the manufacturer, without exceeding the temperature limit specified for the current-carrying components.
Short-circuit relay tripping current setting value (Im): The short-circuit tripping relay (instantaneous or short-delay) is used to quickly trip the circuit breaker when a high fault current value occurs, with a tripping limit Im.
Rated short-circuit breaking capacity (Icu or Icn): The rated short-circuit breaking current of a circuit breaker is the highest (expected) current value that the circuit breaker can break without being damaged. The current values provided in the standard are the RMS values of the AC component of the fault current, and the DC transient component (always present under the worst-case short circuit) is assumed to be zero when calculating the standard value. The rated values for industrial circuit breakers (Icu) and household circuit breakers (Icn) are usually given in kA RMS.
Short-circuit breaking capacity (Ics): The rated breaking capacity of a 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 controlgear 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 the circuit breaker: The breaking capacity under the conditions specified in the specified test procedure, excluding the ability of the circuit breaker to continue carrying its rated current;
2. Rated operating short-circuit breaking capacity of the circuit breaker: The breaking capacity under the conditions specified in the specified test procedure, including the ability of the circuit breaker to continue carrying its rated current;
3. The test procedure for the 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 (e.g., 380V, 50kA), and the test button is not pressed, the circuit breaker is in the closed position, press the test button, the circuit breaker passes through the 50kA short-circuit current, and the circuit breaker immediately trips (open, abbreviated as O). The circuit breaker should be intact and able to close again. t is the interval time, generally 3min, at this time the line is still in hot standby state, the circuit breaker performs another closing (close, abbreviated as C) and immediately tripping (O), (the closing test is to test the electric and thermal stability of the circuit breaker under peak current). This procedure is CO. If the circuit breaker can completely trip, 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 Icn test procedure. After the test, if the circuit breaker can completely trip and extinguish the arc, its rated operating short-circuit breaking capacity is considered qualified.
Therefore, it can be seen that the rated ultimate short-circuit breaking capacity Icn refers to the ability of a low-voltage circuit breaker to operate normally and trip the short-circuit current again after breaking the maximum three-phase short-circuit current at the circuit breaker output terminal. As for whether it can operate normally and trip again afterwards, the circuit breaker does not guarantee; while the rated operating short-circuit breaking capacity Ics refers to the ability of the circuit breaker to trip normally multiple times when the maximum three-phase short-circuit current occurs at its output terminal.
IEC947—2 "Low-voltage switchgear and controlgear Low-voltage circuit breakers" standard stipulates: The Ics of Class A circuit breakers (referring to circuit breakers with only overload long-delay and short-circuit instantaneous protection) can be 25%, 50%, 75%, and 100%. The Ics of Class B circuit breakers (circuit breakers with three-stage protection of overload long-delay, short-circuit short-delay, and short-circuit instantaneous) can be 50%, 75%, and 100% of Ics. Therefore, it can be seen that the rated operating short-circuit breaking capacity is a breaking current value smaller than the rated ultimate short-circuit breaking current.
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 main lines (including the output terminals of transformers) use them as main protective switches. Circuit breakers without short-circuit short-delay function (only with overload long-delay and short-circuit instantaneous two-stage protection) cannot perform selective protection, and they can only be used for branch circuits. IEC92 "Ship Electrical" points out: For circuit breakers with three-stage protection, the emphasis is on their operating short-circuit breaking capacity value, while for circuit breakers used in branch circuits, it should be ensured that they have sufficient ultimate short-circuit breaking capacity value.
Regardless of the type of circuit breaker, although both Icu and Ics are important technical indicators. However, for circuit breakers used on branch lines, it is sufficient to meet the rated ultimate short-circuit breaking capacity. A common bias is to choose a larger value rather than the right one, believing that a larger value is safer. However, choosing a value that is too large will cause unnecessary waste (for the same type of circuit breaker, the H-type - high breaking capacity type is 1.3 to 1.8 times more expensive than the S-type - ordinary type). Therefore, it is not necessary to pursue the operating short-circuit breaking capacity indicator for branch line circuit breakers. For circuit breakers used on main lines, not only the rated ultimate short-circuit breaking capacity requirements should be met, but also the rated operating short-circuit breaking capacity requirements should be met. If only the rated ultimate short-circuit breaking capacity Icu is used to measure whether its breaking capacity is qualified, it will bring safety hazards to users.
A circuit breaker is a basic low-voltage electrical appliance that 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 depending on different applications, and there are many varieties and specifications, and many specific technical indicators.
Circuit breaker free tripping: At any moment during the closing process of the circuit breaker, if the protective action connects the tripping circuit, the circuit breaker can reliably disconnect, which is called free tripping. Circuit breakers with free tripping can ensure that when a short-circuit fault occurs during the closing of the circuit breaker, it can be quickly disconnected, which can avoid expanding the scope of the accident.
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