(1) Rated Voltage
The rated voltage specified on the circuit breaker nameplate refers to the rated voltage of the main contacts; it is the voltage level at which the contacts are guaranteed to operate normally over the long term.
(2) Rated Current
The rated current specified on the circuit breaker nameplate refers to the rated current of the main contacts; it is the current level at which the contacts are guaranteed to operate normally over the long term.
(3) Tripping Current
The tripping current is the preset current value that triggers the overcurrent release mechanism. When a short circuit occurs or the load is severely overloaded-causing the load current to exceed the tripping current-the circuit breaker's main contacts open.
(4) Overload Protection Current-Time Curve
The overload protection current-time curve is an inverse-time characteristic curve; the higher the overload current, the shorter the time required for the thermal release mechanism to actuate.
(5) Rated Voltage of the Undervoltage Release Coil
The rated voltage of the undervoltage release coil must match the rated voltage of the circuit.
(6) Rated Voltage of the Shunt Release Coil
The rated voltage of the shunt release coil must match the control power supply voltage.
(7) Rated Ultimate Short-Circuit Breaking Capacity (Icu)
There are two indicators for a circuit breaker's breaking capacity: rated ultimate short-circuit breaking capacity (Icu) and rated service short-circuit breaking capacity (Ics).
Rated ultimate short-circuit breaking capacity (Icu) represents the limit of the circuit breaker's breaking capability; the breaking capacity will decrease after interrupting a certain number of short-circuit faults.
Rated service short-circuit breaking capacity (Ics) is a performance indicator signifying that the circuit breaker can still operate normally after interrupting a certain number of short-circuit faults.
For molded-case circuit breakers, an Ics value greater than 25% of Icu is considered acceptable; however, most circuit breakers on the market have an Ics between 50% and 75% of Icu.
(8) Current-Limiting Breaking Capacity
Current-limiting breaking capacity refers to the circuit breaker's ability to limit the fault current when tripping during a short-circuit event. When a short circuit occurs, the circuit breaker contacts open rapidly, generating an arc. This acts effectively like inserting a rapidly increasing arc resistance into the circuit, thereby limiting the rise of fault current. This reduces the adverse electromagnetic, electrodynamic, and thermal effects of the short-circuit current on the circuit breaker and connected equipment, thereby extending the circuit breaker's service life. The shorter the circuit breaker's opening time, the better the current-limiting effect, and the closer the rated service short-circuit breaking capacity (Ics) becomes to the rated ultimate short-circuit breaking capacity (Icu).
(9) Tripping characteristics of miniature circuit breakers (MCBs)
Circuit breaker tripping characteristics are categorized into types such as A, B, C, D, and K; their respective meanings are as follows:
Type A tripping characteristic: Tripping current is (2–3)In; suitable for protecting semiconductor electronic circuits, measurement circuits with low-power transformers, or systems with long lines and low short-circuit currents.
Type B tripping characteristic: Tripping current is (3–5)In; suitable for residential power distribution systems, protection of household appliances, and personal safety protection.
Type C tripping characteristic: Tripping current is (5–10)In; suitable for protecting distribution lines, as well as lighting circuits and motor circuits with relatively high inrush currents.
Type D tripping characteristic: Tripping current is (10–20)In; suitable for protecting equipment with very high surge currents, such as transformers and solenoid valves.
Type K tripping characteristic: Features a thermal tripping operating current of 1.2 times the rated current (In) and a magnetic tripping operating range of 8–14 times In; suitable for protecting motor circuit equipment and offers high resistance to surge currents .
