Frame circuit breakers are mainly used for protection and operation of overload, short circuit, over current, loss of voltage, under voltage, grounding, leakage, automatic switching of dual power supplies, and infrequent starting of the motor.
1) The rated voltage of the frame circuit breaker shall not be lower than the rated voltage of the line;
2) The rated current of the frame circuit breaker and the rated current of the overcurrent release shall not be less than the calculated current of the circuit;
3) The rated short-circuit breaking capacity of the frame circuit breaker shall not be less than the large short-circuit current in the line;
4) Selective air circuit breakers should consider the inter-stage coordination of short-time short-circuit making and breaking capacity and time-delay protection;
5) The rated voltage of the undervoltage release of the frame circuit breaker is equal to the rated voltage of the line;
6) When used for motor protection, the starting current of the motor should be considered when selecting the circuit breaker, and it should not act during the starting time;
7) The selection of the circuit breaker should also consider the selective coordination between the circuit breaker, the circuit breaker and the fuse.
Main points of circuit breaker engineering design
(1) When the circuit breaker and the circuit breaker cooperate, the instantaneous tripping action value of the upper-level circuit breaker should be considered, which should be greater than the expected short-circuit current at the outlet of the lower-level circuit breaker. If the short-circuit current value is not much different due to the short-circuit impedance value of the circuit components at the two-stage frame circuit breaker, the upper-level circuit breaker can choose a short-delay trip.
(2) When the short-circuit current of the current-limiting circuit breaker is greater than or equal to its instantaneous tripping setting value, it will trip within a few milliseconds. Therefore, lower-level protection appliances should not use the circuit breaker to achieve selective protection requirements.
(3) When the time limit of the short-delay circuit breaker is set to delay, its making and breaking capacity will decrease. Therefore, in the selective protection circuit, the short-delay on-off capability of the frame circuit breaker should meet the requirements.
(4) It should also be considered that the short-circuit delay reversible characteristic of the upper-level circuit breaker should not intersect the action characteristic time curve of the lower-level air circuit breaker, and the short-delay characteristic curve should not intersect the instantaneous characteristic curve.
(5) When the circuit breaker and the fuse are used together, the upper and lower level coordination should be considered, and the ampere-second characteristic curve of the frame circuit breaker and the ampere-second characteristic curve of the fuse should be compared to have the protection selectivity at the time of short-circuit current.