Selected according to the maximum operating current of the circuit breaker rated current; need to select the type of release, attachment type and size. Specific requirements are:
① breaker rated voltage ≥ circuit rated voltage;
② breaker rated short-circuit breaking capacity ≥ calculate the load current;
③ breaker rated short circuit breaking capacity ≥ maximum possible short-circuit current (generally based on RMS calculation);
④ opposite end of the line single-circuit current ≥ 1.25 times the circuit breaker instantaneous (or short delay) tripping setting current;
⑤ breaker under voltage release voltage rating equal to the rated line voltage;
⑥ shunt trip circuit breaker rated voltage equal to the control power supply voltage;
⑦ electric transmission mechanism rated working voltage equal to the control power supply voltage;
⑧ breaker for lighting circuits, transient electromagnetic release setting current generally take the load current of 6 times.
(2) taken as a single motor circuit breaker short-circuit protection, instantaneous tripping setting current for the motor starting current of 1.35 times (DW series circuit breaker) or 1.7-fold (DZ series circuit breaker).
(3) using the breaker as a multi-stage motor short-circuit protection, instantaneous tripping setting current of 1.3 times the maximum starting current of a motor coupled with the rest of the motor operating current.
(4) used as a distribution transformer low voltage side of the circuit breaker main switch when its breaking capacity should be greater than the low-pressure side of the transformer short-circuit current value, release of not less than the rated current of the transformer rated current, short circuit protection setting current is generally 6-10 times the rated current of the transformer; overload protection setting current is equal to the rated current of the transformer.
(5) initially selected circuit breaker type and grade, they still had the upper and lower switch with protective characteristics, so as not to override trip, expanding scope of the accident.
2, motor protection circuit breaker selection
Motor protection circuit breaker can be divided into two categories:
A class is only for circuit breaker protection without the burden of normal operation; another means breaker protection and do not require frequent operation doubles purposes. After the class of cases to be considered operating conditions and electrical life. Motor protection circuit breaker selection principles are:
(A) long delay current setting equal to the motor rated current.
(2) Instantaneous setting current: the protection cage motor circuit breaker instantaneous setting current is equal to (8-15) times the rated motor current, depending on the model of the motor to be protected, capacity and start conditions; wound rotor motor for the protection of breaker instantaneous setting current is equal to (3-6) times the rated motor current, depending on the wound rotor motor protected model, capacity and launch conditions.
(3) six times the current setting of the long delay to return time is greater than equal to the actual motor startup time. Severity of the load at startup, you can choose to return to the time of 1,3,5,8,15 S in a file.
3, the conductor selection circuit breakers
Lighting, living with a wire protection circuit breaker is used to protect the life of the building distribution system circuit breaker, the choice should be considered:
(A) time-lapse setting value less than or equal line calculation load current.
(2) instantaneous action setting value equal to (6-20) times the line calculated load current.