Selection Of Miniature Circuit Breakers

1. Selection of Ordinary Residual Current Circuit Breakers

The following principles should be followed when selecting residual current circuit breakers:

(1) The rated voltage and current of the circuit breaker should be greater than or equal to the normal operating voltage and current of the line equipment;

(2) The residual current to be protected by the line should be less than or equal to the specified residual current protection current of the circuit breaker;

(3) The ultimate breaking capacity of the circuit breaker should be greater than or equal to the maximum short-circuit current of the circuit;

(4) The rated current of the overload trip unit should be greater than or equal to the maximum load current of the line;

(5) It should have a short breaking response time to protect the line and equipment.

2. Selection of Four-Pole Circuit Breakers

The following principles should be followed when selecting four-pole circuit breakers:

(1) According to IEC 465.1.5, a four-pole circuit breaker should be used for the transfer circuit between the normal power supply and the standby generator;

(2) A four-pole circuit breaker should be used for a dual power supply transfer circuit breaker with residual current protection. Two upstream circuit breakers are equipped with leakage protection, and the downstream power transfer circuit breaker should be a four-pole circuit breaker;

(3) The power switching circuit breaker between two different grounding systems should be a four-pole circuit breaker;

(4) Four-pole circuit breakers are strictly prohibited in TN-C systems;

(5) TN-S and TN-C-S systems generally do not need to be equipped with four-pole circuit breakers. However, whether four-pole circuit breakers are not required in some special cases of TN-S systems (severe three-phase imbalance, high zero-sequence harmonic content, etc.) needs further research;

(6) The power incoming circuit breaker of TT system should be a four-pole circuit breaker;

(7) When there is a neutral line in the IT system, a four-pole circuit breaker should be used .