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Discussion on selectivity of low voltage circuit breaker
2016/7/25 SUMVICT421
Discussion on selectivity of low voltage circuit breaker

Low-voltage circuit breaker is a kind of electrical appliance with the functions of circuit breaking protection, overload protection, control and isolation. It has been widely used in industrial and civil terminal low-voltage distribution system. At present, intelligent low-voltage circuit breaker has also appeared. In the distribution system, if its design is improper, it will affect the normal operation of the power supply loop. Low voltage circuit breakers should pay attention to the following problems in the design of distribution system.

In order to ensure the normal operation of low-voltage distribution system, the selectivity of low-voltage circuit breaker has become an important part of terminal low-voltage distribution system design. In the distribution system protected by circuit breakers, when an electrical fault occurs, the closest circuit breaker action to the fault point removes the fault, while the other levels of circuit breakers do not operate, thus limiting the power failure caused by the fault to a minimum, so that the fault-free power supply circuit can still maintain normal power supply, which is to low-voltage circuit breakers. The required selectivity.

Non-selective low-voltage circuit breaker refers to the operation of the nearest low-voltage circuit breaker to the fault point when an electrical fault occurs, and other levels of circuit breakers also operate, are in an open state, can not guarantee the normal power supply of other fault-free circuits. The selectivity of low-voltage circuit breaker plays an important role in the design of low-voltage distribution system. It can bring convenience to users and ensure the continuity of power supply circuit. Therefore, household appliances without selective protection, once electrical failures occur, the continuity of the distribution circuit can not be guaranteed, so that household appliances such as refrigerators, fume exhausters, etc. in the standby state of shutdown, affecting the daily life of users. The moulded case circuit breaker can cut off the current automatically after the current exceeds the setting off.

In low-voltage distribution system, if the upper circuit breaker is selective circuit breaker, then the next stage needs to use non-selective circuit breaker or selective circuit breaker, it mainly uses the short delay tripper delay action current or time difference to obtain selectivity. But there are several points for attention:
1. Whether the next stage is a selective or non-selective low-voltage circuit breaker, the setting current of the instantaneous overcurrent tripper of the upper stage circuit breaker should not be less than 1.1 times of the maximum three-phase short-circuit current at the outlet of the next stage.
2. When the next stage is a non-selective low-voltage circuit breaker, in order to prevent short-circuit current from occurring in the protective circuit of the next stage, the short-delay over-current tripper of the upper stage acts first because of the inadequate sensitivity of the instantaneous operation of the first stage, which makes it lose selectivity. The fixed current should not be less than 1.2 times of the instantaneous overcurrent release of the next stage.
3. When the next stage is a selective circuit breaker, in order to ensure its selectivity, the short delay operation time of the upper stage should be at least 0.1s longer than that of the next stage.
4. Generally speaking, in order to ensure the selective operation between the upper and lower circuit breakers, the over-current tripper with short delay should be selected at the upper stage, and its operating current should be greater than at least one stage of the next stage, and the operating current Iop1 of the upper stage should be at least 1.2 times that of the next stage, i.e. Iop1 < 1.2 Iop 2.
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