How to make short circuit protection and overload protection for miniature circuit breakers

August 6, 2019

First, overload protection:


The overload protection function of the circuit breaker is realized by the principle that the bimetal is oriented and curved according to the temperature rise. After the small circuit breaker is closed, the inner bimetal is heated due to a certain current in the normal working state. The sheet metal has a different coefficient of thermal expansion and causes bending.


The normal current (1.13In) has a small bending angle, so the thrust is not enough to trip the tripping mechanism. When the line has a general overload, when the overload current (1.45In) is reached, the bimetal bending angle is large and the tripping is triggered. The lever in the mechanism, the thrust is enough to push the trip mechanism, so that the tripping of the miniature circuit breaker acts as an overload protection.


The current flowing through the miniature circuit breaker is different, and the bimetal produces a different degree of bending.


In the general overload of the line, the tripping time of the circuit breaker is generally longer because the overload current is not too large. In the current time-current characteristic of the current GB10963.1-2005 standard, the overcurrent trip current is 1.45 times of the rated current. The trip time should be within 1 hour.


Second, short circuit protection:


The short circuit protection function of the circuit breaker is realized by the instantaneous release.


According to F = IN (suction is proportional to the product of current and the number of turns), the instantaneous trip unit is connected in the circuit because the number of turns of the instantaneous trip unit is small (generally only 10 匝 or less).


When the circuit is working normally, due to the small number of turns, the suction generated by the normal working current is not enough to overcome the reaction force of the spring, so the line can work normally.


When the line is short-circuited or severely overloaded, a very high current flows through the induction coil to generate a strong magnetic field, which is several times or even several tens of times larger than the normal operating current.


The number of turns of the coil has not changed, but the current is increased several times or even several times, so the suction is also increased several times or even several times. The push lever is used to make the circuit breaker quickly trip. Due to the large current, the trip time of the circuit breaker is generally Within 0.1s.


And as long as the reaction force spring is selected reasonably, it can meet the setting requirements of the B-type, C-type, and D-type instantaneous release. Another important component is the arc chute. At the moment when the circuit breaker is closed, a large arc is generated between the two contacts.


The role of the arc chute is to reduce the arc quickly by increasing the heat dissipation area and the arc release space, thereby reducing the adverse effects of the arc.