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Analysis and Selection of Miniature Circuit Breakers: Tripping Causes During Use

Time:2024-02-29   Author:As Beam   Browse:

In modern life, circuit breakers play a crucial role as an electrical device and are among the most important safety devices in both commercial and residential circuits. When the current in a house's wiring becomes too high, this simple device cuts off the power supply until the fault is resolved. Since China's reform and opening up, the household appliance industry has developed rapidly with increasing production and variety, greatly satisfying and enriching consumer needs. Circuit breakers have been widely used in low-voltage distribution systems at all levels of feeders, and for power control and protection of various machinery equipment and electrical terminals, being visible everywhere. With the development needs of various industries, the misuse of circuit breakers due to a lack of understanding of technical parameters has led to unnecessary power outages or equipment disconnections, reducing power supply reliability and causing economic losses. The structure of a C45 circuit breaker, which currently occupies a significant market share, and C65 and L7 structures are gradually becoming popular in the market.

MCB Structure.jpg


1. Working Principle

Minature Circuit Breaker Working Principle.jpg

Miniature Circuit breakers can not only connect and disconnect normal load current and overload current but also short-circuit current. Despite the variety of circuit breakers, their basic structure and working principle are essentially the same. The bimetallic strip is very important as it drives the contact to open when there is an overload current. When an overload current occurs, the bimetallic strip heats up and deforms to a certain extent, pushing the traction rod and causing the operating mechanism to drive the contact open. In the event of a short-circuit, the current can reach tens of times the rated current, causing the circuit breaker's electromagnetic trip unit to act quickly, pushing the traction rod, and disconnecting the contact. The tripping of Miniature Circuit breakers seen in daily life is primarily due to the heating and bending of the bimetallic strip.


2. Fault Analysis

In normal commercial and residential use, circuit breakers trip when devices consuming high power are used simultaneously, causing the actual working current at the load end of the circuit breaker to exceed the rated current, especially after several hours of continuous use. The National Standard GB10963.1-2005 specifies the time-action characteristics of circuit breakers, which can protect the supply lines from overload or short-circuit faults by disconnecting the circuit. The table shows that when the current at the load end reaches a certain value, it will cause the circuit breaker to trip within a specified time range, effectively protecting the supply line.



Besides the impact of load current, the rise in temperature is also an important indicator of circuit breaker performance. AC operation leads to eddy currents and hysteresis losses in ferromagnetic materials and dielectric losses in insulators, turning almost all these losses into heat. If the circuit breaker's connection terminals are not tightened properly, increased contact resistance due to reduced contact area will decrease the current capacity and cause rapid temperature increases at the terminal, affecting the circuit breaker's performance and lifespan. Factors such as using poor-quality wires or improper installation can also lead to temperature increases and tripping. Continuous heating softens copper at about 100-200°C, and this threshold increases to around 300°C for short-term heating.


To ensure reliability and lifespan, GB 10963.1-2005 specifies that the temperature rise at the external wire connection terminals of Miniature Circuit breakers should not exceed 60 K when the ambient air temperature does not exceed 40°C, with an average temperature within 24 hours not exceeding 35°C. Exceeding this temperature rise can cause the bimetallic strip to bend and the device to trip, affecting normal operation. Thus, temperature rise is an important criterion for assessing the reliability and safety of circuit breakers.



3. Selection

Based on the analysis above, when installing circuit breakers, select based on the following criteria: the rated current should be greater than or equal to the actual working current in the load; the rated voltage should be greater than or equal to the rated voltage of the power source and load; the breaking capacity should be greater than or equal to the maximum expected short-circuit current in the line; and the cross-sectional area of the copper conductor corresponding to the rated current. Proper selection of these aspects and using the appropriate wire cross-sectional area can reduce the incidences of circuit breaker tripping, enhancing effectiveness, safety, and reliability in commercial and residential use.

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TAG:   Circuit Breaker MCB Miniature Circuit Breaker