In power systems, primary and secondary fusion circuit breakers, as critical protection devices, have their overload protection action sequence designed in a coordinated manner, directly impacting equipment safety and system stability. When the circuit load exceeds the rated value, the overload protection mechanism triggers tripping action through thermal or current accumulation effects. The action sequence of primary and secondary fusion circuit breakers needs to be differentiated based on equipment level, load characteristics, and protection configuration to achieve graded protection and fault isolation.
The main circuit breaker typically undertakes the overload protection task of the system's main circuit, with its action logic centered on "monitoring first, then responding." When the current continuously exceeds the rated value, the thermal trip unit within the main circuit breaker gradually accumulates deformation energy through the physical characteristic of the bimetallic strip bending under heat. If the load current drops back to a safe range within a short time, the bimetallic strip returns to its original shape, preventing false tripping; if the current continues to exceed the limit, and the deformation energy reaches a critical value, the tripping mechanism is triggered to open the main contacts, cutting off the main circuit current. This design makes the main circuit breaker the first line of defense against overload, and its action time is typically relatively long to accommodate short-term overload scenarios such as motor starting. Auxiliary circuit breakers are primarily used for independent protection of branch circuits or critical loads, with their operating logic emphasizing "rapid response and precise isolation." When the auxiliary circuit breaker detects an over-limit current in the protected circuit, its electromagnetic trip unit immediately activates—the current flowing through the trip coil generates a magnetic field, attracting the armature to overcome the spring tension and directly push the trip lever to open the contacts. Due to the extremely short response time of the electromagnetic trip unit, the auxiliary circuit breaker can quickly disconnect the faulty branch before the primary circuit breaker operates, preventing the overload current from spreading to other circuits. For example, in a system with multiple motors operating in parallel, if one motor experiences a surge in overload current, the auxiliary circuit breaker will trip first, preventing the primary circuit breaker from malfunctioning due to the combined effects of all loads, thus avoiding a system-wide power outage.
The operating sequence of primary and secondary fusion circuit breakers must follow the principle of "graded protection," meaning the operating priority is determined based on the fault location and its impact range. If an overload fault occurs within the protection range of the secondary circuit breaker, the secondary circuit breaker should operate first, disconnecting only the faulty branch, while the primary circuit breaker remains closed to maintain power supply to other circuits. If the fault current exceeds the rated capacity of the secondary circuit breaker, or if the secondary circuit breaker fails to operate due to a fault, the primary circuit breaker will activate as backup protection, disconnecting the main circuit to prevent equipment damage. This hierarchical protection mechanism is achieved through differentiated design of operating times—the tripping time of the secondary circuit breaker is typically shorter than that of the primary circuit breaker, creating a time gradient to ensure accurate fault location and isolation.
In complex systems, the coordination between primary and secondary fusion circuit breakers also requires consideration of the selection and parameter settings of protection devices. For example, the primary circuit breaker's long-delay overload protection (inverse-time characteristic) needs to complement the secondary circuit breaker's instantaneous short-circuit protection (definite-time characteristic): the primary circuit breaker allows for short-term overloads to avoid frequent tripping, while the secondary circuit breaker has zero tolerance for short-circuit faults, ensuring rapid disconnection. Furthermore, by adjusting the trip current threshold, precise division of protection range can be achieved—the operating current threshold of the secondary circuit breaker is typically lower than that of the primary circuit breaker, forming a hierarchical strategy of "small faults, small isolation; large faults, large protection."
In practical applications, the operating sequence of the primary and secondary fusion circuit breaker may be adjusted depending on the system configuration. In a dual-power supply system, if the primary circuit breaker detects an overload on the power supply side, it may prioritize disconnecting from the faulty power supply while simultaneously switching to the backup power supply via the secondary circuit breaker to achieve uninterrupted power supply. In busbar segmented operation scenarios, the primary circuit breaker may control the operating sequence of the secondary circuit breakers through interlocking devices to prevent busbar undervoltage due to misoperation. These scenarios all require mechanical interlocking, electrical interlocking, or intelligent control algorithms to ensure that the operating sequence of the primary and secondary fusion circuit breakers in overload protection meets system safety requirements.
The operating sequence of the primary and secondary fusion circuit breakers in overload protection is a comprehensive reflection of hierarchical protection, precise isolation, and system stability. By differentiating the design of thermal trip units and electromagnetic trip units, configuring the gradient of their operating times, and finely setting the protection parameters, the two form a complementary protection system. This avoids the expansion of the power outage area caused by the operation of a single protection device, while ensuring rapid current interruption in the event of a fault, minimizing equipment damage and system risks. This collaborative mechanism is one of the core guarantees for the safe operation of the power system.
