Controlling the primary and secondary fuse time difference in primary and secondary deep fusion circuit breakers is a core technology for ensuring selective protection during overload or short circuits. Its core objective is to ensure that when a circuit fault occurs, only the circuit breaker closest to the fault point operates, disconnecting the local circuit, while the upstream circuit breaker remains closed, thus avoiding widespread power outages and improving power supply reliability. This control process requires multi-dimensional technical means, including time-current characteristic matching, I²t value coordination, differential time setting, and protection characteristic optimization.
Time-current characteristic matching is the foundation for controlling the primary and secondary fuse time difference. The fuse time of a primary and secondary deep fusion circuit breaker has an inverse time-limit relationship with the current magnitude; that is, the larger the current, the shorter the fuse time. By designing the fuse characteristic curves of the primary and secondary circuit breakers, the secondary circuit breaker is prioritized to operate under smaller overload or short-circuit currents, while the primary circuit breaker operates only under larger currents or longer overload durations. For example, when a circuit experiences a minor overload, the secondary circuit breaker will blow first due to the smaller current but longer duration; if the fault persists and the current increases further, the primary circuit breaker will blow, creating a time difference.
Coordination of the I²t value is a key parameter for controlling the blowing time difference. The I²t value represents the product of the square of the current and time, reflecting the thermal effect of the fuse under short-circuit current. To ensure selectivity, the I²t value of the secondary circuit breaker must be less than the minimum tripping I²t value of the primary circuit breaker. This means that under the same short-circuit current, the secondary circuit breaker will reach its blowing threshold first due to the absorbed heat, while the primary circuit breaker, with its larger I²t value, will require a longer time or higher current to blow. This design avoids simultaneous operation of the primary and secondary circuit breakers, ensuring the locality of fault isolation.
Tiered time settings are a direct means of controlling the blowing time difference. In short-circuit protection, if the primary circuit breaker uses a short-delay trip unit, its delay time must be longer than the fusing time of the secondary circuit breaker, and the difference between the two is typically no less than 0.1 seconds. For example, if the secondary circuit breaker is instantaneous, the primary circuit breaker can be set with a short delay of 0.1 or 0.2 seconds, allowing the secondary circuit breaker to quickly interrupt the current in the early stages of a fault, while the primary circuit breaker operates with a delay after confirming that the fault has not been cleared. This difference in timing avoids overlap between the operating times of the upper and lower level circuit breakers, ensuring selectivity.
Optimizing protection characteristics is an effective way to improve the control accuracy of the fusing time difference. The protection characteristics of primary and secondary deep fusion circuit breakers must avoid overlap; that is, the operating range of the secondary circuit breaker should completely cover its protection area, while the operating range of the primary circuit breaker should maintain a safe distance from that of the secondary circuit breaker. For example, in overload protection, the long-delay trip setting current of the secondary circuit breaker should preferably be no less than 1.3 times the setting value of the primary circuit breaker to ensure that the secondary circuit breaker operates preferentially in the initial stage of overload. In short-circuit protection, the instantaneous trip setting current of the primary circuit breaker should be greater than the maximum expected short-circuit current at the output terminal of the secondary circuit breaker to prevent false tripping.
Environmental factors and installation conditions also affect the control of the fuse-breaking time difference. Temperature, humidity, and installation methods may alter the heat dissipation performance of the circuit breaker, thus affecting its fuse-breaking time. For example, high-temperature environments accelerate fuse heating and shorten the fuse-breaking time; while compact installation may lead to poor heat dissipation, causing the primary circuit breaker to operate prematurely due to temperature rise. Therefore, it is necessary to select an appropriate circuit breaker model according to the actual operating conditions and ensure that the installation complies with specifications to maintain the stability of the fuse-breaking time difference.
Controlling the primary and secondary fuse time difference in primary and secondary deep fusion circuit breaker requires multi-dimensional technical means, including time-current characteristic matching, I²t value coordination, differential time setting, protection characteristic optimization, and environmental adaptability design. These measures collectively ensure that only the circuit breaker closest to the fault point operates during a fault, achieving selective protection and improving power supply reliability.
