Solution for the Scientifically-Based Selection of Fuses in Low-Voltage Distribution Systems

 I.Background and Current Issues​
This solution aims to provide a scientific basis for the design, selection, and procurement of electrical protection devices by objectively comparing the technical characteristics of fuses and circuit breakers. It highlights the irreplaceable advantages and application scenarios of fuses in modern distribution systems, enabling optimal configuration that ensures safety, reliability, and cost-effectiveness.

​II. Analysis of Core Advantages of Fuses (Compared to Circuit Breakers)​​
Fuses are not outdated products; they offer distinct advantages over circuit breakers in specific applications:

1. ​Excellent Selectivity: Achieving full selective protection between upstream and downstream fuses is straightforward—it only requires meeting the 1.6:1 overcurrent selectivity ratio specified by national/IEC standards (i.e., the rated current of the upstream fuse link ≥ 1.6 times that of the downstream fuse). This characteristic makes fuses highly advantageous for protecting intermediate distribution branches, enabling precise fault isolation and minimizing the scope of power outages.
2. ​Strong Current-Limiting and Breaking Capacity: Fuses operate extremely quickly during short-circuit faults, effectively limiting the peak current and energy of short-circuit currents. Their breaking capacity is generally high (often exceeding 100 kA), ensuring reliable interruption of various short-circuit faults and safeguarding circuits and equipment.
3. ​Cost-Effectiveness and Compactness: At equivalent rated currents and breaking capacities, fuses are significantly more economical than circuit breakers (especially selective circuit breakers). Their compact size also helps optimize the spatial layout of distribution cabinets.
4. ​High Reliability and Maintenance-Free Operation: As one-time protection devices, fuses have a simple and direct operation mechanism without complex mechanical components. They offer high reliability and avoid risks such as mechanical jamming or electronic component failures that may occur in circuit breakers.

​III. Typical Application Scenarios and Solutions for Fuses​
Based on their technical features, fuses are ideal solutions for the following scenarios:

1. ​Intermediate-Level Branch Protection:
• Scenario: Distribution branches located between the main switch and end circuits in a distribution system.
• Solution: Using fuses in these positions leverages their perfect selectivity to coordinate with upstream selective circuit breakers or fuses, ensuring localized fault isolation and preventing unintended tripping. This maintains power continuity for other parts of the system while significantly reducing overall costs due to the economic advantages of fuses in large-scale applications.
2. ​Protection of Small to Medium-Capacity Main Feeders or Radial Lines:
• Scenario: Radial lines or main feeders with smaller current capacities (e.g., below 300 A) extending from low-voltage distribution panels.
• Solution: Employing high-breaking-capacity gG-type fuses provides reliable overload and short-circuit protection. Their high breaking capacity ensures safe fault interruption even when installed close to transformers.
3. ​Motor Circuit Protection:
• Scenario: End circuits supplying motors, such as those for fans and pumps.
• Solution: It is strongly recommended to use aM-type (motor protection) fuses instead of gG-type fuses. aM-type fuses are specifically designed to handle motor starting currents and short-circuit currents. Their rated current can be selected at a lower value, significantly improving protection sensitivity for short-circuit faults and ensuring better coordination with the overload protection characteristics of thermal relays.
4. ​Backup Protection:
• Scenario: Used in conjunction with non-selective circuit breakers or load switches.
• Solution: Leveraging the high breaking capacity of fuses compensates for the limited breaking capacity of certain circuit breakers (cascading technology) or provides protection functionality for load switches, forming an economical and practical protection combination.

​IV. Implementation Recommendations and Considerations​

1. ​Correct Selection:
• Use gG-type fuses for general line protection.
• Use aM-type fuses exclusively for motor protection.
• Strictly adhere to the selectivity ratio (1.6:1) for coordinating upstream and downstream devices to ensure selective protection.
2. ​Addressing Inherent Limitations:
• Single-Phase Fusing: For critical three-phase equipment, use fuse bases equipped with striker pins and alarm microswitches. These devices signal when one phase fuse blows, triggering a relay to cut off the upstream three-phase power supply and prevent phase-loss operation of motors.
• Inconvenience of Replacement: Install fuses in easily accessible locations and keep spare fuse links on hand. The need for replacement after a fault also provides clear fault indication.
3. ​Product Development:
• Standard Updates: Promptly revise national fuse standards to align with the latest IEC standards, promoting technological upgrades.
• Product Diversification: Develop more new types of fuses to offer a wider selection.
• Integrated Solutions: Provide more standardized distribution cabinet/box solutions incorporating fuses for designers and users to choose from.

​V. Conclusion​
Fuses hold a significant position in modern low-voltage distribution systems due to their unique advantages, including excellent selectivity, high breaking capacity, cost-effectiveness, and high reliability. They are not meant to "replace" circuit breakers but rather to "complement" them.

The scientific solution is to use powerful selective circuit breakers at the system’s front end and critical circuits while actively employing high-performance fuses for numerous intermediate-level branches and specific end circuits (e.g., motors). This hybrid, hierarchical configuration of protection devices ensures the construction of an optimal low-voltage distribution system that is both safe and reliable as well as economically efficient.

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