| Transformer protection | Expulsion fuse | A fusible element melts when current exceeds its operating characteristics. The resulting arc is extinguished inside a tube, commonly through gases produced by the tube lining. | Used in some distribution transformer protection schemes. Confirm voltage rating, interrupting rating, available fault current, and coordination with upstream and downstream devices. |
| Transformer protection | Current-limiting fuse | When its element melts, an arc forms and is quenched within the fuse body, often using a granular arc-quenching material. A suitable current-limiting fuse can reduce peak let-through current and fault energy for high-current faults. | Check the fuse’s voltage and interrupting ratings, current-limiting performance, and coordination. Some protection arrangements use it with another device to cover a wider range of fault currents. |
| Liquid-filled transformer protection | Bayonet fuse | A removable fuse assembly installed in certain liquid-filled transformers. Its fuse link melts under specified overcurrent conditions; the assembly and its surrounding equipment are designed for the transformer’s installation environment. | Verify compatibility with the transformer design, insulating liquid, voltage class, and protection scheme. Some designs coordinate a bayonet fuse with a separate backup current-limiting fuse. |
| Secondary-side protection | Low-voltage HRC fuse | A high-breaking-capacity fuse interrupts overcurrent on a low-voltage circuit. Its enclosed construction is designed to contain and extinguish the arc within the fuse body. | Select for the secondary circuit’s voltage, load current, prospective fault current, and coordination requirements. It does not replace appropriate primary-side transformer protection. |
| Electrical rating | Voltage and continuous-current ratings | The voltage rating must suit the system, while the current rating and time-current behavior must suit the protected equipment and operating conditions. | Check system voltage, transformer rated current, installation conditions, and applicable utility or project specifications. Do not select by amperage alone. |
| Fault interruption | Interrupting rating | This rating indicates the maximum prospective fault current a fuse is designed to interrupt safely under specified conditions. | Confirm that the rating is adequate for the available fault current at the installation point and is appropriate for the system voltage. |
| Coordination | Time-current and inrush coordination | Fuse response depends on current magnitude and duration. Transformer energization can produce temporary magnetizing inrush, so protection must distinguish expected inrush from damaging faults as required by the design. | Review time-current curves alongside transformer data and upstream and downstream protective-device curves. Use an engineering study for the actual system. |
| Supplier assessment | Test documentation and traceability | Reliable procurement requires verifiable product ratings, test documentation, clear identification, and traceable manufacturing information. | Request applicable test reports, datasheets, batch or serial traceability, and confirmation that the offered configuration matches the project specification. |
| Supplier assessment | Engineering and application support | Technical support helps verify fuse selection, compatibility, coordination, and replacement requirements for the intended transformer and system. | Ask whether the supplier can review transformer nameplate data, system fault levels, fuse curves, applicable standards, and installation requirements. |