The FLRN36-12D SF6 Load Switch-Fuse Combination Unit extends the functionality of the base FLN36-12D by integrating high-voltage current-limiting fuses into the SF6 sealed housing. This combination creates an all-in-one switching and protection device specifically optimized for distribution transformer protection, offering a cost-effective alternative to circuit breakers in 12kV ring main units and compact substations .
When protecting a distribution transformer, the FLRN36-12D handles two distinct fault regimes. For overload and low-level fault currents up to the transfer current of 1700A, the load switch performs the interruption, utilizing SF6 arc-quenching capability. For higher short-circuit currents, the current-limiting fuse operates within milliseconds, clearing the fault before it reaches its full peak value. The fuse's expected short-circuit breaking current of 50kA provides robust protection for transformers up to 1250kVA rating .
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The coordination between the switch and fuse is managed through the striker pin mechanism. When a fuse operates, its striker activates a trip bar that mechanically opens the load switch in all three phases. This ensures three-phase tripping even if only one fuse has operated, preventing single-phasing conditions that would severely damage a transformer. The transfer current value of 1700A defines the boundary where the fuse and switch coordination is optimized for transformer protection .
The combination unit is fully compliant with GB16926 (high-voltage load switch-fuse combinations) and IEC60420 standards. The fuse characteristics are specifically matched to transformer inrush currents—the unit can safely switch off no-load transformers up to 1250kVA without nuisance operation. The fuses are accessible for replacement after operation without breaking the SF6 seal, simplifying maintenance .
Therefore, for cost-effective, reliable transformer protection in ring main units and prefabricated substations, the FLRN36-12D combination unit delivers integrated switching and fault clearing in a compact footprint. Putai also supports customization and the supply of related medium-voltage accessories to meet system integration needs. For assistance selecting the correct fuse rating for your transformer application, contact our technical specialists.
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