FUSE                                  

fuse

These protective elements are integral components of power electronics design. They range from the tiny glass cartridge fuses for control circuits to long, medium-voltage types. Each has characteristics that are tailored for the particular applications. Control fuses should be rated for about 125% of the expected load current. Standard types can be used for most control circuits, but slo-blo fuses should be used for loads such as small motors and contactor coils that may draw inrush currents. Semiconductor fuses are a special type that can limit the fault current by clearing subcycle, and they often protect power semiconductors from load faults. Made with multiple thin, silver links embedded in sand with a binder, they melt very quickly on faults and extinguish the arc by evaporating the binder and melting the sand. They are available in a wide range of currents, voltages, and case styles. Most have a ceramic case, and many are designed to fi t directly into buswork. Some high-current types are built as matched units, paralleled by the vendor. In pulsed applications, they should not be loaded with an rms pulse current more than 60 to 70% of the melting current for the pulse duration. Steady-state current should not exceed 80% of rated. If protection of semiconductors is a design objective, the fuse I 2 t rating should be well under the I 2 t rating of the semiconductor. Better coordination can be obtained in SCR converters if each SCR path is fused rather than the supply lines. This arrangement also offers protection from internal bus-to-bus faults when the load can source power. Medium-voltage fuses are available as “E” rated for transformers and general-purpose applications, and “R” rated for use with applications such as motors with high starting currents. Most mount in clip assemblies. These fuses may be matched in resistance and paralleled by the vendor for higher currents. All high-current fuses should be bolted into sanded buswork with joint compound and suffi cient pressure to ensure a minimum resistance. Fuses are rated under the assumption that the buswork to which they are mounted will sink heat from the fuse and not source heat into it.