FuseLoad BreakMV AC Switchgear

Coordination between fuse and load break switch in MV grid

Load currents are interrupted with the load break switch and short circuit currents are interrupted with the fuse.
Under short circuit currents, the fuse quickly interrupts. The striker trips but the load break switch acts only as a disconnector switch.
Fuse/load break switch combination depend on the magnitude of the current. For large currents the fuse interrupts (the fuse breaking time = the fuse melting time + the fuse arcing time). For small currents the breaker interrupts (the breaker interruption time = the fuse melting time + the striker operation time + the breaker operation time).
Consequently, if the interruption time for the fuse is shorter than for the load break switch, the fuse interrupts and vice versa. Thus, the breaker must be able to interrupt currents up to the transfer current, while the fuse must be able to interrupt all currents larger than this.
The various fuse/load break switch interactions are shown in Figure.
The interruption time for the breaker is closely correlated to the fuse melting characteristic. The fuse time/current characteristic is for high currents coincident with the melting characteristic.

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