Switch disconnectorsSwitch disconnectors
Derating values of 3KF fuse switch disconnectors when using SITOR fuses according to IEC constraints
The 3KF switch disconnectors with fuses are suitable for all fuses in LV HRC design. When using fuses for semiconductor protection (SITOR) in LV HRC design, it must be noted that they have a higher power loss and can become considerably hotter during operation than fuses for cable protection. For this reason, the fuse must be operated below the rated current In of the device (derating) when installed in a closed switching device.
The 3KF SITOR series is optimized for high heat dissipation and can usually be loaded with higher current values when fuses for semiconductor protection are used.
The following table shows the permissible load currents of the SITOR semiconductor fuses for installation in 3KF LV HRC and 3KF SITOR according to IEC constraints. The specified values apply to a 3KF mounted in accordance with the test conditions of IEC 60947-1 (open installation, length of the incoming and outgoing cables 1 m) installed vertically and with insulated cables, cross section corresponding to the rated operational current of the 3KF switch disconnector (acc. to Table 9 and 10 of the IEC 60947-1).
Further derating is necessary if a combination of 3KF and SITOR fuses is used under different conditions that impair heat dissipation.