Semiconductor protection with 3KF SITOR

Switch disconnectors with fuses protect the load from overload and short-circuit by means of the fuses and are used to switch the specified rated current on and off under load. At the same time, they provide a safety isolating function and isolating distance in all low-voltage circuits.

Conventional fuse switching devices, such as 3KF, 3NP1 are designed to use LV HRC fuses that comply with the fuse standard IEC 60269-2. This standard describes the technical constraints on fuses for the protection of cables and motor feeders, including the maximum permitted power loss of the fuses.

Conventional fuse switching devices are designed in accordance with the maximum values permitted according to this fuse standard with respect to possible heat dissipation. Fuses for semiconductor protection (SIEMENS trade name SITOR) must disconnect much faster than standard fuses to protect the semiconductor load as reliably as possible and by virtue of their design have power loss values that in some cases significantly exceed the limit values permitted in IEC 60269-2.

To permit simple installation, not only are many application-specific special designs available but also very many fuses for semiconductor protection in the standard-compliant designs, such as the LV HRC design.

If a fuse of this kind is used in a conventional fuse switching device for standard-compliant fuses for semiconductor protection, the maximum permissible fuse power loss defined by the switching device must be complied with. If the rated power loss of the fuse for semiconductor protection is above that permitted by the switching device, the fuse can only be operated in the partial load range, i.e. the load current must be reduced until the actual power loss is within the limit values defined by the switching device. The fuse manufacturer provides the data required for this (power loss as a function of the load current).

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Power losses depending on the operational current for SIEMENS fuses for semiconductor protection

If the maximum power losses of the fuse defined by the switching device are not complied with, the switching device may overheat or, in the worst case, even cause a fire in the switchgear.

3KF SITOR is a variation of the proven 3KF LV HRC fuse switch disconnector, provides optimized heat dissipation and permits the use of fuses with substantially higher power losses. In this way, fuses for semiconductor protection can be loaded with higher currents than conventional fuse switching devices.

For use of SIEMENS fuses for semiconductor protection in 3KF SITOR and 3KF LV HRC, the possible load values are available for each possible combination of a fuse and circuit breaker type (see chapter Derating values of 3KF fuse switch disconnectors when using SITOR fuses according to IEC constraints)

Fuses from third-party manufacturers can also be used, in which case the possible load values must be determined based on the technical data of the fuse.