Switch-Fuse Combination

Combination of medium voltage fuses with on-load switches have proved to be a very successful solution both for short-circuit protection and disconnection of the power transformers for many years.




Fuse switch combinations are full-range switching devices which are capable of breaking both over currents and short-circuit currents up to the rated breaking current of the fuses



For switch-fuse combinations used in air insulated switchgears, two technical terms shall be considered

1- Take over current
Where switches are equipped with overcurrent release, our fuses are supposed to provide backup protection for the switch. The breaking function is exchanged from the switch to the fuse at the take-over current Ito , which can be read from the intersection point of the two curves



2- Transfer Current
The fuses are fitted with strikers in order to open automatically all three poles of the switch on the operation of one fuse-link. This combination is widely used in transformer substations, as it ensures correct operation.

The fuse selection for transformer protection has to take into account both, the transformer bolted secondary short-circuit current and the striker initiated opening time of the switch.

For practical uses, below table might be of assistance. Fuses having characteristics that pass below the intersection of the opening time of the switch (T0) and the transformer secondary short-circuit current (Isc) basically fulfils the requirements.

Let’s have a look at Fig 4 as an example. In this case, while Fuse 1 fulfills the requirements, Fuse 2 seems inappropriate for use.



Note: The selection might be very confusing for many users/manufacturers both due to complexity of the challenge and the partial discrepancy between IEC 60282-1 and IEC 62271-105. In some cases, none of the fuses seem to be complying with the requirements on paper for these reasons. You are more than welcome to contact us for technical assistance in this regard.

IMPORTANT: Fuses with Thermal Protection feature is state of art in the switch-fuse combinations. Similarly, in motor protection and capacitor protection applications, thermally protected fuses have become the global practice. We recommend this feature whenever fuses work in combination with an on-load switch or contactor.

Related LINKS:

Product Range
Thermal Protection
Gas Insulated Switchgear
Air Insulated Switchgear