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Terms and definitions

Back-up fuse-links

Back-up fuse-links have a “rated minimum breaking current” from which the fuse-links are able to interrupt current. Backup fuse-links are not supposed to operate below their “minimum breaking current” (below l3). Their operating range is from l3 to the “maximum rated breaking current” (l1).
For the assignment of back-up fuse-links, it is important to note that the lowest short circuit current is higher at the site of the HV back-up fuse-link than l3 (IKmin > I3). If the short circuit current is lower than the minimum breaking current, additional protection must be provided.

Back up fuse links graphics


Rated voltage range

It is important for HV fuse-links that they must be operated at the voltage for which it has been rated. Accordingly, the operating voltage corresponds to the maximum rated voltage of the fuse-link. Owing to the switching voltage occurring during arcing, the fuse-link cannot be used at lower voltages without limitation. A lower operating voltage at which the fuselink can still be used without exceeding the system insulation level during extinction must therefore be taken into account.

From these two values results the permissible voltage range of the fuse-link, which is shown on the fuse-links or in the technical data, e.g. 10/24kV.

Breaking capacity I1

The breaking capacity is also referred to as the “rated maximum breaking current”. This clearly indicates that this is the maximum current which can be interrupted by the fuselink. I1 of the fuse-link must be greater than the maximum short circuit current at the site of the fuse-link (I1 > IKmax).

Minimum breaking current I3

The minimum breaking current is referred to as the “rated minimum breaking current”. This value must be specified for back-up fuse-links. From this current, back-up fuse-links are capable to breaking fault currents. The fuse-links must be assigned to the system so that no fault current below I3 can occur at the site of the fuse-link (due to the system parameters or other protective devices).

Power dissipation of a fuse-link Pwarm

The power dissipation of a HV fuse-link is specified at the rated current of the fuse-link. For protection with HV fuse-links, it should be noted that the operating current is normally half the rated current. Because of the physical relationships, the actual power dissipation is less than a quarter of the value Pwarm for HV fuse-links shown in the technical data table.

Time-current characteristic (I/t characteristic)

The time-current characteristic shows the correlation between current and time up to the melting of a fuse-element. The virtual time (tvs) is specified to enable a comparison of the l/t characteristics of fuse-links below 100ms. For co-ordination with other protective devices, e.g. switches or circuit breakers, the melting integral I2t must be referred to for melting times below 100ms.

Current limitation

At high short circuit currents, HV fuse-links interrupt current within several milliseconds. That means, the sinusoidal current does not reach its peak value and that HV fuse-links are current limiting devices. This is a significant advantage compared to mechanical switches whose contacts take longer to open and interrupt currents at natural zero. During this time, the peak short-circuit current is able to freely develop its dynamic force. By using HV fuse-links, this surge current is limited within several ms to a fraction of its peak value and the design of the subsequent system can be reduced in terms of dynamic forces.

Switching voltage

So that HV fuse-links perform a current-limiting action, the short circuit current must be limited and reduced as it increases. This requires a switching voltage that exceeds the driving system voltage and forces the current to zero. This switching voltage must not exceed the specified permissible value of 2.2 times the peak value of the maximum rated voltage. EFEN HV fuse-links are within this limit.


HV fuse-links in this product list conform to DIN 43 625. The contact cap dimensions defined in this standard are shown in Fig. Dimensions acc. to DIN 43 625 in mm. The dimension “e” varies depending on the rated voltage of HV fuse-links, which is shown as a dimension for fuses in the technical data tables. The diameter “d” also varies with the rated current, whereby this dimension is also shown in the tables.

Dimension HV fuse-links for voltage transformer