Selectivity implemented by combinations of protective devices
4.4 Fuse and downstream fuse
Selectivity for 3VA molded case circuit breakers
54 Configuration Manual, 08/2016, A5E03603181010-01
Example
① Tolerance range of characteristic
Figure 4-1 Current-time diagram for LV HRC fuse tripping characteristics
The characteristics and their tolerance ranges do not touch. Total selectivity is assured.
Since the rated currents of the two affected fuses in this example are graded in a ratio of
100:160 = 1:1.6, it can be assumed that the system is totally selective even without analysis
of the current-time diagram.
Sources of information and other documentation
Fuse Systems
BETA Low-Voltage Circuit Protection
Technology Primer
Order No.: E10003-E38-10T-G3021
3VA2 molded case circuit breaker with ELISA
The 3VA2 molded case circuit breaker with the electronic trip unit (ETU) 340 ELISA has
been specially developed for implementing selectivity with LV HRC fuses.
As described in previous chapters, there are many conditions that need to be fulfilled in order
to implement selectivity between an LV HRC fuse and a molded case circuit breaker
because the characteristic curves of these two devices are different.
Siemens has now succeeded in developing a tripping characteristic for the ETU 340 of the
3VA2 molded case circuit breaker that has a similar characteristic curve to LV HRC fuses
over the entire over-current zone. It has thus been possible to minimize the margin between
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