Selectivity implemented by combinations of protective devices
4.2 Fuse and downstream molded case circuit breaker
Selectivity for 3VA molded case circuit breakers
52 Configuration Manual, 08/2016, A5E03603181010-01
A safer, and in most cases, higher selectivity limit value (I
s
) can be calculated from the
I
2
t diagram.
The maximum let-through I
2
t
a
value of the circuit breaker is compared to the minimum
pre-arcing I
2
t
S
value of the fuse in this diagram. Since this diagram only compares extreme
values, tolerances are not taken into account.
① Pre-arcing energy of the fuse
② Let-through energy of the circuit breaker
It is possible to read a far more precise selectivity limit current (I
s
) value from the I
2
t diagram.
This value is approximately 7300 A in the above example.
Further information
The pre-arcing time-current characteristics for Siemens 3NA LV HRC fuses can be found in
the Configuration Manual "Fuse Systems", order number 3ZW1012-3NW10-0AB1.
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