Current Selectivity - Siemens SENTRON Configuration Manual

3.3

Current selectivity

Current selectivity is a form of selectivity in which the (instantaneous) protective devices trip
at different over-current values.
In the event of a fault, the following currents flow:
● A high current flows through the circuit breakers close to the infeed.
● A low current flows through the circuit breakers at a far distance from the infeed.
The tripping characteristics are plotted in a graph in order to determine current selectivity.
Advantages and limitations
Advantages of current selectivity:
● Simple to implement
● Inexpensive
Limitations of current selectivity:
● Equipment exposed to extreme current loads as a result of potentially prolonged flow of
short-circuit currents:
If the fault location is situated between the two protective devices, IEC 60364-4-41 states
that it might take up to 5 seconds for the protective devices in TN systems to trip at a
rated current of > 32 A.
● In order to reduce the thermal and mechanical loading of equipment by high short-circuit
currents, the selectivity limit current is normally set to a relatively low value. As a general
rule, only partial selectivity can be achieved with current selectivity.
Area of application
Circuit breakers can be combined to provide current selectivity in applications in which the
maximum short-circuit currents between the upstream and downstream circuit breakers are
very different.
Circuit breakers suitable for this application
Current selectivity can be implemented using air circuit breakers or molded case circuit
breakers with an electronic and a thermal-magnetic trip unit.
The circuit breakers require neither short-time delay over-current releases nor dynamic
selectivity coordination.
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
Methods of implementing selectivity using circuit breakers
3.3 Current selectivity
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