Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 130)

Selectivity between a circuit breaker and downstream fuse
Time
t
Fig. 4.1-23
Selectivity between circuit breaker and downstream fuse
1 = Circuit breaker
2 = Fuse
Current I (r.m.s.)
Selectivity in the tripping range of the short-circuit release of the circuit breaker is given when
the cut-off current of the fuse is smaller than its trip value.
Selectivity and undervoltage
In a short-circuit the supply voltage breaks down at the short-circuit location. The size of the
residual voltage depends on the impedance of the fault. If an electric arc is produced, the
voltage is appr. 30 V to 70 V.
As the short-circuit current flows over the entire power line up to power source, along this line
there is a voltage drop whose size is determined by the impedances lying between the two
points. All connected electrical consumers are affected by the voltage drop and the closer they
are to the fault location the greater is this effect. Devices such as contactors or undervoltage
releases of circuit breakers may trip depending on the amount and duration of the voltage drop.
In order to guarantee operational continuity, suitable off-delays or remaking equipment should
be provided. When short-circuits are broken by current limiting circuit breakers, voltage break-
downs are so short that no disruptions should be expected.
4.1.3.3.4 Short-circuit coordination
Short-circuit coordination determines the extent of damage and consequences with respect to
the operational interruption as a consequence of a short-circuit for motor starters and load
feeders.
See Section 2.3.4.5.2 .
4.2 Protective devices
4.2.1 Fuses
Fuses as the oldest protective devices in electrical engineering still have a wide area of
application, although circuit breakers are continually gaining currency in plant construction and
are supplanting fuses.
4.2.1.1 Principle of operation
Protection by fuses is based on letting a piece of electrical conductor melt or fuse, providing so
to speak an intentional weak point in a circuit. To this end, a certain current-generated tempera-
ture is required. Full-range fuses have a soldered joint that is the intended weakpoint and
bottleneck of the conductor for small overcurrents and a constricted area of the conductor that is
LVSAM-WP001A-EN-P - April 2009 4-22