Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 131)

broken by high overcurrents (short-circuit currents) by the Joulean heat impulse I 2 t. Part-range
fuses are exclusively designed for short-circuit protection.
4.2.1.1.1 Current limitation
Cut-off current (let-through current)
Fuses trip at very high currents so quickly that the circuit is broken before the short-circuit
current can reach its prospective peak value. The highest instantaneous value of the current
that is attained during the circuit breaking process is known as the cut-off (let-through) current
I
D
. The current limitation is specified by means of cut-off current diagrams ( Fig. 4.2-1 ). These
state the peak value of the current that may flow through the fuse as a function of the prospec-
tive short-circuit current.
I
D
Current p
eak v
a
lue (l
og)
Prospective current (r.m.s. value) (log)
Prospective current
peak value
Fig. 4.2-1
Cut-off current diagram for fuses
I
n1
, I
n2
, I
n3
= rated currents of fuses
I
D
= max. cut-off current
Let-through - I 2 ·t value
The I 2 ·t value (correct ∫ i 2 ·dt) represents the heat energy that the fuse lets through and that
stresses the circuit up to the location of the fault. The better the current limitation, the more the
fault current is reduced and the smaller is the destructive effect of a short-circuit. The faster the
short-circuit is cleared, the smaller is the I
2 ·t value.
Distinction is made between the melting I 2 ·t, which occurs up to melting of the fuse-conductor,
and the total I
2
·t value which represents the total energy until quenching of the electric arc of the
fuse. The two values only start to diverge significantly with large short-circuit currents or when
the total breaking times are shorter than a half-cycle. The I
2 ·t–values of the fuses are critical for
the mutual selectivity of fuses at high currents.
4.2.1.1.2 Breaking capacity
The effective current limitation and the associated very high breaking capacity are specific
properties of fuses that assure their continued use in certain applications in short-circuit
protection.
4.2.1.2 Standards and utilization categories
So that fuses can be adapted to the respective requirements, a variety of models with various
current-time characteristic curves have been developed. The parameters and tests are defined
in various standards. The applicable standards for low-voltage fuses are
IEC 60269 – 1 (General requirements)
IEC 60269 – 2 (Fuses intended for use by authorized persons)
IEC 60269 – 3 (Fuses intended for use by unskilled persons)
IEC 60269 – 4 (Fuses intended for the protection of semiconductor devices).
LVSAM-WP001A-EN-P - April 2009 4-23