Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 38)

u
B
u i
p
i
K
i
D
t
0
t
V t
A
t
K
t
I
2
t=
∫
tk
k
dt i .
2
u,i
Fig. 2.3-1
Basic characteristic of current and voltage when clearing a short-circuit with a current limiting circuit
breaker
u System
voltage
u
B
Electric arc voltage
i
B
p
Prospective peak short-circuit current
i
K
Limited short-circuit current
i
D
Cut-off current
t
0
Inherent system delay
t
V
Electric arc hesitation time
t
A
Rise time
t
K
Total break time
2.3.4.1 Joule integral I
2
t
The I 2 t-value is a measure of the thermal loading of the electrical equipment in the shorted
circuit. Fuses and current limiting circuit breakers limit the short-circuit current to values
significantly below those of the uninfluenced current and thus reduce the thermal loading of the
devices in the shorted circuit, for example of the contact system of a contactor connected
downstream. The rule of thumb is that the Joule integral of the short-circuit protective device
must be smaller than the permissible I
2 t-value of the conductor and of the electrical equipment
to be protected.
2.3.4.2 Cut-off current I
D
The cut-off current is the largest instantaneous value of the current that a current limiting short-
circuit protective device allows through. As the action of the force of the electrical current is
proportional to the square of the current, the cut-off current is critical in ensuring the required
mechanical strength of connected electrical equipment. This is particularly relevant for the
design of bus systems (number and strength of the supports). IEC 60439 takes this circum-
stance into account by dispensing from the requirement of verification of the short-circuit
withstand capacity for cut-off currents
≤ 17 kA.
2.3.4.3 Rated short-time withstand current l
CW
Like the Joule integral, the short-time withstand current l
CW
is a measure of the thermal load
capacity. It is important for circuit breakers of category B (suitable for selectivity) and is usually
stated as the 1s-current (preferred values under IEC 60947-2 are 0.05, 0.1, 0.25, 0.5 and 1 s). A
conversion of I
cw
currents for other durations is permitted according to the equation
I
cw1
2 · t
1
= I
cw2
2 · t
2
= const.
The I
cw
-value is of importance when for selectivity reasons the breaking action of circuit
breakers is delayed. The circuit breakers in the shorted circuit must be able to carry the short-
circuit current until the delay time has expired and then shut down the shorted circuit. For circuit
LVSAM-WP001A-EN-P - April 2009 2-10