User Manual Manual

3-43 Functions
Publication 825-UM001B-EN-P January 2001
Tripping is delayed by 50 ms. This enables the circuit breaker to be actuated rapidly while
preventing unnecessary tripping by current peaks. In the event of a short-circuit, the separate
output relay #1 trips, regardless of the other protective functions. The output relay #1
actuates a circuit breaker with adequate breaking capacity. To prevent the contactor from
opening under short-circuit conditions, relay MR remains blocked at currents ≥ 12 I
e
. If a
thermal trip occurred shortly before the short circuit, relay MR assumes the tripped position
as soon as the current has dropped to < 12 I
e
.
Figure 3.25 Interruption of a Short-Circuit
Application
• Medium/high-voltage motors
ATTENTION
!
The short-circuit protection function must not be used for switching
off the contactor.
I
t
Q
1
M
8
2
5
-
M
I
A
I
A
I
12
I
e
I
e
I
I
e
I
e
I
1
2
3
t
v
t
v
t
t
Q
t
Q
t
t
LB
t
LB
t
R
e
l
a
i
s
#
1
t
BL
t
BL
t
R
e
l
a
i
s
M
R
Q1M Circuit-breaker (tripping relay)
 Current curve

A
Pickup value

e
Rated service current
t
v
Tripping delay 50 ms
t
Q
Operating time of the breaker
t
LB
Arc duration
1 Short-circuit
2 Contact separation
3 Short-circuit interruption
t
BL
Relay MR blocked at ≥ 12 
e