Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 41)

a) b) M3~ M3~ M3~ c) d) M3~
I > I >
I >
M3~
Fig. 2.3-4
Types of load feeders (with electromechanical switchgear)
a) Fuse, contactor, motor protection relay
b) Circuit breaker with magnetic release, contactor, motor protection relay
c) Circuit breaker with motor protection characteristic, contactor
d) Operational switching and circuit breaker function combined in one contact system
For starters that are protected by circuit breakers with motor protection characteristic, no
coordination with respect to the overcurrent selectivity is required, as circuit breakers switch-off
in the event of overloads and short-circuits.
0.001
0.01
0.1
1
10
100
1000
10000
1 10
xI
e
t
100
7
1
2
4 3
5
6
Fig. 2.3-5
Short-circuit coordination of switching and protective devices. Circuit breakers with motor protection
characteristic are used as an alternative to fuse/overload relay.
1
Motor starting current
2 Trip characteristic overload relay
3 Destruction limit curve overload relay
4 Trip characteristic circuit breaker with motor protection characteristic
5 Time/current-characteristic fuse (alternative to circuit breaker)
6 Rated breaking capacity of the contactor
7 Welding area of the contactor
2.3.4.5.2 Coordination with respect to the operability after a short-circuit
The coordination of contactor and overload relay, if any, with a short-circuit protective device
with respect to the operability of starters after a short-circuit is determined by the destructive or
damaging effect of the short-circuit current on the starter components. Basic requirement –
regardless of coordination type – is that neither persons nor equipment may be endangered.
LVSAM-WP001A-EN-P - April 2009 2-13