Chapter A General rules of electrical installation design

N - Characteristics of particular sources and loads
Circuit
breaker
Magnetic
relay
Contactor
Thermal
relay
Cable
Motor
End of
start-up
period
1 to
10 s
20 to
30 ms
Fig. N64 : Tripping characteristics of a circuit-breaker + contactor + thermal relay (1)
(1) In the majority of cases, short-circuit faults occur at the
motor, so that the current is limited by the cable and the wiring
of the starter and are called impedant short-circuits
t
In Is
5 Asynchronous motors
b Better protection for the starter in case of overcurrent and in particular for impedant
short-circuit (1) corresponding to currents up to about 30 times In of motor (see Fig. N64).
b Possibility of adding RCD:
v Prevention of risk of fire (sensitivity 500 mA)
v Protection against destruction of the motor (short-circuit of laminations) by the
early detection of earth fault currents (sensitivity 300 mA to 30 A)
I" magn.
Schneider Electric - Electrical installation guide 2008
1.05 to 1.20 In
Operating curve
of thermal relay
Cable thermal withstand limit
Limit of thermal relay constraint
Short circuit current breaking capacity
of the association (CB + contactor)
Operating curve of the
MA type circuit breaker
I
Short circuit current breaking capacity
of the CB
Conclusion
The combination of a circuit-breaker + contactor + thermal relay for the control and
protection of motor circuits is eminently appropriate when:
b The maintenance service for an installation is reduced, which is generally the case
in tertiary and small and medium sized industrial sites
b The job specification calls for complementary functions
b There is an operational requirement for a load breaking facility in the event of need
of maintenance.
Key points in the successful combination of a circuit-breaker
and a discontactor
Standards define precisely the elements which must be taken into account to
achieve a correct coordination of type 2:
b Absolute compatibility between the thermal relay of the discontactor and the
magnetic trip of the circuit-breaker. In Figure N65 the thermal relay is protected if
its limit boundary for thermal withstand is placed to the right of the circuit-breaker
magnetic trip characteristic curve. In the case of a motor control circuit-breaker
incorporating both magnetic and thermal relay devices, coordination is provided by
design.
Compact
type MA
Icc ext.
t
2
1
1 Operating curve of the MA type circuit breaker
2 Operating curve of thermal relay
3 Limit of thermal relay constraint
Fig. N65 : The thermal-withstand limit of the thermal relay must be to the right of the CB
magnetic-trip characteristic
3
I
N49
© Schneider Electric - all rights reserved