Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 65)

Fig. 2.4-9
Reversing of a two step motor with separate windings
2.4.4.3 Applications of switchgear with 3 NO and 1 NC contact
Devices with three NO and one NC contact are used in applications in which, when the main
load is switched off – for example the motor –, another single-phase load must be switched on.
Such applications could include:
ƒ Safety
circuits
ƒ Direct current brake systems that are activated when a drive is switched off
ƒ Clutches that must be released when the drive is switched off
2.4.5 Application of circuit breakers in IT networks
IT supplies are used to prevent that a ground fault leads to immediate disconnection of the
affected circuit like in a grounded system. Although a first ground fault results in a displacement
of the potential of the entire supply, continued operation is still temporarily possible. Special
ground fault monitoring equipment reports any ground faults and hence makes it possible to
quickly rectify the fault – often without disruption to the operation of the plant. The situation is
similar in supplies with high impedance grounding.
If however a second ground fault occurs in another phase, there is a short-circuit that must be
immediately cleared by the short-circuit protective device. The voltage to be switched off varies
depending on the locations of the short-circuits (
Fig. 2.4-10 ). This results in different voltage
levels to be switched off by the short-circuit protective device and in the case of circuit breakers
to different required breaking capacities.
Fig. 2.4-10
Double ground faults on the load side of the circuit breakers do not cause increased stress. If, however,
there is one ground fault on the supply side and the other on the load-side, a significantly higher breaking
capacity is required because of the increased voltage load.
LVSAM-WP001A-EN-P - April 2009 2-37