Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 52)

When switching motors – assuming that the motor is correctly rated for the stated frequency of
operation – it should be checked whether the overload protection device is suitable for the high
frequency of operation and that it does not release early or late. See also Section
4.1.2 .
Note
Inadvertently exceeding of the permissible frequency of operation is the most frequent cause of
prematurely eroded contactor contacts. If the contactors are made to “chatter” by rapidly
recurring interruptions of the coil current and at the same time switch high currents – e.g.
starting currents of motors – this results in heavy wear that can lead to welding or destruction of
the contacts.
It is often difficult to identify faults in switching circuits as they can have a variety of causes, for
example:
- Loose terminals
- Gradually opening contacts of thermostats, pressure sensors, limit switches etc.
- Strongly bouncing control contact
- Slowly activated control switch
- Incorrectly
programmed PLC
- Drop of control voltage
2.3.8 Rated frequency and harmonics
See also Section 2.4.3 .
The normal supply frequencies for all catalog data are 50 and 60 Hz. Also for direct current
applications corresponding values are provided in the technical data. With other frequencies,
whether higher (for example 400 Hz in military and aviation applications) or lower (for example
16 2/3 Hz on railroads) the loading of switching and protective devices changes. An examination
of the suitability for the respective application and the determination of the performance data for
the specified loadings are essential prerequisites for correct device selection.
Also in applications in which harmonic contents of the current occur – for example in variable
speed drives – the performance data of switching and protective devices may be affected.
2.3.9 Safety clearances
For devices that generate electric arcs, especially for circuit breakers, safety clearances to
adjacent devices, conductors or conductive surfaces may be required, as the arc gases
(plasma) can be ejected with very high temperature and speed. The safety clearances specified
in the manufacturer documentations must be observed to avoid risks to persons and equipment.
If the required safety clearances are not observed a secondary short-circuit may be created on
the input side of a circuit breaker – for example when conductive gas is emitted during switching
off a short-circuit. Such short-circuit would be switched off by the next short-circuit protective
device on the supply side, whose rated current usually would be significantly higher. The
destructive energy of the electric arc and the danger to persons and material are correspond-
ingly high.
The safety clearances are usually stated in the dimensional drawings for the devices (see
example
Fig. 2.3-15 ).
LVSAM-WP001A-EN-P - April 2009 2-24