Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 63)

Fig. 2.4-6
The switching of the output voltage (above) results in a harmonic content of the output current of
frequency converters (below) that affects the performance of protective devices on the load side.
The temperature rise is not only dependent on the r.m.s. value of the currents, but also on the
induction effects of the higher frequency currents in the metal parts of the devices. The addi-
tional heating effect results in a reduction of the ultimate tripping current of the overload relays,
whose extent must be individually determined for the respective combination of frequency
rectifier / overload relay / connecting lines / motor. As a consequence of these effects, the
current settings should be increased for overload relays with bimetals, the choice of a higher
current range may even be necessary. Depending on the device combination, the factor can be
up to 150 %.
As the determination of the shift of the ultimate tripping current is very time consuming in
individual cases and for all possible device combinations, it is recommended in practice to set
the motor protective devices by physical testing. To this end the drive with a rated load is run for
around one to two hours and the overload relay initially adjusted so that there is no risk of it
being released. At the end of this operating period, the current setting on the device is succes-
sively reduced until the relay trips. The final setting is around 10 % above the trip value. If
interruption to operation by tripping during the test is undesired, the release contact can be
temporarily bypassed. The current setting also provides the basis for checking the required size
of the contactor or other switchgear in the circuit.
Also the performance of electronic overload relays may depend on and be affected by the
harmonic content of inverter currents. It is not possible to make a general statement due to the
differences in the modes of operation. In electronic overload relays with current transformers it
should be remembered that use at low frequencies is limited because of converter saturation.
2.4.4 Application of four-pole switchgear devices
The majority of low-voltage switchgear is equipped with three contacts in the main circuit, which
switch three-phase loads in all poles. In some applications switchgear with four main poles is
required, either for safety reasons or for an optimum solution of the application. This may
require various device configurations.
2.4.4.1 Applications of switchgear with 4 NO contacts
Four NO contacts are required or at least very advantageous for the below applications
ƒ Applications which require the interruption of the neutral line for switching off or disconnect-
ing loads. This can be the case in supplies with adverse grounding conditions, in TT sup-
plies, for protective disconnection in IT or impedance-grounded networks. Attention has to be
paid that the neutral pole closes before or at the same time as the other poles and opens
after them or at the same time. When switching non-linear consumers, specific attention has
to be paid regarding the current loading of the neutral line. See also Section
2.4.3 .
LVSAM-WP001A-EN-P - April 2009 2-35