Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 57)

Overload release units
The reaction of bimetal strips heated by the operating current depends on the heat generated in
the bimetal strips and in their heating coil, if any. This applies equally for alternating current and
direct current. The trip characteristic can be somewhat slower with direct current as there are no
hysteresis and eddy current losses. With overload releases that are sensitive to phase failure,
all three circuits should always be connected in series to prevent premature tripping.
Overload releases heated via current transformers are not suitable for direct current. Also
electronic overload relays in most cases cannot be used in direct current applications as the
current is measured via current transformers and their functionality is tailored to alternating
current.
Short-circuit releases
Electromagnetic overcurrent releases can be used with direct current. However the tripping
threshold current is somewhat higher than with alternating current.
Undervoltage and shunt-trip releases
Undervoltage and shunt-trip releases operate with magnet circuits. Special designs are required
for direct voltage.
2.4.3 Applications at supply frequencies < 50 Hz and > 60 Hz.
Effect of harmonics
Low-voltage switchgear is designed for a supply frequency of 50 ... 60 Hz. If it is desired to use
them for other rated frequencies the following device characteristics should be checked:
- thermal load capacity of the circuits,
- switching capacity,
- life span of the contact system,
- release characteristics,
- operating characteristics of magnetic and motor drives.
The effect of higher frequencies on the performance of low-voltage devices should be consid-
ered both in networks with higher basic frequencies (for example 400 Hz) and also in cases
where current-harmonics occur. Such current-harmonics occur if the supply voltage contains
harmonics or if non-linear consumers are connected. Such consumers may for example be
compensation devices for luminescent lamps that operate in the range of saturation or devices
with phase angle control. With consumers with phase angle control and with frequency convert-
ers (Inverters; see Section
3.10 ) harmonics with frequencies up to several kHz may arise in the
supply. The harmonic content can be increased by capacitors connected to the supply, whose
current consumption increases with increasing frequency. Special attention should be paid to
this factor in individually compensated motors and a correction of the current settings of the
protective relay may be required.
In applications in which current-harmonics arise, the effect of the harmonics (for example
additional heating effects) is added to that of the basic frequency. This can be especially critical
in devices that contain coils or ferromagnetic materials (bimetal heating coils, magnetic releases
etc.).
In the case of loads with connection to the neutral conductor (e.g. single-phase loads such as
luminescent lamps, small power adapters etc.), a high harmonic content can result because of
the formation of a zero-sequence-system that may lead to thermal overloading. This should also
be taken into account in the use of 4-pole switchgear.
2.4.3.1 Effect of the supply frequency on the thermal load
In contrast to direct current, with AC the current does not flow evenly through the cross-section
of a conductor. The current density falls from the surface inward. This effect – known as the skin
effect – increases with increasing frequency so that at very high frequencies the core of the
LVSAM-WP001A-EN-P - April 2009 2-29