Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 56)

These advantages are exploited by using three-pole contactors and circuit breakers for switch-
ing single-phase alternating current and above all direct current. The limit for higher operating
voltage is determined by the rated insulation voltage that may in no event be exceeded. The
permissible current loading of poles connected in series is the same as for individual poles.
1-pole 3-pole 2-pole
Fig. 2.4-1
Examples of diagrams for poles connected in series. Where grounded power supplies are used (top
graph) with loads switched on both sides, it should be noted that ground faults can lead to bridging of
contacts and hence to a reduction in the breakable voltage.
The overload trip characteristics of devices with thermally delayed bimetal tripping mechanisms,
such as circuit breakers and overload relays, apply when all three bimetal strips are equally
loaded. This is guaranteed by connecting the circuits in series. In devices that are sensitive to
phase failure, series connection of all circuits is compulsory. With electronic motor protection
relays, it may be necessary to deactivate the phase failure protection.
The impact of series connection of circuits when switching direct currents is dealt with in
Section
2.4.2 .
For the effect of series connection of circuits on switching frequencies < 50 Hz and > 60 Hz see
Section
2.4.3 .
2.4.2 AC switchgear in DC applications
Switchgear designed for alternating current can carry at least the same rated continuous
operational DC current. With direct current the skin effect in the circuits disappears and none of
the specific effects associated with alternating currents such as hysteresis or eddy current
losses occur.
DC devices that are operated at low voltage can be switched by AC switchgear without difficulty,
as their direct current switching capacity at low voltages is practically the same as for alternating
current.
With voltages in excess of around 60 V, the direct current switching capacity of AC switchgear
with double-breaking contacts (for example contactors) decreases strongly. By connecting two
or three circuits in series (
Fig. 2.4-1 ) this limit can be raised to twice or three times the voltage.
The reason for the reduced switching capacity with DC compared with AC is the absence of the
current zero crossover that with AC supports the quenching of the electric arc. The electric arc
in the contact system can continue to burn under larger direct voltages and thus destroy the
switchgear. With direct voltages, the contact erosion and hence also the contact life span differ
from those at alternating voltage. The attainable values for direct current are specifically tested
and documented.
With direct current, the load affects the switching capacity more strongly than with alternating
current. The energy stored in the inductance of the load must largely be dissipated in the form of
an electric arc. Hence with a strongly inductive load (large time constant L/R) the permissible
switching capacity for the same electrical life span is smaller than with an ohmic load due to the
much longer breaking times.
LVSAM-WP001A-EN-P - April 2009 2-28