Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 35)

2.3.1 Rated isolation voltage U
i
U
i
is the voltage on which the selection of creepage distances of electrical equipment and the
dielectric tests are based. U
i
must always be bigger than (or at least the same as) the voltage
that is applied to the electrical equipment and is thus always larger than or the same size as the
rated operational voltage U
e
. With the selection of U
e
is thus U
i
(in the responsibility of the
device manufacturer) correctly selected, whereby the pollution degree and the overvoltage
category should be taken into account.
2.3.2 Rated operational voltage U
e
, rated operational current I
e
and utilization
category
The rated operational voltage U
e
is always to be considered in association with the correspond-
ing rated operational current I
e
and the utilization category. These three variables determine the
suitability of an item of electrical equipment for a certain application. For utilization categories
see Section
1.1 .
Corresponding to the universal applicability, an item of electrical equipment can be assigned a
variety of different datasets (for example I
eAC-3
for various operational voltages or I
eAC-3
and I
eAC-4
for a certain operational voltage). Common values for rated operational voltages for switchgear
can be seen in
Tab. 2.3-2 . With 3-phase supply systems, the delta (phase to phase) voltage of
the power supply applies.
Supply voltages Supply voltages for three-phase
4-wire or 3-wire systems
d.c.
V
a.c.
V
50 Hz
V
60 Hz
V
6 6
12 12
24 24
36
48 48
60
72
96
110 110 120/208
220 230 230/400 240
277/480
440 400/690 347/600
600
1000
Tab. 2.3-2
Preferred rated voltages for supply systems in accordance with IEC 60038 ed. 6.2 and industrial practice
(L-N/L-L). Copyright © IEC, Geneva, Switzerland. www.iec.ch
2.3.3 Rated impulse withstand voltage U
imp
The rated impulse withstand voltage U
imp
is a measure for the dielectric strength. From the point
of view of the user it is important, as the required dielectric strength among other things
depends on the pollution degree of the installation site and the overvoltage category, i.e. the
proximity to the feeding supply network. With respect to the pollution degree see
Tab. 2.3-3 .
Tab. 2.3-4 shows as an excerpt from Table H.1 of IEC 60947-1 Annex H the influence of the
overvoltage category on the applicable impulse withstand voltage U
imp
. The rated impulse
withstand voltage is for example important in circuit breakers that are often deployed on the
distribution level or also on the supply level.
LVSAM-WP001A-EN-P - April 2009 2-7