538 / 582
Appendix
Glossary
15/12
Siemens ET B1 · 10/2008
15
Time/current characteristic curve
This is the characteristic curve that specifies the virtual time for
specific operating conditions (e.g. melting time or switch-off
time) as a function of the prospective breaking current.
Total operating time
The time from the start of the switch off command to the end of
the arcing time.
Touch voltage
The voltage that can occur between two simultaneously ex-
posed components in the event of an insulation fault.
Trip-free mechanism
Design feature of key-operated switching devices and releases.
The trip-free mechanism prevents any obstruction of the release
process – even when the drive is blocked or the device is man-
ually held in the "ON" position.
Tripping characteristic
The graphical representation of the connection between the trip-
ping time and the influencing variable is shown in the character-
istic curve. The time/current diagram shows, e.g. how long the
release or tripping relay takes to respond to a specific current.
Tripping time
Current value at which a release trips within a specified time.
Types of networks and systems
Electrical networks are categorized according to voltage, con-
figuration and topology. AC voltages are categorized according
to:
•
Low-voltage networks up to 1000 V,
•
High-voltage networks over 1 KV to 380 kV,
•
e.h.v. systems over 380 kV.
The topology of the networks are distinguished as follows:
•
Open network (e.g. radial network) and
•
Closed network (e.g. ring network, meshed network)
There are the following system types for low voltages:
•
TN system (network)
In the TN system, one point of the network (star point or outer
conductor) is directly grounded. The frames of the electrical
equipment are linked to the grounded network point over a
protective or PEN conductor. The TN system is available in the
following versions:
-
TN-S system (network)
Neutral conductor (N) and protective conductor (PE) are
separated throughout the network.
-
TN-C system (network)
The neutral conductor and the protective conductor function
are combined in a single conductor, the PEN conductor,
throughout the network.
-
TN-C-S system (network)
The neutral conductor and protective conductor function are
combined in a single conductor, the PEN conductor, in part
of the network only; in the rest of the system, the neutral and
PE conductor are installed separately.
•
TT system (network)
In the TT system, one point of the network is directly grounded;
the frames of the electrical equipment are linked to ground
electrodes.
•
IT system (network)
The IT system has no direct connection between live conduc-
tors and grounded components: the frames of the electrical
equipment are grounded.
Voltage drop U
R
(
V); U
R
(%);
U (V);U (%)
The difference between the no-load voltage U
0
and the rated
voltage U
2
r of a transformer at rated power, in relation to 20 °C
ambient temperature. The degree to which the voltage drop will
affect the power depends on the length, cross-section and
which material is used for the conductors. When covering large
distances, the expected voltage drop must be taken into ac-
count.
© Siemens AG 2008