Fundamental Principles and System Description
Engineering Information
SINAMICS Engineering Manual – November 2015
Ó Siemens AG
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1.2 Supply systems and supply system types
1.2.1 General
The low-voltage products in the SINAMICS series with line supply voltages of ≤ 690 V are normally connected to
industrial supply systems that are supplied from the medium-voltage distribution system via transformers. In rare
cases, however, these devices may be directly connected to the public low-voltage supply systems or to separate
supply systems, such as those supplied by diesel-electric generators.
IEC 60364-1 stipulates that supply networks are classified as either TN, TT or IT systems depending on the type of
arrangement of the exposed-conductive parts and the grounding method. The classifications and letters are
explained in brief below.
First letter: Relationship of the supply system to ground:
T = Direct connection of one point to ground.
I = All live parts isolated from ground, or one point connected to ground through an impedance.
Second letter: Relationship of the exposed-conductive parts (enclosures) of the installation to ground:
T = Direct electrical connection of the exposed-conductive parts (enclosures) to ground, independent of
whether one point of the supply system is already grounded.
N = Direct electrical connection of the exposed-conductive parts (enclosures) to the grounded point of the
supply system (the grounded point of the supply system is generally the star point in three-phase
systems, or one of the three phases if the system has no star point).
In TN systems, one point is directly grounded and the exposed-conductive parts (enclosures) of the electrical
installation are connected to the same point via a protective conductor (protective earth PE).
Example of a TN supply system with grounded starpoint
In TT systems, one point is directly grounded and the exposed-conductive parts (enclosures) of the installation are
connected to ground electrodes which are electrically independent of the ground electrodes of the supply system.
Example of a TT supply system with grounded starpoint
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