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Symmetra MW 800 kW 3 × 400/230 V Installation - 990-1510D 39
IT Systems
Characteristics
In IT systems the installation is insulated from earth or connected to earth through a sufficiently high
impedance. Exposed conductive parts are earthed individually, in groups, or collectively.
Reference to IEC 60364-4-41 413.1.5
In IT systems the installation must be insulated from earth or connected to earth through a sufficiently
high impedance. This connection must be made either at the neutral point of the system or at an
artificial neutral point. The latter may be connected directly to earth if the resulting zero-sequence
impedance is sufficiently high. In installations where no neutral point exists, a phase conductor can
be connected to earth through an impedance. In case of a single fault to an exposed conductive part or
to earth, the fault current will be low and disconnection will not be imperative.
Exposed conductive parts must be earthed individually, in groups or collectively and the condition
must be fulfilled.
In the condition:
In systems where an IT system is used for continuity of supply, an insulation monitoring device must
be provided to indicate the occurrence of a first fault from a live part to the exposed conductive parts
or to the earth. It is recommended to eliminate a first fault as soon as possible.
Depending on whether all exposed conductive parts are interconnected by a protective conductor
(collectively earthed) or are earthed in groups or individually, after a first fault, the disconnection
conditions of the supply for a second fault must be as follows:
a. In installations where the exposed conductive parts are earthed in groups or individually, the
protection conditions for TT systems apply (see 413.1.4.1)
b. In installations where the exposed conductive parts interconnected by a protective conductor
collectively earthed, the conditions for TN systems apply
In installations where the neutral is not distributed, the following conditions must be fulfilled:
is the resistance of the earth electrode for exposed conductive parts
is the fault current of the first fault of negligible impedance between a phase conductor and an
exposed conductive part. The I
d
value takes the leakage currents and the total earthing impedance
of the electrical installation into account
R
A
I
d
× 50V≤
R
A
I
d
Z
s
3U
0
×
2I
a
×
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≡