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27Symmetra MW II 1200 kW 3 × 400/230 V Installation - 990-1979E-001
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:
1. 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)
2. 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:
In installations where the neutral is distributed, the following conditions must be fulfilled:
In the condition:
Protective devices in IT systems
The following protective devices are recognized in IT systems:
• Insulation monitoring devices
• Overcurrent protective devices
• Residual current protective devices
The following diagram shows a Symmetra
®
MW installed in a IT system:
• Earthing arrangements and protective conductors - Symmetra
®
MW in “IT installation”
is the nominal A
C
RMS voltage between phase and neutral
is the impedance of the fault loop comprising the phase conductor and the protective conductor of
the circuit
is the impedance of the fault loop comprising the neutral conductor and the protective conductor of
the circuit
is the operating current of the protective device. The disconnecting time is 5 seconds (distribution
circuits)
Z
s
3 U
0
×
2 I
a
×
--------------------≡
Z′
s
U
0
2 I
a
×
-------------≤
U
0
Z
s
Z′
s
I
a