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Symmetra MW II 1200 kW 3 × 400/230 V Installation - 990-1979E-00124
If there are other effective earth connections, the protective conductors must be connected to such points
when it is possible. It may be necessary to earth at additional points to ensure that the potentials of
protective conductors remain as close as possible to that of earth in case of a fault.
Additional requirements for generating sets (IEC 60364-5-55 551.4.2)
To be used when the generating set provides a switched alternative to the public supply.
Protection by automatic disconnection of supply must not rely on the connection to the earthed points of
the public supply system when the generator is operating as a switched alternative to a TN system. A
suitable earth electrode must be provided.
Protective devices in TN systems
The following protective devices are recognized in TN systems:
• Overcurrent protective devices
• Residual current protective devices (not to be used in TN-C systems)
When a residual current protective device is used in a TN-C-S system, a PEN conductor must not be
used on the load side. The connection of the protective conductor to the PEN conductor must be made on
the source side of the residual current protective device (see below illustration):
The characteristics of protective devices and the circuit impedances shall be such that, if a fault of
negligible impedance occurs anywhere in the installation between a phase conductor and a protective
conductor or exposed conductive part, automatic disconnection of the supply will occur within 5 seconds
(valid for distribution circuits), the following condition fulfilling this requirement:
In the condition:
is the impedance of the fault loop comprising the source, the live conductor up to the point of the
fault, and the protective conductor between the point of the fault and the source
is the current causing the automatic operation of the disconnecting protective device within a
conventional time not exceeding five seconds
is the nominal AC RMS voltage to earth
L1 L1
L1 L1
L1 L1
PEN N
SOURCE LOAD
PE
Residual Current Sense
Z
s
I
a
× U
0
≤
Z
s
I
a
U
0