25Symmetra MW II 1200 kW 3 × 400/230 V Installation - 990-1979E-001
If a fault occurs close to the UPS (before the power distribution) while the UPS system is in Battery
Operation and Bypass is unavailable, the available power is unable to activate the protective device. In
that situation the Inverter will shut down in five seconds (IEC 60364-4-41 413.1.3.5 norm). If a residual
current protective device is used, this device will disconnect the supply.
The four diagrams show the Symmetra MW installed in four different TN systems:
• Earthing arrangements and protective conductors - Symmetra® MW in “TN-S installation”
• Earthing arrangements and protective conductors - Symmetra® MW in “TN-S installation” (Legal
in DK - special cases)
• Earthing arrangements and protective conductors - Symmetra® MW in “TN-C-S installation”
• Earthing arrangements and protective conductors - Symmetra® MW in “TN-C installation”
TT Systems
Characteristics
TT systems have one point connected directly to ground and all exposed conductive parts of the
installation must be connected to an earth electrode. This earth electrode is independent of the power
system earthed point.

Reference to IEC 60364-4-41 413.1.4

All exposed conductive parts that are protected collectively by the same protective device must be
connected to a common earth electrode together with the protective conductors. In installations where
several protective devices are utilized in series, the requirement applies separately to all exposed
conductive parts protected by each device.
The neutral point or, if a neutral point does not exist, a phase conductor of each generator station or
transformer station must be earthed.

Protective devices in TT systems

The following protective devices are recognized in TT systems:
• Overcurrent protective devices
• Residual current protective devices
Overcurrent protective devices are only applicable for protection against indirect contact in TT systems
where a low RA value exists (see specification b elow).
The condition must be fulfilled.
In the condition:
is the sum of resistance of the earth electrode and the protective conductor for the exposed
conductive parts
is the current causing the automatic operation of the protective device.
When the protective device is a residual current protective device, Ia is the rated residual operating
current I n
RAIa
×50V≤
RA
Ia
Δ