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Symmetra MW 800 kW 3 × 400/230 V Installation - 990-1510D 37
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 R
A
value exists (see specification below).
The condition must be fulfilled.
In the condition:
For discrimination purposes, S-type residual current protective devices may be used in series with
general type residual current protective devices. To provide discrimination with S-type residual
current protective devices, an operating time not exceeding 1 second is permitted in distribution
circuits.
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, I
a
is the rated residual operating
current I
n
R
A
I
a
× 50V≤
R
A
I
a
Δ