Symmetra MW II 1200 kW 3 × 400/230 V Installation - 990-1979E-00126
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.
When the protective device is an overcurrent protective device, it must be either:
• a device with inverse time characteristics and Ia must be the current causing automatic operation
within 5 seconds, or
• a device with an instantaneous tripping characteristic and Ia must be the minimum current causing
instantaneous tripping
The following diagram shows a Symmetra® MW installed in a TT system:
• Earthing arrangements and protective conductors - Symmetra® MW in “TT installation”
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.
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 Id value takes the leakage currents and the total earthing impedance
of the electrical installation into account
RAId
×50V≤
RA
Id