APC MW II manual IT Systems, Characteristics

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 RA ⋅ Id ≤ 50V must be fulfilled.

In the condition:

RA	is the resistance of the earth electrode for exposed conductive parts
Id	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

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.

26 Symmetra MW II 1200 kW 3 × 400/230 V Installation - 990-1979E-001