APC Symmetra MW Installation Guide download pdf (Page 43)

Symmetra MW 800 kW 3 × 400/230 V Installation Guide - 990-1510B 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

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

is the rated residual operating

current I

× 50V≤

∆

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APC Symmetra MW Installation Guide Page 43