TT systems have one point connected directly to earth and all exposed conductive parts of the installation must be connected to earth electrodes. These earth electrode are independent of the power system earthed point.
Reference to IEC/EN 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 below).
The condition “RA x Ia ≤ 50V” must be fulfilled. In the condition:
RA is the sum of resistance of the earth electrode and the protective conductor for the exposed conductive parts
Ia 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
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