TT Systems

Characteristics

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 R_A value exists (see specification below).

The condition “R_A x I_a ≤ 50V” must be fulfilled. In the condition:

• R_A is the sum of resistance of the earth electrode and the protective conductor for the exposed conductive parts

• I_a 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

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 I_a must be the current causing automatic operation within 5 seconds, or

• a device with an instantaneous tripping characteristic and I_a must be the minimum current causing instantaneous tripping