Signalling instead of switching – Residual current monitoring in earthed systems

TN-S/TT-System

In TN systems, the star point of the feeding transformers is connected to earth through a low impedance and the exposed conductive parts of the electrical installation are connected to the protective earth (PE) of the system. In TT systems, the star point is also connected to earth through a low impedance but the exposed conductive parts of the electrical installation are earthed independently from the system earthing.

TT system

In TT systems, one point is connected directly to earth (functional earthing). The exposed-conductive parts of the electrical installation are connected to earth electrodes which are electrically isolated from the earth electrode for earthing the system.

Permissible protective devices:

  • Overcurrent protective device
  • Fault current protective devices (RCDs)