Residential electrical installation - The power network

From Electrical Installation Guide

The power distribution utility connects the LV neutral point of its MV/LV distribution transformer to earth.

All LV installations must be protected by RCDs. All exposed conductive parts must be bonded together and connected to the earth.

The vast majority of power distribution utilities connect the low voltage neutral point of their MV/LV distribution transformers to earth.

The protection of persons against electric shock therefore depends, in such case, on the principle discussed in chapter Protection against electric shocks and electrical fires. The measures required depend on whether the TT, TN or IT earthing system is adopted.

For fault protection, RCDs are essential for TT and IT earthed installations. For TN installations, high speed overcurrent devices (circuit breakers) or RCDs may provide fault protection of the electrical circuits. Nevertheless, to extend the protection to flexible leads beyond the fixed socket outlets and to ensure protection against fires of electrical origin high sensitivity RCDs shall be installed. AFDDs (Arc Fault Detection Devices) are also recommended (mandatory in some countries) to extend even more the protection against fire risks.