Why Earthing Of An Electrical Installation Is Important For Safety.

Earthing is the primary level of protection and the PE conductors providing they are intact will be the path of any fault current. PE = protective earthing conductor.ME= Main earthing conductor.

Earthing and bonding are not the same, bonding maintains the equipotential value to conductive metal that enter the ground some distance away, that distance is generally regarded as a minimum of 2 meters, ie full arms length. These two points is where the touch voltage exists with an earth fault. Touch voltage only exists under earth fault conditions.

Bonding is NOT required on isolated metal work, ie sinks etc. Isolated metal work is where there is no DIRECT connection with the general mass of earth.

The auto disconnection requirement is to prevent undue temperature rise in the cable under fault conditions and to disconnect supply from the fault.

This disconnection time must be less than 0.4 secs. In practice it will be far less than this.

An RCD/RCBO will provide additional protection where the impedance of the earthing system is not low enough to opert a conventional protective device ie MCB in the required time.

On an MEN (TN-C-S) system the fault path is through the PE conductor to the main neutral back to the source of supply. The current in the main earth is negligible and may be disregarded.

On a TT system the fault path is through the PE conductor , the Main Earth conductor, electrode, any equipotential bonding to the general mass of earth, then to the source of supply by way of the distribution earthing system. As this resistance is generally high ( relative to an MEN connection) a conventional protective device, ie. an MCB, will not operate within the required time and RCD protection MUST be utilised.

The same earthing resistance values apply irrespective of an MEN or TT system within an electrical installation. (less than 1.0ohm in practice 0.5 ohms, PE + ME)

Yes TT systems predominate in Thailand, (and many cases earthing is not utilised at all or it is nor compliant with any recognised standard).