The following are the main types of installation used in Ireland:
i) TN-C-S system.
The standard installation system in Ireland. (See National Rules for Electrical Installations, Third Edition, ET101, Figure 2A). RCDs are mainly used on this system for protection against Direct Contact, and they also provide protection against Indirect Contact. The rated residual operating current of RCDs intended for protection against Direct Contact must not exceed 30mA.
ii) TT system.
The use of TT systems in Ireland is relatively rare. The TT system differs from the TN-C-S system in that there is no connection between the protective conductor (PE) and the neutral, and instead protection relies on local earth electrodes. Resultant impedances in the earth return path between the load and the origin of the supply may prevent the operation of overcurrent protection devices in the event of an earth fault. This can result in bonded metalwork reaching dangerous touch voltages and also give rise to fire hazards due to the sustained flow of earth fault currents if the overcurrent protection device fails to operate. The use of RCDs on TT systems for earth fault protection is therefore essential in all cases. RCDs are used to provide protection against Direct Contact and Indirect Contact in TT systems.
iii) IT system.
Generally used where continuity of supply is required. The use of IT systems in Ireland is confined to special applications such as hospital operating theatres, mines, etc., where continuity of supply under a first earth fault condition is of paramount importance. IT systems are characterised by the absence of a direct connection of the supply to earth or connection to earth via a relatively high impedance which negates the use of RCDs on such systems.
[This message has been edited by djk (edited 01-21-2006).]
