TT, TN-C, TN-S, TN-C-S Systems

The earthing type for a TN system is marked by using 2, 3, or 4 letters:

• The first letter, T, is defined by “Terre” which defines how the source or transformer neutral is connected to the earth.

• The second letter indicates how the load chassis is connected:
   
  • T is defined by “Terre”, which gives a TT system, which is the how the load chassis is grounded to earth using an earthing electrode.
  • N is defined by “Neutral” which defines the load chassis as connected to neutral. This gives a TN system, which is how the load chassis is connected to earth using either a neutral or a protective earthing (PE) conductor.   
• The third letter indicate how the neutral and PE conductors are utilized      
  • C is defined by “Combined” which means that the neutral and PE conductors are combined into one conductor (PEN conductor) that acts as the current return conductor during both steady state and fault conditions.
  • S is defined by “Separated” which means that the neutral and PE conductors are separated and independent from each other.    
• The TN-C-S earthing type has four letters and the fourth letter, just like the third letter, indicates how the neutral and PE conductors are utilized. The TN-C-S start from the source (such as a utility company) with a combined PEN conductor until a service entry point, e.g. a residential unit, is reached. As soon as the customer’s service entry point is crossed, the earthing conductor will separate into two separate conductors, which are the neutral and the PE conductors.

IT System

The earthing type for an IT system is marked by using 2 letters and a term:       

• The first letter, I, is defined by “Isolated” which defines the neutral as is either not connected to earth or indirectly connected to earth through a high impedance.
• The second letter, T, is defined by “Terre” which defines the load chassis as connected to earth using an earthing electrode.
• The term that follows the IT earthing types are defined as follows:
• Individual: The chassis of each load is earthed separately from the neighboring load chassis.
• In Groups: Different loads are separated into groups and then the chassis of each load in a group is interconnected with the other chassis within the same group. The different groups are then earthed individually.
• Collective: All of the chassis of all the loads are interconnected and then earthed.

Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. The combined PEN conductor typically occurs between the substation and the entry point into the building, and separated in the service head. In the UK, this system is also known as protective multiple earthing (PME), because of the practice of connecting the combined neutral-and-earth conductor to real earth at many locations, to reduce the risk of broken neutrals - with a similar system in Australia being designated as multiple earthed neutral (MEN).