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System Grounding and Earthing Types Software


ETAP system grounding or earthing system feature is an integrated tool in ETAP one-line diagram that automatically detects the system earthing configuration based on source and transformer grounding or earthing type selection. The resulting earthing types are displayed both on the one line diagram and for the various connected cables.

System grounding and earthing tool considers 3-phase and 1-phase AC systems that are less than 1 kV as well as DC systems that are less than 1.5 kV.

ETAP system grounding and earthing system tool is used for sizing of Protective Earthing (PE) Conductors and for Electric Shock Protection calculations.

Distinction can be made between grounding system and earthing types by using the following definitions:

Grounding System

There are four grounding systems available in ETAP:

  • Solid Grounded
  • Low Impedance Grounded (Low Z)
  • High Impedance Grounded (High Z)
  • Ungrounded

The system grounding is determined by the connected energized sources in the system. Grounding types can also be determined if multiple sources exist the system. The selection of a grounding system enables the selection of earthing types.

Earthing Type

For solidly grounded system the following earthing types are available:

  • TN-C
  • TN-C-S
  • TN-S
  • TT
  • NEC
  • For other grounding systems (including ungrounded systems) the following earthing types are available:

Grounding System Earthing Type
Open IT-Collective, IT-Individual, and IT-In Groups
Resistor IT-Collective, IT-Individual, and IT-In Groups
Reactor IT-Collective, IT-Individual, and IT-In Groups
Xfmr-Reactor IT-Collective, IT-Individual, and IT-In Groups
Xfmr-Resistor IT-Collective, IT-Individual, and IT-In Groups
Delta IT-Collective, IT-Individual, and IT-In Groups

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:

     
    TN-S Grounding Type
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    TT Grounding Type
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    • 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

     
    TN-C-S Grounding Type
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    TN-C Grounding Type
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    • 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.

    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).

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:

       
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    • 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.

Fault path impedance
If the fault path between accidentally energized objects and the supply connection has low impedance, the fault current will be so large that the circuit overcurrent protection device (fuse or circuit breaker) will open to clear the ground fault. Where the earthing system does not provide a low-impedance metallic conductor between equipment enclosures and supply return (such as in a TT separately earthed system), fault currents are smaller, and will not necessarily operate the overcurrent protection device. In such case a residual current detector is installed to detect the current leaking to ground and interrupt the circuit.