Earthing tab is used to set the protective conductor and earthing details for the selected circuit. The screen is divided into three main sections: 1- CPC settings, 2- Adiabatic check data, and 3- Local earthing settings which is available only for distribution nodes.

Depending on the feeder cable or conductor settings, CPC options will vary. For example, if the feeder is a cable type without an armour, then, using the cable armour as a CPC will not be an option. The CPC arrangement can be selected by using the Installation  drop down list which is a dynamic list and contents will vary depending on the cable or conductor type selected by the user.

  • CPC Settings

The CPC installation type can be selected using the Installation drop down menu. Installation options will vary depending on the selected cable or conductor. After selecting an installation option, ElectrcialOM will do various things depending on the selection.

    • Cable armour/Cable sheath options will set the armour or the sheath of the selected cable to be used as a CPC. In this case, ElectricalOM will calculate the copper equivalent cross section area and the impedance of the armour or sheath, and display it under the Installation drop down list.

    • Separate conductor option will create a separate conductor external to the cable, or as an extra cable to single phase conductors, and use this as a CPC. When this option is selected, ElectricalOM will display a selection list to set the cross section area of the separate conductor. The user can select a cross section area manually or let ElectrcialOM to select a suitable size based on calculations by using the Find button next to the drop down list.

The user may not want to base the CPC cross section area on calculations but can opt to select the size the same as/half the size of the line conductors, or according to Table 54.7 Minimum cross-sectional are of protective conductor in relation to the cross-section area of associated line conductor of BS7671. In this case, Selection... button will help the user to select the CPC size accordingly and automatically

It is also possible to use multiple parallel conductors as CPC. This option is initiated by ticking the Use parallel conductors check box and choosing the quantity by the help of the drop down list.

    • Core of the cable option will set one of the cores of the selected multicore cable as a CPC. The cross section setting will not be available with this option however the number of the cores associated with CPC can be set by ticking the Define the cores for CPC box and selecting a number from the drop down list. Selecting multiple cores for CPC will also increase the core number of the cable and will be indicated on the schematics.

    • Earth conductor of the cable option will be available with cables which already have an associated earth conductor, like twin flat cables. There will be no settings available for this option and ElectricalOM will set the CPC cross sections as shown below:

ElectricalOM's cross-section area settings for earth conductor of a

Flat twin 70C PVC with CPC cable

Line conductor cross-section area

Earth conductor cross-section area













However, if the user selects a flat twin cable to Irish Wiring Regulations, then the CPC will be set to the same cross-section as the live conductors.

    • Metallic Conduit option will be available only when a cable or conductor is run within a metallic conduit (See Ph/N Conductor Tab). ElectrcialOM will calculate the copper cross section area equivalent, and the resistance of the metallic conduit and display it under the Installation drop down menu

    • Other CPC arrangement options include:
      • Cable armour and separate conductor
      • Cable armour and core of the cable
      • Cable armour, core, separate conductor
      • Core of the cable and separate conductor
      • Earth conductor and separate conductor
      • Metallic conduit and separate conductor
  • Adiabatic check data

This section does not include any settings but is purely informative where the calculated minimum CPC cross section area and the actual cross section area of the selected circuit is shown and compared. ElectricalOM will calculate the required minimum CPC cross section area, and will display the result next to the Minimum CPC C.S.A (mm2). ElectricalOM will also consider all the components set as CPC by the user, and calculate a total cross section area and display it in the box next to the Total CPC C.S.A. (mm2). Any issues will be indicated by a red marker.

In the case of regulations 419.2, and 419.3, the user must tick the box next to Provisions of Regulations 419.52 and 419.3 area applicable so ElectrcialOM will not classify the warning associated with the CPC of the selected circuit as Critical but reduce it to Simple, however, the warning will still be visible on the Warnings page of a report.

  • Local Earthing Settings

Sometimes, especially in TT systems, a distribution node may be earthed locally and further more, it can be isolated from the feeder side. ElectricalOM's Local Earthing tick box is designed to model this occasions. Once ticked, the resistance and reactance settings becomes available, so, the user can type in the values in to calculate the impedance at the point of local earthing. If the distribution node is not isolated from the feeder side then this value will be considered together with the rest of the system during the calculations, however, if the Isolated box is ticked, ElectricalOM will consider only the local earthing value for the rest of the system fed from this node ignoring the feeder side value.

Local Earthing section also accommodates the setting for the Class II Equipment. When this box is ticked, Earthing settings will be completely removed.

Similar to Ph/N Conductor tab, Busbar Trunking System component also has a special Earthing tab, however, options below are only for Generic busbar trunking systems.

Installation drop-down list has three options:

  • Busbar trunking case option will use the casing as the protective conductor.
  • Internal busbar option will use one of the busbars as the protective conductor. The user may choose the quantity of busbars will be used for earthing by ticking the Define cores for cpc tick box.

  • Busbar trunking case and Internal busbar option will consider both options used at the same time. The user may choose the quantity of busbars will be used for earthing by ticking the Define cores for cpc tick box.