Earthing Arrangements, Protective Conductors and Bonding
Earthing arrangements
The system type (TN-S, TN-C-S, TT, IT) determines how fault current returns and therefore what protection is needed. In TN-S the earth is a separate conductor from the source; in TN-C-S the neutral and earth are combined in the supply and separated at the origin (PEN); in TT the installation uses a local electrode and needs an RCD. IT is isolated or impedance-earthed and needs insulation monitoring.
The earthing conductor and electrode
Earth electrodes (rods, tapes, plates) must have low enough resistance for the protective device to operate (TT) and be protected against corrosion and mechanical damage. The earthing conductor connects the main earthing terminal (MET) to the electrode.
Main protective bonding
Main protective bonding connects extraneous conductive parts (metal water/gas pipes, structural steel, other services) to the MET. It must be sized per the table (related to the supply neutral/earthing conductor), be continuous, and be connected as near as practicable to the point of entry. Supplementary bonding is additional, used where disconnection times cannot be met or in special locations (e.g. bathrooms), connecting exposed and extraneous parts together.
Traps
- Bonding is not the same as earthing - bonding equalises potential; earthing provides a fault path.
- A gas or water pipe must not be used as an earth electrode.
- Bonding conductors must not be removable without a tool and must be labelled.
- TN-C-S requires the supply PEN to be reliable; bonding is critical because a broken PEN is dangerous.
Worked rule of thumb
For a TN-C-S supply with a 25 mm² neutral, the main protective bonding conductor is typically 10 mm² (per the table relating bonding to supply conductor size). Always check the table for the exact size rather than assuming.