How TN-S, TN-C-S (PME) and TT systems earth an installation, and why we bond extraneous-conductive-parts.
Why we earth and bond
Earthing creates a low-impedance path for fault current so that an overcurrent device or RCD disconnects the supply quickly (automatic disconnection of supply). Bonding equalises potential: it ties metalwork together so that, during a fault, parts a person can touch at the same moment stay at substantially the same potential, which limits the touch voltage.
Earthing system types
- TN-S: the supply provides a separate protective earth throughout, traditionally through the metallic sheath or armour of the supply cable returning to the source.
- TN-C-S (PME): the supply network combines neutral and earth as a PEN conductor, then separates them at the origin of the installation. Protective Multiple Earthing earths the PEN at several points along the network.
- TT: no supply earth is provided, so the installation is earthed through a local earth electrode; the loop impedance is high and an RCD is needed for fault protection.
- TN-C is not used inside consumer installations: with neutral and earth combined throughout, loss of that single conductor can make exposed metalwork live, and an RCD cannot function.
The PEN conductor and the MET
A PEN conductor performs the role of both the neutral and the protective conductor. The main earthing terminal (MET) is the common point where the earthing conductor, the circuit protective conductors and the main protective bonding conductors all connect.
Main protective bonding
Main bonding connects extraneous-conductive-parts (incoming metallic water, gas and oil pipes, and structural steel) to the MET, within 600 mm of the meter where practicable. On a PME (TN-C-S) supply the minimum is normally 10 mm2 copper, related to the supply neutral size; on TN-S or TT it is half the cross-sectional area of the earthing conductor, subject to a 6 mm2 minimum.
Supplementary bonding
Supplementary bonding links simultaneously accessible metalwork locally. The minimum size is 2.5 mm2 copper where mechanically protected, or 4 mm2 where it is not. In a room containing a bath or shower it may be omitted only when all three conditions are met together: every circuit meets the required disconnection times; every circuit has 30 mA RCD additional protection; and all extraneous-conductive-parts are connected to the main protective bonding.