An earthing system is coded with letters: the first describes the source (T for directly earthed, I for isolated or impedance-earthed), the second the installation's exposed metalwork (T for its own earth electrode, N for a connection back to the source neutral). The main types are TN, TT and IT, with TN split into TN-S, TN-C and TN-C-S. The choice decides fault current magnitude, how earth faults are detected and which disconnection devices are needed.
Before you size the RCDs, find out the earthing system, a rural TT supply and a city TN-C-S service need different protection.
Most North American services are TN-C-S in all but name, though the letter codes are rarely used there.
Also heard as
- earthing arrangement
- system earthing
- system grounding
Do not mix it up with
- Equipotential bonding: Earthing connects metalwork to the ground; bonding connects metal parts to each other so they share one potential.
Related terms
TN system
Earthing arrangement where the source is earthed and installation metalwork connects back to it by a protective conductor.
TT system
Earthing arrangement where the supply and the installation each have their own separate earth electrode.
IT system
Earthing arrangement in which the source is isolated from earth or earthed only through a high impedance.
TN-C-S system
Supply earthing in which a combined neutral and earth conductor splits into separate ones at the installation.