The classification uses two or three letters. The first describes the relationship of the supply source to earth; the second, the relationship of the exposed conductive parts of the installation to earth. Where further letters follow, they describe the arrangement of the neutral and protective conductors.
| Position | Letter | Meaning |
|---|---|---|
| First | T | Source has a direct connection to earth |
| I | Source is isolated from earth, or connected through an impedance | |
| Second | T | Exposed parts connected to a local earth electrode, independent of the source |
| N | Exposed parts connected to the earthed point of the source | |
| Further | S | Neutral and protective functions are separate conductors throughout |
| C | Neutral and protective functions are combined in a single conductor |
Neutral and protective conductors are separate all the way from the source. Fault current returns through a conductor that never carries load current, so the protective conductor stays close to earth potential in normal operation. This is the arrangement most people picture when they say “earthed system”.
The functions are combined in the supply and separated within the installation. It is economical and widespread. The price is that the combined conductor carries load current, and a break in it can raise the potential of every earthed part in the installation. This is why such installations place strict conditions on where the separation is made and on bonding of incoming metallic services.
The installation has its own earth electrode, electrically independent of the source electrode. The fault loop then includes two electrodes and the general mass of earth, giving a loop impedance far too high for an overcurrent device to clear a fault in the required time. Residual current protection is therefore not an optional refinement here; it is the means of disconnection.
The source is isolated from earth or earthed through a deliberately high impedance. A first fault produces only a small current and need not interrupt the supply, which is the point: continuity matters more than immediate disconnection. The arrangement requires insulation monitoring, because the protection against a second fault depends on the first one having been found and cleared.