Installation technology · Topic 2 of 5
Supply systems and earthing arrangements
The DNO delivers electricity to the building. How the installation gets its connection to earth depends on the type of supply.
- V – Voltage
- I – Current
- R – Resistance
- f – Frequency
The UK supply
- Single phase: 230 V between line and neutral.
- Three phase: 400 V between lines (230 V × √3 ≈ 400 V), and 230 V from any line to neutral.
- Frequency: 50 Hz.
- Under ESQCR, the voltage can vary from 10 % above to 6 % below 230 V.
Conductor colours
| Conductor | Colour |
|---|---|
| Line (single phase) | Brown |
| Lines L1, L2, L3 (three phase) | Brown, black, grey |
| Neutral | Blue |
| Protective conductor (earth) | Green and yellow |
Earthing arrangements
The letters tell you how the system is earthed:
- First letter T: the supply is connected directly to earth (Terre) at the transformer.
- Second letter N: the installation's earth is connected to the supply's neutral or earth conductor. T: the installation has its own earth electrode.
- S: separate neutral and earth conductors. C: combined neutral and earth.
| System | How the installation is earthed | Typical maximum Ze |
|---|---|---|
| TN-S | Separate earth, usually the lead sheath or armour of the supply cable | 0.8 Ω |
| TN-C-S (PME) | Combined neutral and earth (PEN) in the supply cable, split at the cut-out | 0.35 Ω |
| TT | The installation's own earth electrode (rod) | 21 Ω |
Ze is the external earth fault loop impedance: the part of the loop outside the installation. The figures above are the long-standing industry values for a 230 V single-phase supply up to 100 A. DNOs aren't obliged to meet them, so always measure Ze.
Exam tip: TN-C-S is the most common supply on new UK homes. TT is common in rural areas with overhead lines, and needs RCD protection because its Ze is high.
Mini test
Pick an answer to see if you're right and why.