8 min read
The earthing system is one of the most fundamental design decisions in an installation and the hardest to change afterwards. The system type determines which protective device will operate, whether an RCD is mandatory, and how much voltage appears on an enclosure during a fault.
What the letters mean
- First letter — the source's (transformer neutral's) relation to earth. T: directly earthed. I: isolated or earthed through an impedance.
- Second letter — the relation of conductive enclosures in the installation to earth. T: connected to their own electrode. N: connected to the source's earthing.
- Later letters — whether the protective (PE) and neutral (N) conductors are separate or combined. S: separate. C: combined (PEN).
| System | Structure | Typical location | Key characteristic |
|---|---|---|---|
| TN-S | PE and N separate throughout | Industrial plants with their own transformer | The cleanest system; best for EMC |
| TN-C-S | PEN up to a point, then separated | Buildings and plants fed from the grid | The most common arrangement in Türkiye |
| TT | Installation on its own earth electrode | Rural sites, construction sites, remote consumers | An RCD is mandatory |
| IT | Source isolated or impedance-earthed | Hospital operating theatres, critical processes | The system does not stop on the first fault; it alarms |
The critical point in TN-C-S: the separation point
In a TN-C-S system the PEN conductor is split into PE and N at the main panel. After that separation point, PE and N must never be joined again. A neutral-earth link accidentally made in a sub-panel causes RCDs either to trip constantly or not to work at all — both are dangerous.
Why an RCD is mandatory in a TT system
In a TT system the fault current returns to the source through the installation's earth electrode. Because earth resistance is high, that current may not be large enough to open a fuse or breaker, leaving a dangerous voltage on the enclosure. An RCD, which sees a difference at milliamp level, is the only effective protection here.
Measurement and acceptance values
Earth resistance should be measured periodically and recorded. The acceptance value depends on the system type and the protective device used; in a TT system it is assessed together with the RCD's rating. Readings differ between dry and wet seasons — acceptance is judged on the worst case.
For earth busbars, insulators and connection materials we work with IRTU and Hüner Elektrik. If you are not sure which system your installation uses, we identify and document it together during the survey.
Frequently asked questions
How do I tell which earthing system my installation uses?
Look at the main panel: does the incoming supply bring neutral and earth as separate conductors, or a single PEN that is split at the panel? Separate means TN-S, split at the panel means TN-C-S. If the installation has its own electrode and is not bonded to the network earth, it is TT. Where there is doubt, measure — protection is not designed on a guess.
Why does earthing matter more in panels with drives?
A drive produces high-frequency switching, part of which flows through earth. With poor earthing that current wanders through signal lines; analogue values drift, communications drop and sensors misread. The remedy is not simply a bigger conductor but a short, low-impedance earth path and 360-degree shield bonding.
Should earth and neutral busbars be separate in a panel?
In TN-S, and in every panel after the separation point in TN-C-S, they must be separate. They are joined only at the main panel where the PEN is split. Separate busbars in sub-panels are both a regulatory requirement and essential for residual current protection to work correctly.
What should be done about earthing when adding a machine?
The machine frame is bonded to the existing system's PE conductor at the appropriate size, determined by the phase cross-section of its supply cable. Do not drive a separate electrode for the machine — that creates two different earth potentials in one installation and is dangerous. After connection, continuity is measured and recorded.