Earthing and earthing systems (TN, TT, IT)
This page has not yet been technically reviewed by a qualified person. Always check the current standard and your own appointment (aanwijzing); this is not a substitute for NEN 3140 or NEN 1010.
§411.3.1 — Earthing and earthing systems
Protective earthing ensures that conductive enclosures of equipment are connected to earth so that an insulation fault cannot leave a dangerous voltage on touchable parts.
Color codes per NEN 1010
- Brown: line (L)
- Blue: neutral (N)
- Green/yellow: protective earth (PE)
For TN-C-S installations: PE and N are separated downstream of the split point.
The five earthing systems
NEN 1010 distinguishes five earthing systems, identifiable by a two- or three-letter code that describes how the supply and the enclosure (the exposed-conductive-part) are connected relative to earth:
| System | Supply relative to earth | PE/N arrangement | Typical use |
|---|---|---|---|
| TN-S | One point directly earthed | PE and N separated throughout the entire installation | New-build, industrial installations — preferred because no current flows over PE and RCDs operate correctly |
| TN-C | One point directly earthed | PE and N combined in a single PEN conductor | Only permitted upstream of the split point; not in final circuits |
| TN-C-S | One point directly earthed | PEN from the grid operator, split into PE and N at the meter cupboard | Most common in Dutch dwellings |
| TT | One point directly earthed | Own earth electrode at the installation, independent of the network earth | Temporary installations, sites without a reliable PEN earth (greenhouses: see example below) — always with an RCD |
| IT | No direct earthing, or via high impedance | Isolated from earth; a first fault does not immediately cause disconnection | Hospitals (operating theatres), industrial continuous processes where loss of supply on a first fault is undesirable |
Exam rule of thumb: "1010 builds, 3140 works" — the system is chosen at the design stage (NEN 1010); TN-S is preferred in practice setups and new-build because the separated PE carries no leakage current, so an RCD trips reliably.
Practical example — greenhouse installation
The system in a greenhouse or horticultural business depends on the supply and the design: with its own transformer TN-S is usual; when connected to the public low-voltage network the system follows from the connection (sometimes TT, especially where no reliable PEN earth is available). Always verify the system on site. For TT, an RCD (§531) is mandatory.
Common mistakes
- Using a metal water or gas pipe as an earth electrode or as the PE conductor. Such pipes must, on the contrary, be connected to the main equipotential bonding (NEN 1010 411.3.1.2); they just must never form the earthing or the protective conductor themselves.
- Re-connecting N and PE downstream of the split point. At the split point itself the connection is normal: the PEN is connected to the PE busbar and bridged from there to N. After that, N and PE must remain separate; otherwise N carries part of the PE current and RCDs no longer work correctly.
- Forgetting the protective conductor on Class I luminaires.
Related
- §411.3.2 — Automatic disconnection of supply
- §531 — Residual current devices
- Conductor colors (NEN-EN-IEC 60445)
Further reading
- §444EMC — separation of power and data cables (§444)
- NEN-EN-IEC 61557-8IT system and insulation monitoring (IMD) — first-fault detection
- §411.6IT system — disconnection on a second fault (§411.6)
- §413.3Electrical separation (§413.3) — an isolating transformer as a protective measure without earthing
- §531Residual Current Circuit Breakers (RCD)
- §542Earth Electrodes — Foundation Earth Electrode, Rod and Ring Electrodes (§542)