Earthing in Belgium: TT vs. TN-S System Explained
Differences between TT and TN-S earthing systems for Belgian electrical installations according to AREI Art. 4.2.3.2 and 4.2.4.3.
Earthing in Belgium: TT vs. TN-S
In Belgium, the TT system is the standard for residential installations. Each building has its own earth electrode (earth rod), which provides earthing independently from the distribution network. The TN-S system, where the protective conductor is supplied by the grid operator, is mainly used in new constructions and industrial buildings.
What Is a TT System?
In the TT system (Terre-Terre, "earth-earth"), the distribution network has its own earth electrode at the transformer, and the consumer installation has a separate, local earth electrode (typically an earth rod or foundation earth). The protective conductor (PE) is not connected to the network neutral.
What Is a TN-S System?
In the TN-S system (Terre Neutre Séparé, "earth, neutral, separate"), the protective conductor (PE) is routed by the grid operator from the transformer separately from the neutral (N). Earthing is achieved through the network connection, not through a local earth electrode.
Comparison Table
| Property | TT System | TN-S System |
|---|---|---|
| Earthing source | Own earth rod/foundation earth | Protective conductor from grid operator |
| Prevalence in Belgium | Standard (existing + new) | New builds, industrial |
| RCD mandatory | 300 mA main RCD + 30 mA group RCDs for sockets/special rooms (Art. 4.2.4.3) | Since AREI 2020: 30 mA RCD for socket circuits also mandatory (Art. 4.2.4.3) |
| Earth resistance | Max. 100 Ohm absolute limit (Art. 4.2.3.2); 30 Ohm is regulatory threshold (Art. 4.2.4.3_b) — if exceeded, additional RCDs required | Guaranteed by grid operator |
| Advantage | Independent from the grid | More reliable earthing |
| Disadvantage | Earth rod must be maintained | Dependent on grid operator |
| Fault current disconnection | Only via RCD | Also via breaker |
AREI Requirements
| AREI Article | Requirement |
|---|---|
| Art. 4.2.3.2 | Earth resistance in residential installations must be below 100 Ohm (absolute limit) |
| Art. 4.2.4.3_b | 30 Ohm is regulatory threshold: if exceeded, at least two 30 mA RCDs with max. 16 sockets each are required |
| Art. 4.2.3.4_c | Formula R_E ≤ U_L / I_A — with 300 mA and U_L = 50 V this yields max. ~166 Ohm |
| Art. 6.5.2 | Inspection visit for residential installations every 25 years — maintaining compliance, incl. earth resistance measurement |
| Art. 4.2.4.3 | RCD (residual current protection) is mandatory — for TT and since AREI 2020 also for TN-S socket circuits |
| Art. 9.1 | The type of earthing system must be documented (documentation requirements) |
Checking the Earth Rod
In a TT system, the earth resistance must be measured at every electrical inspection (keuring). AREI Art. 4.2.3.2 sets an absolute limit of 100 Ohm for residential installations. Art. 4.2.3.4_c defines the formula R_E ≤ U_L / I_A (with a 300 mA main RCD and U_L = 50 V this yields max. ~166 Ohm). The value 30 Ohm is a regulatory threshold (Art. 4.2.4.3_b): if exceeded, the AREI requires additional 30 mA RCDs. If the resistance is too high, an additional earth rod or deep-driven earth electrode may be required.
Typical causes of excessive earth resistance:
- Dry, sandy soil
- Corroded earth rod
- Earth rod too short (minimum 1.50 m depth recommended)
TT or TN-S: Which Is Better?
Both systems are safe when correctly installed. In practice, the TT system has the advantage of independence from the distribution network — a fault on the network does not affect the local earthing. Since AREI 2020, a 30 mA RCD for socket circuits (Art. 4.2.4.3) is also mandatory in TN-S systems. The exemption from the RCD requirement in TN-S only applies to certain older installations (Part 8).
TN-C-S System
In Belgium, the TN-C-S system also occurs (common in older installations). In this system, the PEN conductor is split into a protective conductor (PE) and neutral (N) at the building's service entrance.
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