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 (Terra-Terra), 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 (Terra-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 |
| Part 6, Art. 6.4.6 | Earthing installation must be inspected regularly (periodic inspection) |
| 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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