TT or TN-S Earthing? The Difference Under AREI (2026)

Short answer: the fault-current path makes the difference

In a TT system, fault current returns to the source through the local earth electrode and the ground; in a TN-S system it returns through the protective conductor. That leads to different disconnection conditions: in TT the RCD does the disconnecting in practice, in TN-S the circuit breaker can also trip in time (AREI Book 1, subsection 4.2.3.4).

Which system your supply uses is established from the connection itself and the network operator's information – not from the building's age or use. AREI does not declare TT the Belgian domestic standard, nor TN-S a system for new builds or industry only.

This article covers domestic AC installations under Book 1, version 06, applicable since 1 April 2026. The new DC systems in version 06 are a separate topic.

Reading the letters correctly

The first letter describes how the source relates to earth: T means one point of the source is directly earthed. The second letter describes how the exposed conductive parts of the consumer installation are connected:

Whether TN-C-S is present is established from the connection and the network operator's information; it is not simply "typical of old houses".

TT system: local earth electrode and RCD

In a fault to an exposed part, current flows from the line conductor through the appliance's exposed part, the protective conductor, the local earth electrode and the ground back to the earthed point of the source. The earth is part of the fault loop; fault current stays well below a short-circuit current. That is why, in TT, automatic disconnection is usually provided by the RCD. The condition is (4.2.3.4 c.2):

IΔn × RE ≤ UL

IΔn is the RCD's rated residual operating current, RE the earthing resistance of the exposed parts and UL the conventional touch-voltage limit (2.4.1, Table 2.3). The calculation 50 V / 0.3 A ≈ 166 Ω does not lift the domestic limit of below 100 Ω (4.2.3.2).

AREI also allows disconnection by overcurrent devices in TT, provided Ia ≤ U0 / Zs is actually met (4.2.3.4 c.2). "TT only works with an RCD" is therefore too absolute – in practice the RCD remains the tool of choice.

TN-S system: return path through the protective conductor

In TN-S, fault current returns through PE to the earthed point of the source. The fault loop must be low enough in impedance for the protective device to disconnect within the time of the safety curve (4.2.3.4 b.2):

Ia ≤ U0 / Zs

Ia is the operating current of the protective device within the permitted time, U0 the voltage to earth, Zs the fault-loop impedance.

This does not mean the network operator guarantees a particular earthing resistance, nor that RCDs become unnecessary: the domestic RCD requirements of 4.2.4.3 apply whatever the system. Conversely, a 30 mA RCD does not replace checking protective-conductor continuity and disconnection conditions.

A local earth electrode in TN too

"The PE comes with the supply" does not mean a home can do without its own earth electrode. In domestic installations the electrode is built according to 5.4.2.1 and its earthing resistance is below 100 Ω (4.2.3.2). Earthing conductor, disconnecting link, main protective conductor and bonding are part of the installation and are made accessible, durable and testable.

Which electrode type fits depends on new build or existing building, foundations and soil. What counts is execution and the measured value, not a standard rod length.

30 Ω and 100 Ω: two different thresholds

The sixteen counts socket outlets, not final circuits. Alongside it, the limit of eight final circuits per high- or very-high-sensitivity RCD still applies. Both counting rules apply at the same time – details in A 30 mA RCD for every socket?.

What the measurement tells you

Earthing resistance is determined by measurement using a suitable method. Soil moisture, temperature, corrosion, loose connections or a damaged earthing conductor change the value; a visual check is not enough. The inspection also covers protective conductors, bonding, RCDs and other protective measures.

The approved body's report states the measured earthing resistance. After the conformity inspection, domestic installations are re-inspected at least every 25 years (6.5.2). That interval does not cover later changes: an important modification or extension needs its own conformity inspection, and if a control visit finds infringements, the body sets a date for a follow-up visit in its report (6.5.7.2 b.6).

Earthing and bonding belong together

The electrode is only part of the protective measure. The earthing conductor connects it to the disconnecting link; from there, protective conductors and main equipotential bonding run to the conductive parts. Water, gas or heating pipes do not replace an earth electrode, but are included in the bonding according to the applicable rules.

A good earthing measurement does not compensate for a broken PE to a socket, and a continuous protective conductor says nothing about earthing resistance. Resistance, continuity, insulation, RCD operation and disconnection conditions are therefore checked separately. During renovation, pay particular attention to old combined neutral/protective arrangements: an impermissible N-PE connection downstream of the separation point distorts RCD operation and readings.

For the dossier, what counts is the measurement date, the part of the installation measured and the report – not a figure passed on by word of mouth. Do not carry an old reading over to a new electrode, an extension or changed connections without re-checking.

What belongs in the diagram and dossier?

Section 9.1.2 requires, among other things, the single-line diagram and situation plan for homes. Show the earthing and protective components actually present correctly: earthing conductor, disconnecting link, main equipotential bonding and protective devices with their assignment.

How to survey an installation:

1. Check the supply connection and the network operator's information.
2. Trace N, PE and, where present, PEN.
3. Record the local electrode, disconnecting link and bonding.
4. Have earthing resistance and protective-conductor continuity measured.
5. Assess RCD and overcurrent protection including disconnection conditions.
6. Compare installation, board labelling, single-line diagram and situation plan.

TT or TN-S: no ranking

Both systems are safe when design, network conditions, conductor routing and protective devices fit together. TT is not automatically better, TN-S not automatically more reliable. What matters is that fault current has a defined return path and disconnection happens in time in the specific case.

Documenting earthing in PlanElec

In PlanElec you set the earthing system (TT, TN-S, TN-C-S or IT) in the network configuration and document earth electrode, bonding and protective devices in the single-line diagram and situation plan, both exportable as PDF. The AREI/RGIE self-check flags missing main equipotential bonding, a missing earth electrode in a TT or IT system and – as a plausibility check – a public 3×230 V supply stored as TN. The scope is in What the self-check verifies. Earthing resistance is measured on site by the electrician; the inspection is carried out by the approved inspection body.

Further reading

• A 30 mA RCD for every socket?
• RCD Type A or Type B?
• Inspection checklist: what the inspector verifies

Legal basis: AREI/RGIE Book 1, version 06, section 2.4.1, subsections 4.2.3.2, 4.2.3.4(b) and (c), 4.2.4.3(b) and 5.4.2.1, section 6.5.2, subsection 6.5.7.2 b.6 and section 9.1.2. The official text published by the FPS Economy prevails.