TT or TN-S: follow the fault path, not a rule of thumb

TT and TN-S describe how the earthed point of the supply and the exposed conductive parts of an installation are connected. That architecture determines the fault-current path and the automatic-disconnection conditions. AREI does not declare TT to be the universal Belgian residential standard, nor does it reserve TN-S for new or industrial buildings. The actual arrangement must be established for the connection concerned; age and building use are not sufficient evidence.

This guide concerns domestic AC installations under AREI/RGIE Book 1, Version 06, applicable from 1 April 2026. V06 also added DC provisions, but those form a separate scope and must not be inferred from these AC examples.

Reading TT, TN-S and TN-C-S

The first letter describes the source-to-earth relationship. T means that one point of the source is directly earthed. The second letter describes the exposed conductive parts of the consumer installation:

• TT: those parts are connected to a local electrode independent of the source electrode;
• TN: they are connected to the earthed source point through protective conductors;
• TN-S: neutral N and protective conductor PE remain separate;
• TN-C-S: a combined PEN exists for one part and is later divided into PE and N.

TN-C-S should not be labelled “common in older homes” without evidence. Inspect the connection, trace the conductors and check the distribution network operator's information.

TT: local electrode and residual-current protection

During an insulation fault, current returns to the earthed source point through the exposed part, protective conductor, local electrode and soil. This loop can have too much impedance for an overcurrent device to trip quickly. Automatic disconnection in TT is therefore normally achieved with residual-current devices.

The basic condition is:

IΔn × R_E ≤ U_L

IΔn is the RCD's rated residual operating current, R_E the installation's earth-electrode resistance and U_L the conventional touch-voltage limit. Calculating 50 V / 0.3 A ≈ 166 Ω does not override the specific domestic requirement of less than 100 Ω. In principle an overcurrent device may provide automatic disconnection in TT if all relevant impedance and timing conditions are demonstrably met. It is therefore too absolute to say that TT can only be protected by an RCD, even though RCD protection is the normal practical solution.

TN-S: a metallic return path through PE

In TN-S, fault current returns to the earthed source point through PE. The loop must be low enough in impedance for the protective device to disconnect within the required time. A simplified check is Zs × Ia ≤ U0: fault-loop impedance multiplied by the current that operates the device is compared with the phase-to-earth voltage.

This does not mean that the network operator “guarantees earth resistance”, and it does not eliminate RCD duties. The domestic requirements in section 4.2.4.3 also apply to TN-S. Conversely, fitting a 30 mA RCD does not replace protective-conductor continuity and disconnection checks.

A local electrode can still be required with TN

Receiving a PE from the supply is not a reason to omit the installation's own electrode without checking the applicable requirements. Book 1 requires domestic installations to have an earth-electrode resistance below 100 Ω. The earthing conductor, test joint, main protective conductor and equipotential bonding must be durable, accessible and testable.

The appropriate electrode depends on new or existing construction, foundations and soil. The relevant provisions do not establish a universal claim that every rod must be “at least 1.50 m”. Compliance of the complete arrangement and the measured result matter, not one generic rod length.

The 30 Ω and 100 Ω thresholds do different jobs

The limit of 16 counts single or multiple socket-outlets, not final circuits. Separately, no more than eight final circuits may be connected to one high- or very-high-sensitivity RCD in a new domestic installation. Both limits can apply at once.

Measurement, inspection and changing conditions

Earth resistance requires suitable instruments and a suitable method. Soil moisture, temperature, corrosion, a loose joint or damage to the earthing conductor can change the result. A visual inspection alone is insufficient. An inspection also addresses protective-conductor continuity, equipotential bonding, RCDs and the other measures against indirect contact.

The approved inspection body's report records the measured earth resistance. A positive report does not guarantee every later alteration or deterioration for 25 years. Twenty-five years is the interval for certain periodic domestic inspections; a negative sale inspection and other inspection triggers can lead to different follow-up periods.

Electrode, PE and bonding form one protection system

The electrode is only one component. The earthing conductor connects it to the main earthing terminal or test joint; protective conductors and main equipotential bonding then connect the relevant conductive parts. Water, gas or heating pipework must not be treated as a substitute for the required electrode. At the same time, extraneous conductive parts must be bonded as required so that a dangerous potential difference does not remain during a fault.

A low electrode resistance does not compensate for a broken PE to a socket. Conversely, protective-conductor continuity does not establish the electrode resistance. Resistance, continuity, insulation, RCD operation and disconnection conditions are therefore separate checks. During alterations, pay particular attention to legacy combinations of neutral and protective functions and any connection after the intended separation point; an improper N-PE link can distort both RCD operation and measurements.

Keep the measurement date, inspected scope and report. An earlier value must not simply be assigned to a new extension or altered electrode. Parallel earth paths or an inaccessible test joint also affect the suitable measurement method and belong in the competent assessment.

What belongs in the plans and dossier?

Chapter 9.1 requires a single-line diagram, position plan and other records. It does not generally say that the words “TT” or “TN-S” must appear as a separate field on every diagram. The actual components must nevertheless be represented correctly where required: earthing conductor, test joint, main equipotential bonding, protective devices and their relevant connections.

A defensible survey therefore includes these steps:

1. check the supply connection and network operator's data;
2. trace N, PE and any PEN;
3. record the local electrode, test joint and bonding;
4. have resistance and continuity measured;
5. assess RCD and overcurrent protection, including disconnection conditions;
6. compare the board, labels, single-line diagram and position plan with reality.

Which system is better?

There is no universal ranking. Either architecture can be safe when the supply, conductors and protective devices are coherently designed and tested. TT is not automatically more independent or better; TN-S is not automatically more reliable. The useful question is whether fault current has a controlled return path and the supply is disconnected in time.

PlanElec can document earthing and protective components in the editor, export a single-line diagram and position plan as PDF, and run an indicative self-check. It does not measure earth resistance, identify the earthing system automatically or issue an official inspection report.

Related guidance

• 30 mA RCDs: circuits, sockets and existing installations
• RCD Type A or Type B without the myths
• What does an electrical inspector check?

Open PlanElec for diagrams, position plans and an indicative self-check →

Basis: AREI/RGIE Book 1 V06, particularly sections 4.2.3, 4.2.4.3, 6.5 and 9.1. The official text, connection data and measurements for the actual installation remain decisive.