Table des matières
TN-S SPD Wiring at a Glance
In a three-phase TN-S system, the surge protective device is connected in parallel with L1, L2, L3, N and PE according to the approved internal circuit of the selected SPD. Neutral and protective earth remain separate during normal operation.
Quick Answer: How Should an SPD Be Wired in a TN-S System?
Where neutral is present, use a manufacturer-approved 4+0 / CT1 or 3+1 / CT2 arrangement. Connect the SPD close to the incoming busbars, use the specified branch protection, and keep the SPD-to-PE connection as short and direct as practicable. Do not select the device from pole count alone.
Fast Selection by TN-S Panel Condition
| TN-S Panel Condition | Common Selection Direction | What the Buyer Must Confirm |
|---|---|---|
| L1 / L2 / L3 without N Three-phase, three-wire circuit | A project-specific 3P SPD may be used. | Confirm that neutral is genuinely absent and check the exact product wiring diagram. |
| L1 / L2 / L3 / N / PE Project specifies CT1 | 4P / 4+0 / CT1 may be selected. | Confirm L-PE and N-PE protection modes, Uc, Up, TOV behaviour and permitted RCD position. |
| L1 / L2 / L3 / N / PE Project specifies CT2 | 3P+N / 3+1 / CT2 may be selected. | Confirm L-N protection, the N-PE component, Itotal, Up and manufacturer-approved TN-S application. |
| The enquiry only says “four-pole SPD” Internal circuit not provided | Do not approve the model yet. | Request the single-line diagram, internal SPD circuit and required protection modes. |
The market descriptions “4P” and “3P+N” do not always provide enough information to confirm the internal protection circuit.
What Is a TN-S Earthing System?
In a TN-S system, the source neutral is earthed, while the neutral conductor and protective earth conductor remain separate from the source to the loads.
A typical three-phase TN-S supply uses five conductors: L1, L2, L3, N and PE. The neutral carries normal return current from phase-to-neutral loads. The PE conductor provides protective bonding and the fault-current path and should not carry normal load current.
This differs from TN-C, where neutral and protective functions are combined in a PEN conductor, and TN-C-S, where a PEN conductor is separated into N and PE at a defined point.
Do not identify TN-S from conductor colours alone
Colours can vary by country, installation age and local practice. Confirm the system from the project single-line diagram, supply documentation and the actual N/PE arrangement.
L1, L2, L3, N and PE in a TN-S Wiring Diagram
| Conducteur | Normal Function | Signification du choix du SPD | Common Mistake |
|---|---|---|---|
| L1 / L2 / L3 Phase conductors | Carry the three-phase supply voltage and load current. | Each phase requires the protection mode defined by the project and SPD circuit. | Choosing a 3P SPD merely because the supply is described as “three-phase.” |
| N Neutral conductor | Carries return current from phase-to-neutral loads. | Confirm whether the device provides L-N protection, N-PE protection, or a combination of both. | Assuming all four-module SPDs protect neutral in the same way. |
| PE Protective earth | Provides protective bonding and the surge discharge path. | The SPD-to-PE conductor should be short, direct and sized according to the product instructions. | Mounting the SPD far from the PE bar and using a long looped lead. |
How to Wire an SPD in a Three-Phase TN-S Panel
The SPD is connected in parallel with the incoming supply. Normal load current does not pass through the SPD.
Confirm the supply arrangement
Verify L1, L2, L3, N and PE from the single-line diagram and confirm that the system is TN-S at the SPD installation point.
Connect the SPD in parallel
Take the required phase and neutral connections from the incoming conductors or busbars according to the SPD terminal diagram.
Install the specified branch protection
Use the fuse, MCB or other backup protection stated in the SPD datasheet after checking the available short-circuit current.
Connect directly to the PE bar
Keep the earth path short, direct and free from unnecessary loops or bends that increase inductive voltage.
Keep N and PE separate
Do not add a permanent copper link between the neutral bar and PE bar in the downstream TN-S distribution board.
Complete the panel verification
Check terminal torque, lead length, backup protection, remote contact, replacement module and the final approved drawing.
Installation dimensions are product-specific
Do not copy terminal positions, conductor sizes or backup-device ratings from a generic internet diagram. Use the exact installation instructions for the ordered SPD model.
3P vs 4P vs 3P+N SPD for a TN-S System
Pole count describes the physical arrangement. It does not fully define the internal surge protection paths.
| Market Description | Common Circuit Logic | Possible TN-S Use | Buyer Confirmation |
|---|---|---|---|
| SPD 3P Three-pole device | Protects three phase conductors without a dedicated neutral path. | A three-phase three-wire circuit or an approved project-specific design. | Confirm that neutral is absent at the SPD location. |
| 4P SPD Four-pole device | Often uses 4+0 / CT1, with L1, L2, L3 and N protected toward PE. | May be suitable where the manufacturer approves the device for TN-S. | Confirm the circuit symbol, protection modes, Uc, Up, TOV behaviour and RCD arrangement. |
| 3P+N SPD Three phases plus neutral | Often uses 3+1 / CT2, with L-N protection plus a separate N-PE path. | Commonly available for TN-S systems. | Confirm the N-PE component, Itotal, Up and the exact certified model. |
This page is about TN-S wiring, not only product comparison
For a deeper comparison of product structures, see the 3P+N SPD vs 4P SPD guide. This TN-S page focuses on how system type, conductor arrangement and installation conditions affect the final wiring decision.
4+0 vs 3+1 Protection in a TN-S System
Both protection arrangements may be available for TN-S systems, but they create different surge paths. [3]
| Comparaison | 4+0 / X+0 / CT1 | 3+1 / X+1 / CT2 |
|---|---|---|
| Phase protection path | Commonly L1-PE, L2-PE and L3-PE | Commonly L1-N, L2-N and L3-N |
| Neutral protection path | N-PE protection element | Dedicated N-PE switching path |
| Typical market description | 4P SPD | 3P+N SPD |
| Main purchasing check | L-PE and N-PE modes, TOV behaviour, Uc, Up and installation position | L-N mode, N-PE technology, Itotal, combined Up and installation position |
| Can it be used in TN-S? | Yes, when approved by the manufacturer and project specification | Yes, when approved by the manufacturer and project specification |
There is no universal winner
The correct arrangement depends on the project specification, SPD type, protection modes, voltage protection level, TOV performance, RCD coordination, lightning-current duty and the manufacturer-approved application.
Does a TN-S System Need N-PE Surge Protection?
Where neutral is present, the selected SPD assembly normally requires a clearly defined neutral-related surge protection arrangement. Depending on the circuit, this may be part of a 4+0 / CT1 or 3+1 / CT2 design.
In a CT1 device, N may have a protection path toward PE. In a CT2 device, the phase protection normally operates from L to N, with a separate N-PE switching path. [4]
N-PE surge protection is not a permanent N-PE connection
The SPD protection element is designed to respond under defined overvoltage conditions. It does not justify installing a permanent neutral-to-earth bridge in a downstream TN-S board.
TN-S System SPD Selection Process
- Confirmer le système de mise à la terre. Verify that the protection point is genuinely TN-S and not TN-C-S before or after an uncertain PEN separation point.
- Confirm the conductor arrangement. Check whether neutral is present at the SPD location.
- Select Type 1, Type 2 or Type 1+2. Base this on lightning exposure, external lightning protection, incoming supply and project risk—not only the earthing system.
- Confirm CT1 or CT2. Review the internal circuit diagram and protection-mode table.
- Check voltage and discharge ratings. Confirm Uc, Up, In, Imax, Iimp and Itotal le cas échéant.
- Check short-circuit coordination. Verify Isccr, prospective short-circuit current and permitted backup protection.
- Check the installation position. Confirm conductor lengths, RCD coordination, PE connection and panel layout.
- Check the exact certificate scope. Ensure the ordered model, voltage and configuration are covered.
Parameters to Confirm Before Ordering
| Paramètres | Pourquoi c'est important | Buyer Question |
|---|---|---|
| Uo / UnTension du système | Defines the normal phase-neutral and phase-phase voltage. | Is the system 230/400 V, 240/415 V or another voltage? |
| UcMaximum continuous voltage | Must suit the continuous voltage and expected system conditions. | Is the Uc approved for the exact TN-S system and market? |
| Haut de la pageNiveau de protection de la tension | Indicates the limiting performance under the defined test condition. | What are the relevant L-N, L-PE and N-PE protection levels? |
| In / ImaxType 2 discharge ratings | Describe nominal and maximum impulse-current performance. | Are the stated ratings per mode, per pole or for the assembly? |
| IimpType 1 impulse current | Required where lightning-current duty is specified. | What is Iimp per mode and for the complete multipole assembly? |
| ItotalTotal discharge current | Important for multipole Type 1 devices and N-PE discharge paths. | Can the N-PE path handle the required total current? |
| Isccr / IscShort-circuit capability | Must coordinate with the available fault current at the panel. | What backup protection is required at the stated fault level? |
| CT1 / CT2Circuit de protection | Defines the internal protection paths. | Can the supplier provide the internal circuit diagram? |
| Contact à distanceStatus signalling | Allows monitoring through PLC, BMS or alarm systems. | What are the NO/NC logic and contact ratings? |
IEC 61643-11 covers requirements and test methods for SPDs connected to AC low-voltage power systems. Installation and selection must also follow the applicable electrical installation requirements. [1] [2]
RCD Position and SPD Backup Protection
RCD coordination cannot be decided from the labels “CT1” or “CT2” alone. Check the SPD’s TOV behaviour, N-PE technology, permitted installation position and the exact manufacturer wiring instructions.
Some products have specific requirements for installation upstream or downstream of residual-current protection. The final position must also comply with the local installation rules and project design. [4]
Do not specify a universal backup breaker
The backup fuse or circuit breaker must be selected from the SPD datasheet after checking the maximum permitted backup protection, conductor size, prospective short-circuit current and selectivity.
- Confirm the manufacturer’s maximum permitted backup protection.
- Check the prospective short-circuit current at the installation point.
- Confirm Isccr or the stated short-circuit coordination arrangement.
- Check selectivity with upstream protective devices.
- Verify the required means of isolation and maintenance access.
Common TN-S SPD Wiring and Selection Errors
- Choosing 3P only because the supply is three-phase. A TN-S panel containing neutral loads may require a neutral-related protection path.
- Treating 4P and 3P+N as identical. Similar module counts can contain different protection circuits.
- Describing L-PE paths as a conventional 3+1 circuit. A conventional CT2 arrangement normally uses L-N paths plus N-PE.
- Reconnecting N and PE downstream. Neutral and protective earth must remain separate in the TN-S installation.
- Using long SPD conductors. Added inductance increases the effective voltage seen by the protected equipment.
- Using a generic backup-device rating. Backup protection must match the exact SPD and panel fault level.
- Selecting Type 1 or Type 2 only from the earthing system. The SPD type depends on lightning exposure, supply and installation level.
- Checking only Imax. Uc, Up, In, Iimp, Itotal, Isccr and protection modes also matter.
- Approving a certificate without checking the model scope. Confirm the exact voltage and configuration listed in the certificate.
TN-S SPD OEM and Bulk Ordering Checklist
Send complete system and project information before requesting a final model, sample or bulk-order price.
Electrical Information
TN-S confirmation, conductor arrangement, Uo/Un, frequency, installation point and prospective short-circuit current.
Configuration des SPD
Type 1, Type 2 or Type 1+2; CT1 or CT2; Uc, Up, In, Imax, Iimp, Itotal and backup protection.
Commercial Requirements
Exact model, quantity, sample approval, logo, label, packaging, certificate scope and replacement-module requirement.
Copy-Ready TN-S SPD RFQ Template
Replace the bracketed information before sending it to the supplier.
Application: [Main distribution board / sub-distribution board / control panel] Earthing system: TN-S Supply conductors: [L1 / L2 / L3 / N / PE] System voltage: [___ V phase-to-phase / ___ V phase-to-neutral] Frequency: [50 / 60 Hz] Neutral available at SPD point: [Yes / No] Prospective short-circuit current: [___ kA] Required SPD type: [Type 1 / Type 2 / Type 1+2] Required protection circuit: [CT1 / 4+0] or [CT2 / 3+1] Required Uc: [___ V AC] Required Up: [___ kV] Required In / Imax: [___ kA / ___ kA] Required Iimp / Itotal: [___ kA / ___ kA] Backup protection: [Please recommend from the exact datasheet] RCD position or project requirement: [___] Remote indication: [Required / Not required] Pluggable module: [Required / Optional] Certificate required: [IEC / EN / TÜV / CB / CE / project-specific] OEM logo and label: [Yes / No] Packaging language: [___] Estimated quantity: [___ units] Required delivery date: [___] Please provide: 1. Exact model number 2. Internal protection circuit 3. Wiring diagram 4. Datasheet 5. Backup protection requirement 6. Certificate and model scope 7. Product and label drawing 8. Sample and bulk-order lead time
Need to confirm a TN-S SPD before ordering?
Send LEEYEE your single-line diagram, system voltage, required SPD type, short-circuit level and preferred protection circuit. We can help confirm the model, wiring logic, backup protection and OEM requirements.
Related Technical Guides
TN-S System SPD Wiring FAQ
Can a 3P SPD be used in a TN-S system?
It may be used where the protected circuit has no neutral conductor or where the approved project design specifically requires three-pole protection. It should not be selected automatically for a five-conductor TN-S panel.
Is a 4P SPD the same as a 3P+N SPD?
Not necessarily. A 4P device often uses 4+0 / CT1, while a 3P+N device often uses 3+1 / CT2. Check the internal circuit diagram.
Can both 4+0 and 3+1 be used in TN-S?
Yes, manufacturer documentation shows that both arrangements may be available for TN-S systems. The final choice must match the project, local rules and the approved application of the exact SPD.
Should the SPD be installed before or after the RCD?
There is no universal answer for every SPD. Check the product’s TOV behaviour, N-PE technology, permitted position, local installation rules and manufacturer instructions.
What backup breaker should be used?
Use the fuse or circuit breaker specified for the exact SPD after checking the short-circuit current, Isccr, conductor size and coordination with upstream protection.
What should an OEM buyer request before approval?
Request the internal circuit diagram, wiring diagram, datasheet, backup-protection requirement, certificate scope, product drawing and label drawing.
Technical References
- IEC 60364-5-53: Low-voltage electrical installations — Selection and erection of electrical equipment. Official IEC publication.
- IEC 61643-11: Low-voltage surge protective devices connected to AC low-voltage power systems — Requirements and test methods. Official IEC publication.
- Legrand, Modular Surge Protective Devices, technical sheet F04648EN-00. Includes CT1 / X+0 and CT2 / X+1 protection modes and applicable earthing systems. Official PDF.
- Legrand, Surge Protective Devices Technical Guide. Explains CT1 and CT2 circuits, N-PE components, TOV behaviour and RCD coordination. Official PDF.
- ABB Furse, ESP 240CDT2 installation instructions for three-phase TN-S, TN-C-S and TT supplies. Official PDF.
- Phoenix Contact, selection guide for lightning and surge protection in industrial buildings, including five-conductor TN-S systems. Official PDF.
- DEHN, Surge Protection in Low-Voltage Switchgear Assemblies. Includes modular SPD solutions for TN-S installations. Official PDF.
