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2026

3+1 vs 4+0 SPD Configuration: Which Is Better for TT and TN S Systems

3+1 vs 4+0 SPD Selection Guide for TT and TN-S Earthing Systems, Protection Mode Comparison and Application Recommendations

4+0 Protection Mode
3+1 Protection Mode
Comparison of N-PE channel operating principles
Selection guidance for TN-S and TT earthing systems
TT earthing system
Key take aways
Frequently Asked Questions (FAQ)
Contact Us

TECHNICAL GUIDE | ELECTRICAL SAFETY

Reference standards: IEC 61643-01:2024, IEC 61643-11:2025

Disclaimer: All wiring conclusions require review by a qualified electrical engineer. There is no universally superior solution. Selection depends on earthing system, fault voltage conditions and site grounding quality.For more application-specific SPD selection guidance and protection solutions, please refer to CRESIN SPD solutions.Core analysis dimension: protection modes (L-N / L-PE / N-PE), rather than subjective performance ranking.

Basic definition of two protection mode topologies,Practical hardware example: The CSMS-B40 SPD supports both 3P+1 and 4-pole mechanical housings, enabling flexible implementation of 3+1 and 4+0 protection modes. More information about product specifications and configuration options is available here.

Note: A 4 pole housing does not equal a 4+0 protection mode; a 3P+N housing does not equal a 3+1 protection mode. The pole count of the housing refers to terminal quantity, while protection mode describes internal component connection paths.

4+0 Protection Mode

Internal components: 4 MOV varistors

Protection paths: L1-PE, L2-PE, L3-PE, N-PE (all live conductors connected directly to PE, common mode protection)

L-PE path: Direct MOV connection with no series components

N-PE path: MOV varistor connected across N-PE terminals

3+1 Protection Mode

Internal components: 3 L-N MOV varistors + 1 N-PE gas discharge tube (GDT)

Protection paths: L1-N, L2-N, L3-N, N-PE, L-PE protection is formed by the series combination of L-MOV-N + N-GDT-PE (combined mode protection defined in IEC 61643-11:2025).

L-PE path: Series connection of L-N MOV and N-PE GDT

N-PE path: Gas discharge tube, which maintains high impedance under power frequency voltage and only conducts during surge transients.

Comparison of N PE channel operating principles

For the 4+0 configuration, the N PE channel uses a MOV varistor. It presents medium high leakage current under normal operating conditions. Its tolerance for power frequency offset voltage is limited; sustained N-PE voltage will accelerate component aging. It delivers fast clamping response to surges and is suitable for scenarios where N-PE potential stays close to zero, such as TN-S systems with transformer side neutral PE bonding.

For the 3+1 configuration, the N-PE channel uses a GDT gas discharge tube. It provides near infinite insulation impedance under normal operation and can withstand large temporary N-PE offset voltages caused by earth faults. Surge response is trigger type discharge. It is suitable for cases where large fault induced N-PE offset may occur, including TT systems and TN-S systems with poor earthing.

Selection guidance for TN S and TT earthing systems

⚠ Engineer review is mandatory before site application. Consider system temporary overvoltage (TOV), earthing quality and matching of backup SCB protection.

TN-S earthing system

Both 4+0 and 3+1 configurations are applicable for TN-S systems.

The 4+0 configuration provides direct L-PE and N-PE MOV paths, with N-PE potential near zero at the transformer bonding point. If neutral PE bonding degrades on site and sustained N-PE offset voltage appears, the N-PE MOV will suffer accelerated deterioration.

The 3+1 configuration delivers full differential mode (L-N) plus common mode (N-PE GDT) combined mode protection complying with IEC 61643-11:2025. When selecting this option, always check the SPD datasheet for the Up value of the series mode L-PE protection.

On site verification shall include measuring N-PE voltage at the main distribution panel under load conditions. A 4 pole SCB shall be used for 4+0, while a 3 pole SCB is required for 3+1. The Uc rating for each protection mode must also be verified.

TT earthing system

3+1 is the preferred choice for TT systems. TT systems feature independent local earth electrodes. During phase to earth faults, the N-PE GDT can withstand large temporary N-PE offset voltages, and L-PE protection is implemented by the standard defined series combined mode topology.

Mis applying 4+0 in TT systems carries critical risks. When earth faults occur, the N-PE MOV is continuously exposed to power frequency offset voltage, which causes premature component failure and potential short circuit fire hazards.

On site verification shall include checking local earth electrode resistance, confirming the N-PE TOV withstand capability of the 3+1 N-PE GDT, and strictly complying with TOV stress requirements in IEC 61643-11.

Key take aways

There is no universal “better” configuration. Selection is determined by protection mode stress within the earthing system, not inherent component superiority.
For TT systems, 4+0 presents high risk due to power frequency stress on the N-PE MOV. The 3+1 topology is preferred, as its N-PE GDT resists earth fault induced N-PE offset voltage.
For TN-S systems, both configurations are viable:
Important note: A physical 4 pole housing is not equivalent to a 4+0 protection mode. Always refer to manufacturer datasheets to confirm internal protection mode declaration and the Up value for combined mode operation.
All SPD wiring and selection must be reviewed by a qualified electrical engineer, in compliance with local installation codes and IEC 61643-01:2024 / IEC 61643-11:2025 normative requirements.

Frequently Asked Questions (FAQ)

What is the main difference between 4+0 and 3+1 SPD protection modes?

The main difference is the N-PE protection path design. A 4+0 SPD uses MOV varistors for all protection paths, including N-PE, providing direct common-mode protection. A 3+1 SPD uses L-N MOV varistors combined with an N-PE GDT, providing combined-mode protection with higher tolerance against temporary N-PE overvoltage.

Is 4+0 or 3+1 better for all earthing systems?

No. There is no universally superior SPD configuration. The selection depends on the earthing system, temporary overvoltage (TOV) conditions, grounding quality, and installation requirements. 3+1 is generally preferred for TT systems, while both 4+0 and 3+1 can be applied in TN-S systems after proper engineering evaluation.

Why is 3+1 recommended for TT earthing systems?

TT systems use independent local earth electrodes. During earth faults, large temporary N-PE offset voltages may occur. The N-PE GDT in a 3+1 SPD provides high insulation impedance during normal operation and can withstand these temporary overvoltage stresses, reducing the risk of premature component failure.

Can a 4-pole SPD housing be considered a 4+0 protection mode?

No. The number of poles only refers to the physical terminal configuration. A 4-pole housing does not automatically mean the SPD uses a 4+0 protection topology. The internal connection design and protection mode must always be confirmed through the manufacturer's datasheet.

What should be checked before selecting an SPD for TN-S or TT systems?

Before selection, engineers should verify:

Earthing system type (TN-S, TT, or other configurations)
System temporary overvoltage (TOV) conditions
N-PE voltage under normal and fault conditions
Ground electrode resistance
SPD Uc and Up parameters
Backup short-circuit protection device (SCB) coordination

Contact Us

Selecting the correct SPD protection mode requires a comprehensive evaluation of the earthing system, fault conditions, and installation environment. CRESIN provides professional surge protection solutions and technical support for different application scenarios.

For customized SPD selection guidance or project consultation, please contact the CRESIN engineering team.

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