AC Surge Protection: 3+1 vs 4+0 SPD Configurations for TT and TN-S Systems

What are four-pole surge protective devices? How are they different from other devices? For a three-phase system, it does not matter how many stages a surge protector is made up of for surge protection. The difference is the path the surge protector is made up to protect.

Understanding protection of a three-phase system shows that a 4+0 Surge Protection Device (SPD) and a 3+1 SPD are completely different from each other.

This helps in designing Surge Protection Systems (SPS) for electric power distribution systems made up of Transformer-Terra Volt (TT) or Transformer-Neutral System (TN-S) protection.

1. How Earthing Systems Affect AC Surge Protection

Before comparing SPD configurations, engineers must identify how neutral (N) and protective earth (PE) are arranged.

System	Earthing Characteristics	Main SPD Consideration
TT	Installation PE uses an earth electrode arrangement independent of the source earthing electrode.	N-PE discharge, TOV behavior and RCD coordination
TN-S	Neutral and PE remain separate throughout the relevant installation.	L-N, L-PE and N-PE protection coordination

In TT installations, earth-potential differences during transient events make the N-PE protection arrangement particularly important.

TN-S systems allow greater flexibility in selecting between suitable 3+1 and 4+0 topologies, provided the declared protection modes satisfy the installation requirements.

2. 3+1 vs 4+0 SPD: Internal Protection Mechanisms

The fundamental difference is how surge currents are routed through the SPD.

3+1 Configuration: MOV and GDT Coordination

A typical 3+1 AC Surge Protection circuit combines three phase-to-neutral elements with one neutral-to-earth discharge element:

•L1-N, L2-N and L3-N: voltage-limiting protection.

•N-PE: additional neutral-to-earth discharge protection.

Telebahn's MOV+GDT designs incorporate metal oxide varistors to limit transient voltages on the phases and neutral. A discharge path made of a gas discharge tube provides a path for the breakdown of the gas in the tube.

GDTs operate as open circuitswitches and become low resistance paths when the sparkover voltage is reached. This is analogous to the operation of an SCR. The GDT design presented limits continuous PE leakage current.

However, GDT selection must also account for follow-current interruption and temporary overvoltage (TOV) behavior.

4+0 Configuration: PE-Referenced Protection

A conventional 4+0 SPD uses four protection paths:

•L1-PE, L2-PE and L3-PE.

•N-PE.

Telebahn also provides MOV-based 4+0 AC Surge Protection arrangements for TN-S applications.

Unlike 3+1, this topology has no dedicated L-N protection element. Differential-mode voltage limitation depends on the behavior of the relevant combined paths.

Technical Comparison

Criteria	3+1 SPD	4+0 SPD
Phase protection	L-N	L-PE
Additional path	N-PE	N-PE
Typical elements	MOV + GDT	MOV-based
Dedicated L-N element	Yes	No
Common applications	TT / TN-S	TN-S
Key verification	Combined L-PE Up, N-PE ratings	L-N combined-mode performance, PE leakage

Neither configuration is inherently superior; the correct choice depends on the electrical network and required protection modes.

3. Which AC Surge Protection Configuration Fits TT and TN-S?

Why is 3 + 1 Adopted in TT Systems?

In TT systems, surge protection and earth faults are arranged using 3 + 1 configurations. The name comes from the number of elements in the configuration.

In the evaluation of these configurations, the following are considered:

•The capability of N-PEs to discharge and withstand TOS.

•The placement of RCDs to prevent unwanted tripping.

•The interruption of current by GDT, if it is downstream of the N-PE.

•Combined protection voltage between L and PE.

A conventional 4+0 SPD should not be substituted without verifying compliance with the applicable TT installation requirements.

TN-S Systems: Evaluating Both Configurations

TN-S networks may use either suitable topology.

The choice depends on the protected equipment's insulation withstand voltage, required protection paths and installation conditions.

In 3+1 arrangements, L-N and N-PE protection elements combine for an L-PE surge. The combined protection level of the surge must be assessed. The individual L-N up-rating does not apply in this case.

Similarly, 4+0 arrangements require verification of differential-mode protection through the available protection paths.

Engineering consideration: Compare the manufacturer's declared combined-mode Up values—not merely the individual module ratings.

4. Critical Parameters for AC Surge Protection Selection

SPD topology alone cannot establish protection suitability. Telebahn recommends evaluating electrical ratings alongside the actual distribution system requirements.

Parameter	Engineering Significance
Uc	Maximum continuous operating voltage; select according to system voltage and connection mode.
Up	Declared voltage protection level; evaluate individual and combined modes.
In	Nominal discharge current under the 8/20 μs waveform.
Imax	Maximum discharge current, relevant to Type 2 SPD testing.
Iimp	Lightning impulse current under the 10/350 μs waveform, relevant to Type 1 applications.
TOV	Withstand or safe-failure behavior during temporary overvoltage.
Ifi	Follow-current interrupt capability, where applicable.
ISCCR	Short-circuit current rating under specified protection conditions.

Why Combined Up Matters

The effective protection voltage at equipment terminals is influenced by both the SPD's declared Up and the inductive voltage developed across connecting conductors.

The latter increases with conductor inductance and surge-current rise rate (V = L × di/dt).

New measurements for combining protection modes have been defined in IEC 61643-11:2025. These enable comparison of protection systems between 3+1 and 4+0 configurations.

The protection level must not exceed the downstream equipment's withstand voltage of impulse.

5. Installation and SPD Coordination

Compromising factors on the correct selection of Surge Protection Devices (SPDs) during installation are:

1) Connection Length and Routing

As stated above, the length of protection system connections must be kept to a minimum.

2) RCD Coordination

Where RCDs are installed adjacent to the SPD (and in accordance with local codes and standards), it may be assumed that coordination of the RCD and SPD is not required.

3) Backup Protection

In case of coordination with an upstream circuit breaker, the ISCCR of the SPD must be evaluated.

4) Multi-Level Protection

Coordinate Type 1, Type 2 and Type 3 SPDs based on installation position, discharge duty, residual voltage and downstream equipment sensitivity.

Remember: 3+1 and 4+0 describe circuit topology; Type 1, Type 2 and Type 3 describe SPD test classifications.

6. AC Surge Protection Procurement and Verification Checklist

Before purchasing, engineers and industrial buyers should confirm:

•Network compatibility: TT or TN-S.

•Nominal voltage, Uc and connection diagram.

•Protection modes and combined Up values.

•In, Imax and applicable Iimp ratings.

•N-PE characteristics, TOV and follow-current requirements.

•RCD arrangement, ISCCR and backup protection.

•Applicable standards, test reports and certification scope.

Requirements for testing AC SPDs are given in IEC 61643-11:2025. Selection, application and coordination of AC SPDs are given in IEC 61643-12:2020.

Explore Telebahn's AC Surge Protection Products

For TT and TN-S systems, Telebahn's product offering includes several MOV and MOV/GDT SPDs, which include modular design, thermal separation, and indicate fault condition.

Prior to specifying a surge protection device, it is recommended that the potential user identify the requirements and topology of surge protection for 3+1 or 4+0 systems and communicate with Telebahn's technical staff to assess the electrical limitations of the systems.

FAQs

Q1. Does Telebahn offer both 3+1 and 4+0 AC Surge Protection?

Yes. Telebahn offers 3+1 SPD configurations for TT/TN-S systems and 4+0 configurations for suitable TN-S applications.

Q2. Which Telebahn SPD configuration is suitable for TT systems?

Telebahn's 3+1 MOV+GDT configuration is designed for TT and TN-S networks, providing three L-N protection paths and one N-PE discharge path.

Q3. Can Telebahn 3+1 SPDs be installed in TN-S systems?

Yes. Selected Telebahn 3+1 SPDs support TN-S installations. Verify voltage ratings, protection modes and installation requirements before selection.

Q4. What is the protection arrangement of Telebahn 4+0 SPDs?

Telebahn's conventional 4+0 design uses four PE-referenced protection paths: L1-PE, L2-PE, L3-PE and N-PE.

Q5. Why does Telebahn use MOV and GDT technology in 3+1 SPDs?

MOVs limit phase-to-neutral transient voltages, while the GDT provides the neutral-to-earth discharge path with high insulation under normal operating conditions.