SPD Selection Wizard: Match System Voltage, Earthing and Application in 6 Steps

• How to Choose an SPD
• The Six-Step SPD Selection Process
• Example: 230/400 V TN-S System
• Common SPD Selection Mistakes
• Practical Selection and Installation Checks
• Conclusion
• FAQ

TECHNICAL GUIDE | ELECTRICAL SAFETY

How to Choose an SPD

An SPD should be selected according to the complete electrical system rather than voltage alone. The key factors include system voltage, earthing system, SPD Type, configuration, Uc, Up, and backup protection. Checking these parameters in the correct order helps buyers avoid selecting an SPD that cannot match the installation.

For a more detailed selection process, refer to the CRESIN SPD Selection Guide for additional guidance on choosing suitable surge protection devices for different electrical systems and applications.

The Six-Step SPD Selection Process

The correct selection of surge protection devices (SPDs) is critical to ensuring electrical system safety. This guide covers the complete six-step selection process, from confirming system voltage to backup protection configuration, helping engineers choose the most suitable CRESIN SPD solution for different application scenarios.

Step 1: Confirm System Voltage

SPD selection must first be determined based on system voltage, grounding method, protection level, and installation location. CRESIN provides a comprehensive range of AC & DC power SPD solutions for different electrical systems and surge protection requirements. 

For a AC 230/400 V three-phase system, the phase-to-neutral operating voltage is 230 V, not 400 V. The SPD's continuous operating voltage Uc should be selected based on the actual system voltage and earthing arrangement. 

For DC systems, selection should consider the maximum operating voltage, while PV applications should also account for the maximum open-circuit voltage under the lowest expected temperature to ensure the SPD can withstand extreme operating conditions.

For systems with communication, control, or data transmission lines, surge protection should also be considered for connected signal circuits. CRESIN offers Signal & Data SPD solutions for protecting communication and data lines against surge-related damage.

After defining the SPD requirements, engineers should also consider the overall protection solution, including the installation environment, protected equipment, power distribution structure, and coordination between different protection stages. CRESIN provides surge protection solutions for different electrical and application scenarios.

Step 2 — Identify the Earthing System

Determine whether the installation site uses a TN-C, TN-S, TN-C-S, TT, or IT grounding system. The grounding method directly affects the SPD wiring configuration and protection mode, and is a critical factor that cannot be overlooked during selection.

TN-C uses L-PEN protection, while TN-S and TT can use L-N and N-PE protection. TT systems require an N-PE protection element, while IT systems generally require higher Uc because phase-to-earth voltage can rise during the first earth fault.

The choice of grounding method directly determines the SPD wiring configuration, so the site grounding system type must be identified before selection.

Step 3 — Select the SPD Type

Based on the type of lightning risk at the installation location, choose the appropriate SPD category. Different SPD types are suitable for different protection scenarios, and correct classification is the basis for ensuring protection effectiveness.

Type 1 is used in locations with external lightning protection systems or direct lightning current risk, capable of discharging the high current generated by a direct lightning strike.

Type 2 is typically installed in main distribution boards and distribution panels, used to handle switching overvoltages and induced lightning overvoltages. It is the most common SPD type.

Type 3 provides additional protection close to sensitive equipment and should be coordinated with an upstream Type 2 SPD and it must not be installed alone.

Proper coordination is essential for multi-stage protection. Type 1, Type 2, and Type 3 SPDs should be selected according to their installation position and declared coordination requirements. Keep SPD connection leads as short as possible; a commonly applied design target is a total lead length of no more than 0.5 m. When the distance between an upstream SPD and sensitive equipment exceeds about 10 m, an additional coordinated SPD may be required.

Step 4 — Select the Configuration

The choice of poles and protection mode depends on the phase arrangement and grounding system. The correct configuration ensures that the SPD fully covers the system’s surge protection requirements while avoiding safety risks caused by incorrect wiring.

Typical three-phase TN-S or TT applications use a 3+1 configuration, meaning three phase lines plus a neutral-to-ground protection
combination, providing comprehensive protection for both the phase lines and the neutral line.

While TN-C systems use 3P protection. Do not install an N-PE element in the TN-C section, because the neutral line and protective ground line are already combined into a PEN line; installing an N-PE element will cause incorrect wiring.

Step 5 — Check Uc and Up

Uc (continuous operating voltage) and Up (voltage protection level) are the two core electrical parameters in SPD selection, and they must match the system conditions and the withstand capability of the equipment being protected.

Uc is the maximum continuous operating voltage of the SPD and must be suitable for both normal voltage and temporary overvoltage conditions. For a 230 V phase-to-earth TN system, Uc should generally be at least 1.1 × Uo, with practical classes such as 275 V or higher depending on the application. 

Uc — Continuous Operating Voltage is the maximum operating voltage that the SPD can continuously withstand. It must be suitable for
both normal voltage and temporary overvoltage conditions. For a 230 V phase-to-ground TN system, Uc should generally be at least 1.1 × Uo, and in practical applications, 275 V or a higher rating may be selected depending on the situation.

Up is the voltage protection level and should remain below the impulse withstand voltage Uw of the protected equipment.

Up — Voltage Protection Level is the maximum residual voltage of the SPD during lightning current discharge. It should be lower than the impulse withstand voltage Uw of the protected equipment to ensure effective protection. The lower the Up value, the better the protection effect, but it must be considered in coordination with Uc.

The selection of Uc and Up must comprehensively consider the system voltage level, grounding method, and the insulation withstand capability of the protected equipment.

Step 6 — Confirm Backup Protection and Signalling

Reliable SPD operation depends on the correct backup protection device. For critical or unattended installations, remote
signaling can provide important status monitoring capability.

Backup Protection Configuration

Refer to the SPD datasheet to confirm the required external fuse or MCB rated value and short-circuit withstand capability. Do not select backup protection based on the specifications of other products; it must be configured strictly according to the technical requirements of the selected SPD.

Remote Signaling Function
For critical or unattended installation sites, a floating remote signaling contact can be configured to issue an alarm signal when the SPD reaches end of life, enabling timely maintenance and replacement and ensuring the system remains continuously protected.

Overview of the Six-Step Selection Process
After determining the SPD selection requirements, engineers should also consider the overall protection scheme at the system level to ensure coordination between protection stages and achieve comprehensive surge protection. A holistic approach is crucial for establishing robust defenses against transient overvoltages, safeguarding critical infrastructure and sensitive electronics across various environments.

The overall protection scheme should take into account installation environment conditions, the characteristics of the protected equipment, power distribution layout, and the coordination between different protection stages. By meticulously evaluating these factors, engineers can design a resilient surge protection system that mitigates risks effectively. 

Example: 230/400 V TN-S System

For a three-phase 230/400 V TN-S commercial installation without an external lightning protection system, a typical selection process would identify Uo as 230 V, select Type 2 protection at the distribution board, and consider a 3+1 configuration. The final Uc, Up, In, Imax, backup protection, and other parameters must then be verified against the selected manufacturer's datasheet and the project's TOV and short-circuit conditions.

Common SPD Selection Mistakes

Common mistakes include selecting Uc from the line-to-line voltage in a standard TN system, ignoring the earthing arrangement, using Type 3 without upstream Type 2 protection, choosing an unsuitable backup fuse or MCB, and allowing excessively long SPD connection leads. These issues can reduce protection performance or cause premature SPD failure.

Practical Selection and Installation Checks

Before finalizing an SPD, engineers should verify the actual electrical conditions at the installation point rather than selecting a model only by nominal voltage. Check the system voltage, earthing arrangement, expected short-circuit conditions, available installation space, and the protection requirements of connected equipment. The SPD wiring should also be kept as short and straight as possible to reduce additional voltage during a surge event. The PE connection is especially important because long or poorly routed grounding conductors can reduce the effectiveness of surge diversion. 

For multi-stage protection, the upstream and downstream SPDs should be properly coordinated according to their electrical characteristics and installation distances. Buyers should also confirm the product's test standards, rated parameters, backup protection requirements, status indication, remote signalling, and replacement method before bulk ordering. These checks help ensure that the selected SPD is not only electrically compatible but also practical for installation, maintenance, and long-term system protection.

For commercial and industrial projects, SPD selection should also consider the installation environment and future maintenance requirements. Factors such as temperature, humidity, enclosure space, accessibility, replacement frequency, and remote monitoring can affect the long-term performance of the protection system. Clear documentation and product traceability can also simplify inspection, maintenance, and future replacement.

Conclusion

Correct SPD selection starts with the electrical system, not the product catalogue. By checking voltage, earthing, SPD Type, configuration, Uc, Up, and backup protection step by step, buyers can prepare a clear specification for suppliers. CRESIN can further verify the selected parameters against the appropriate SPD datasheet before quotation and ordering. Contact CRESIN to discuss your SPD requirements and confirm the suitable protection solution for your application.

FAQ

What information is needed for SPD selection?

Provide system voltage, phase number, earthing system, installation location, lightning exposure, required SPD Type, equipment Uw, short-circuit current, and quantity.

What is the difference between Uc and Up?

Uc is the maximum continuous operating voltage the SPD can withstand, while Up is the voltage protection level during a surge.

Does a TT system need N-PE protection?

Yes. TT systems generally require an N-PE protection element and appropriate TOV verification.

When is Type 3 required?

Type 3 is used near sensitive equipment as an additional protection stage, particularly when the equipment is sufficiently far from the upstream SPD or requires a lower Up.