Choosing the right surge protective device starts with understanding your electrical system, not simply comparing product specifications.
A practical surge protective device selection process follows this path: Application → System configuration → Installation location → SPD type → Voltage rating → Discharge capacity → Voltage protection level → Installation coordination
Step 1 — What Are You Protecting?
Step 2 — AC Supply Configuration
Step 3 — SPD Protection Level
Step 2 — DC Voltage Range
Step 2 — PV System Voltage
Step 2 — Signal Line Type
Recommended Surge Protectors
Select a Surge Protective Device: First Identify What You Are Protecting
Your first selection criterion should not be the surge protective device type. Instead, you should start with the electrical circuit you need to protect.
The electrical conditions in AC distribution systems, photovoltaic arrays, low-voltage DC systems, and communication networks are very different. Each application places different requirements on the SPD, including voltage rating, discharge capability, protection level, and circuit configuration.
| Application | Typical Systems | Primary Selection Parameters |
| AC Power | Residential, commercial, industrial distribution | Phase, earthing system, Uc, Type 1/2/3 |
| DC Power | Battery, BESS, telecom, control systems | DC operating voltage, Uc, polarity |
| Solar PV | PV strings, combiner boxes, inverter DC input | Ucpv, PV voltage, Type 1+2/Type 2 |
| Signal & Data | Ethernet, PoE, RS-485, CCTV, RF | Signal voltage, interface, bandwidth, discharge capability |
How to Select an AC Surge Protective Device
For an AC system, selection should normally answer four questions: What is the phase configuration? → What is the earthing system? → Where will the AC surge protector be installed? → What level of surge current must it withstand?
Single-Phase vs Three-Phase Surge Protector
In a typical residential or light-commercial installation, you may be working with a 230 V single-phase system, while industrial and commercial installations commonly use 400 V three-phase systems.
In general, a single phase surge protector is selected for a single-phase system, while a 3 phase surge protector is selected for a three-phase system.
However, you should not determine the number of surge protective device poles based on system voltage or phase configuration alone. You also need to identify the earthing arrangement of your electrical system.
For example, if your system uses TN-C, TN-S, TN-C-S, or TT earthing, the required SPD configuration may be different even when the nominal system voltage is the same.
Depending on your phase configuration and earthing system, you may need an SPD configuration such as: 1P, 2P, 1P+N, 3P, 4P, or 3P+N.
Select Type 1, Type 2 or Type 3 According to Installation Location
One of the most important surge protective device selection rules is: SPD type is closely related to where the device is installed and the surge energy expected at that location.
Type 1 at the service entrance, Type 2 at distribution boards, Type 3 close to sensitive equipment, and Type 1+2 where combined protection is required.
Type 1 SPD - Main Service Entrance
Type 1 SPDs are intended for locations where a portion of lightning current may enter the electrical installation.
Typical locations include:
- Main distribution board
- Building service entrance
- Installations with an external Lightning Protection System (LPS)
- LPZ 0 → LPZ 1 boundary
The critical parameter is Iimp, tested using a 10/350 μs impulse waveform.
Type 2 SPD - Distribution Board Protection
Type 2 is the most commonly used SPD for general low-voltage electrical installations.
It is normally installed in:
- Distribution boards
- Sub-distribution boards
- Industrial control panels
- Commercial buildings
- Residential distribution boards
Type 2 surge protection devices are characterized primarily by the 8/20 μs current waveform.
Type 3 SPD - Protection Close to Equipment
Type 3 surge protector is positioned close to sensitive loads.
Typical applications include:
- Electronic equipment
- Control systems
- Sensitive power supplies
- Equipment-level protection
It is generally used as the final stage of a coordinated SPD system rather than as the only SPD protecting an entire building.
A typical protection concept is therefore: Service entrance → Distribution board → Sensitive equipment
The objective is not merely to install more SPDs, but to progressively reduce the surge energy and residual voltage reaching the final equipment.
When Should You Select a Type 1+2 SPD?
A Type 1+2 SPD combines lightning-current and surge-current protection functions in one device.
It can be particularly useful where:
- Space inside the distribution board is limited
- The main incoming panel requires both Type 1 and Type 2 characteristics
- A compact coordinated solution is preferred
How to Select the Key Parameters of a Surge Protective Device
Once the general surge protective device type has been determined, the next step is to select the correct electrical parameters for the specific system.
For example, after deciding that an installation requires a Type 2 SPD, this alone is not enough to specify the final product. The engineer still needs to determine parameters such as Uc, Up, In, Imax, Iimp and Uoc.
These parameters answer different questions: Can the surge protection device operate continuously at the system voltage? How much surge current can it withstand? What voltage will remain across the protected equipment during a surge?
The same basic parameter-selection logic also applies to AC, DC, PV and signal SPDs, although the applicable parameter names, test methods and acceptable values differ according to the SPD type and relevant standard.
Uc - Maximum Continuous Operating Voltage
Uc is the maximum RMS AC voltage or DC voltage that may be continuously applied to the surge protective device without causing it to operate incorrectly under normal conditions.
Uc should therefore be selected according to the maximum continuous voltage that can occur at the SPD installation point, rather than simply matching the nominal system voltage.
For example, a 230 V AC system does not necessarily mean that a 230 V Uc SPD should be selected. The SPD must tolerate normal voltage variations and the conditions associated with the system configuration and earthing arrangement.
The basic selection principle is: Maximum continuous system voltage ≤ Uc
However, selecting an unnecessarily high Uc is not always beneficial. A higher Uc may also be associated with a higher voltage protection level.
The objective is therefore to select a Uc that is high enough to avoid unwanted operation during normal system conditions, but not unnecessarily high for the application.
For a typical 230 V AC system, an SPD with a Uc of 275 V is commonly selected.
For photovoltaic surge protection devices, the corresponding parameter is normally Ucpv, which must be coordinated with the maximum voltage that can occur in the PV array, including the effect of low temperature on string open-circuit voltage.
Up - Voltage Protection Level
Up indicates the voltage protection level of the SPD under specified test conditions.
When a surge occurs, an SPD does not reduce the voltage to zero. A residual voltage still appears across the SPD and therefore across the protected side of the installation.
In practical terms: The lower the Up, the lower the surge voltage presented to downstream equipment, provided the surge protective device is correctly coordinated with the system and installed properly.
The selected Up should be below the impulse withstand voltage of the equipment being protected, with an appropriate coordination margin.
Therefore, two Type 2 SPDs with the same In of 20 kA and Imax of 40 kA may not provide the same level of protection if their Up values differ significantly.
Up should never be evaluated independently, however. Uc and Up need to be considered together.
Choosing an excessively high Uc simply to increase voltage tolerance may result in a less favorable protection level.
In - Nominal Discharge Current
In is the nominal discharge current that an SPD is designed to withstand under the specified test waveform.
For Type 2 power SPDs, In is normally associated with an 8/20 μs current waveform.
For example: In = 20 kA means that the SPD has been tested at a nominal discharge current of 20 kA under the applicable standardized test conditions..
In is particularly useful when evaluating the SPD's ability to withstand repeated surge stresses.
It should not be confused with:
- The normal load current of the electrical circuit
- The short-circuit current of the installation
- Imax
- Iimp
These parameters describe different electrical characteristics.
Imax - Maximum Discharge Current
Imax is the maximum discharge current associated with a Type 2 SPD, normally using an 8/20 μs waveform.
A common specification may be: In = 20 kA, Imax = 40 kA
In this example, 20 kA is the nominal discharge current, while 40 kA represents the maximum discharge-current capability under the applicable test conditions.
This distinction is important because comparing two SPDs only by the largest “kA” number on the datasheet can be misleading.
Always determine whether the stated current is: In, Imax or Iimp.
A larger Imax may provide additional surge-current capability, but it does not automatically mean that the SPD provides a lower Up or is better suited to every installation.
Iimp - Impulse Discharge Current
Iimp is one of the key parameters for Type 1 surge protectors.
Unlike In and Imax, which are commonly associated with the 8/20 μs waveform, Iimp is associated with a high-energy lightning impulse test, typically represented by the 10/350 μs waveform.
Typical ratings may include: Iimp = 12.5 kA/pole or Iimp = 25 kA/pole
Iimp becomes particularly important where the SPD may be required to discharge a portion of lightning current, such as at the entrance of a building protected by an external lightning protection system.
Therefore: Type 1 selection → pay particular attention to Iimp, while: Type 2 selection → pay particular attention to In and Imax
A Type 2 surge protection device with a high Imax should not automatically be treated as a substitute for a Type 1 SPD when lightning-current capability is required.
Uoc - Open-Circuit Voltage of the Combination Wave Generator
Uoc is particularly relevant to Type 3 surge protector testing and describes the open-circuit voltage of the combination wave generator used for the test.
The combination-wave generator is commonly characterized by: 1.2/50 μs open-circuit voltage and 8/20 μs short-circuit current.
For example, a Type 3 SPD may specify a Uoc value in kilovolts.
Uoc should not be confused with Uc:
| Parameter | Meaning | Main Purpose |
| Uc | Maximum continuous operating voltage | Determines whether the SPD is suitable for continuous system voltage |
| Uoc | Open-circuit voltage of the combination wave generator | Characterizes the test severity, particularly for Type 3 SPD testing |
The two parameters may look similar, but they describe completely different characteristics.
How to Select a DC Surge Protector
DC surge protective device selection follows the same basic principles as other surge protectors, but particular attention should be paid to the continuous DC operating voltage and the SPD’s suitability for DC operating conditions.
The first step is to determine the maximum DC operating voltage of the system. The SPD’s maximum continuous operating voltage (Uc) must be compatible with the actual DC circuit to ensure stable operation under normal conditions.
Typical DC applications include:
- 12 VDC control circuits
- 24 VDC automation systems
- 48 VDC telecom systems
- Battery systems
- Battery Energy Storage Systems (BESS)
- Industrial DC buses
- EV-related DC systems
According to the operating voltage of different DC circuits, DC SPDs can generally be classified into 12 V DC Surge Protectors, 24 V DC Surge Protectors, and 48 V DC Surge Protectors to meet the protection requirements of different DC voltage systems.
However, an AC SPD with a similar voltage rating should not be assumed to be suitable for DC use. Unlike AC current, DC current does not have a natural zero crossing every half-cycle. Therefore, the SPD design and its failure-disconnection mechanism must be specifically suitable for DC operating conditions.
How to Select a Solar PV SPD
Photovoltaic systems require a separate selection process because the DC voltage can be much higher than ordinary low-voltage DC control circuits.
PV systems can be broadly classified by DC voltage as: ≤600 VDC → 1000 VDC → 1500 VDC, corresponding approximately to smaller, commercial, and utility-scale PV applications.
For engineering selection, however, the more important concept is: Select Ucpv according to the maximum possible PV array open-circuit voltage—not simply the nominal operating voltage.
This is especially important because module open-circuit voltage changes with temperature.
The array maximum voltage should therefore be determined from the string design and the lowest expected ambient temperature.
According to the maximum DC voltage of different PV systems, PV SPDs can generally be classified into 600 V PV Surge Protectors, 1000 V PV Surge Protectors, and 1500 V PV surge protectors to meet the surge protection requirements of different PV system voltage levels.
How to Select a Signal Surge Protector
How to Select an Ethernet or PoE Surge Protector
Unlike AC, DC, and PV power SPDs, Ethernet and PoE surge protectors must provide surge protection while maintaining normal data transmission and, where applicable, power delivery. The first step is therefore to determine whether the Ethernet cable carries data only or both data and power.
For a data-only Ethernet connection, an Ethernet Surge Protector can be selected according to the network interface and data rate. When Power over Ethernet (PoE) is used, however, the same cable carries both network data and DC power, so a PoE surge protector should be selected rather than an Ethernet SPD designed only for data-line protection.
Once the basic surge protective device type has been determined, the RJ45 interface, data rate, PoE power conditions, operating voltage, surge protection level, grounding method, and installation location should also be considered before selecting the final model.
How to Select an RS-485 / RS-232 SPD
RS-485 and RS-232 are widely used industrial communication interfaces, and the correct signal surge protector should not be selected solely by the name of the communication protocol. The selection should also consider the signal operating voltage, interface configuration, number of conductors, data rate, and grounding method.
First, determine whether the protected circuit uses RS-485 or RS-232 and select an SPD with an operating voltage suitable for the normal signal level. The SPD's maximum continuous operating voltage should be high enough for normal communication, while its voltage protection level should be low enough to limit surge voltage reaching PLCs, sensors, instruments, SCADA systems, and other connected equipment.
For RS-485, the wiring configuration, such as two-wire or four-wire communication, should also be considered; for RS-232, the required signal-line configuration must be identified. In addition, SPD capacitance and insertion loss should not significantly interfere with normal data transmission.
Once these requirements are established, the final signal surge protector can be selected according to its surge discharge capability, response time, grounding method, and installation location.
Conclusion
Selecting the right surge protective device is not about choosing the device with the highest kA rating. You need to match the SPD to your electrical system, installation location, expected surge exposure, and the equipment you want to protect.
The correct SPD is therefore not necessarily the one with the largest surge-current rating. It is the one whose voltage rating, discharge capability, protection level, circuit configuration, and installation position are properly coordinated with your system.
If you are unsure which SPD is right for your application, contact LSP. Our team can help you evaluate your system voltage, circuit configuration, installation location, and protection requirements and recommend a suitable surge protection solution for your project.
Frequently Asked Questions
Is a higher kA rating always better for an SPD?
No. A higher kA rating does not automatically mean better protection. You first need to determine whether the value represents Iimp, In, or Imax, because these parameters describe different surge-current capabilities and use different test waveforms. You should also evaluate Uc and Up rather than comparing SPDs only by their largest kA value.
Can I use an AC surge protector in a DC system?
You should not assume that an AC SPD is suitable for a DC circuit simply because the voltage ratings appear similar. DC systems have different operating and interruption characteristics, including the absence of a natural current zero crossing. You should use an SPD specifically designed and rated for the actual DC operating conditions.
Does the earthing system affect surge protective device selection?
Yes. Your earthing arrangement is an important part of SPD selection because it determines how the SPD is connected between the active conductors, neutral, and protective earth. Before selecting the final SPD configuration, identify whether your installation uses TN-C, TN-S, TN-C-S, TT, or another applicable system.
How do I select an SPD for sensitive electronic equipment?
For sensitive equipment, you should pay particular attention to the voltage protection level Up and the coordination between upstream and downstream SPDs. Depending on the installation, this may require a Type 2 SPD upstream and additional Type 3 protection close to the equipment.
References and Sources
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