Figure 1. Three-phase SPD selection starts with system facts, not the largest kA number.
A three-phase surge protector should not be selected by the largest kA number on its label. First confirm the system voltage, whether a neutral is present, the earthing arrangement, the installation point, and the required SPD type. Then compare Uc, Up, and the applicable discharge-current ratings on the same test basis. Finally, verify the pole configuration, protection modes, backup-protection instructions, and short-circuit information in the exact model’s data sheet.
If the project information and product documents do not align clearly, pause the selection and request written model confirmation. This article is a specification-reading and quotation-preparation guide, not a wiring or installation instruction.
The label is useful, but it cannot answer every selection question. Use these seven checks in order.
| Check | Why it matters | Evidence to request |
| 1. Three-phase system | The product must be reviewed against the actual voltage, conductors, earthing arrangement, and installation point | Single-line diagram, panel specification, system voltage, and site information |
| 2. Pole and protection configuration | Similar module counts may represent different protection modes or terminal arrangements | Full model code, connection diagram, terminal designations, and protection modes |
| 3. SPD type | Type 1, Type 2, and combined classifications describe different test and application roles; they are not quality grades | Project requirement, risk assessment where applicable, and manufacturer classification |
| 4. Uc | The maximum continuous operating voltage must suit the voltage applied to each protection mode | Label value plus the exact model data sheet |
| 5. Up | The voltage protection level must be considered with equipment withstand, connection conditions, and coordination | Declared Up and the conditions under which it applies |
| 6. In, Imax, and Iimp | All may be shown in kA, but they do not describe the same test duty | Parameter definitions, waveform labels, SPD type, and values per mode or pole |
| 7. Backup protection and documents | The front label rarely contains the complete coordination and short-circuit information | Installation instructions, backup-device requirements, short-circuit data, and declared standards |
For a broader explanation of SPD types, AC/DC distinctions, and system selection, see OOHMAGE’s SPD types and system selection guide.
Figure 2. Confirm the system voltage, neutral, earthing, location, SPD role, and exact model evidence before selection.
A product label describes a device, not its electrical system. Collect the project facts before comparing products.
| Project fact | Where to find it | Why the supplier needs it |
| Nominal system voltage and actual connection | Single-line diagram, panel schedule, or project specification | To compare the system conditions with the voltage ratings of each SPD protection mode |
| Phase and neutral arrangement | Diagram and terminal schedule | To determine which pole and protection configuration requires review |
| Earthing arrangement | Design documents prepared by qualified personnel | To assess the required protection modes and manufacturer application conditions |
| Installation point | Main switchboard, distribution board, machine panel, or another defined location | To relate the SPD type and coordination role to the project design |
| Protection objective and downstream equipment | Project specification and equipment documentation | To review the required voltage protection level and coordination |
| Prospective short-circuit conditions | Engineering calculation or project record | To verify the product’s declared short-circuit and backup-protection conditions |
Do not assume one common three-phase voltage or earthing system applies worldwide. Even projects with the same line-to-line voltage may differ in neutral arrangement, voltage to earth, installation point, and protection design. Ask which documented SPD configuration can be reviewed against the exact system—not which product has the highest rating.
Labels such as 3P, 4P, 3P+N, or 3P+NPE do not replace a circuit description. Depending on the product, they may refer to modules, protected conductors, terminal arrangement, or protection modes.
Similar-looking devices can use different internal configurations, and enclosure width alone does not confirm suitability.
Where a product includes a neutral-related module or protection mode, that element forms part of the declared configuration. It should not be added, removed, or treated as interchangeable on appearance alone.
Use the exact model’s connection diagram, mode-of-protection information, and terminal designations. A qualified reviewer should compare them with the project diagram. This article provides no terminal-by-terminal wiring instructions.
Figure 3. Verified 3P and 3P+N variants; visible module count does not replace the connection diagram.
Type 1, Type 2, and Type 1+2 markings identify tested classifications and an SPD’s intended role. They are not a “good, better, best” scale.
| Label | Buyer-level interpretation | What still needs confirmation |
| Type 1 | The SPD has an impulse-current test classification associated with higher-energy lightning-current duty | Whether the project requires this role, the applicable Iimp value, system configuration, and coordination |
| Type 2 | The SPD has a nominal discharge-current test classification commonly used in distribution-level surge protection | The required In and Imax values, Up, Uc, installation point, and coordination |
| Type 1+2 | The selected product declares combined Type 1 and Type 2 test classifications | The values and protection modes declared for the exact product and how it fits the project design |
The current IEC framework places common SPD requirements in IEC 61643-01:2024, AC-specific requirements in IEC 61643-11:2025, and selection principles in IEC 61643-12:2020. A standard reference alone does not establish suitability for every system or market.
Uc is the maximum continuous operating voltage declared for an SPD or a mode of protection. In buyer terms, it answers: “What voltage can be continuously applied to this part of the SPD under its stated conditions?”
The value should be checked against the actual system voltage applied to the relevant protection mode, including the effects of the system and earthing arrangement. Do not automatically equate the nominal system voltage with Uc, and do not select the closest-looking number without understanding what conductors or terminals the value refers to.
Do: identify the voltage object—line to neutral, line to earth, line to line, or another declared mode—and compare it with the exact data sheet.
Do not: copy a Uc value from another model, another earthing arrangement, or a competitor’s selection table.
If the label shows several voltage values, record each symbol and unit exactly. A plain “385 V” is incomplete without its parameter and protection mode.
Up is the declared voltage protection level associated with the SPD’s limiting performance under specified test conditions. It is not the same parameter as Uc.
A lower Up value cannot be treated as a universal guarantee of better protection. The comparison must use the same protection mode and appropriate test basis, and it must still account for installation and coordination conditions. A catalogue value alone does not promise that every downstream device will experience that exact voltage in a real installation.
Ask the supplier to state Up for the relevant mode and identify the controlling document. Send any project equipment-withstand requirement for technical review rather than requesting only “the lowest Up.”
In, Imax, and Iimp can all appear as kA values, but equal units do not make them interchangeable. The waveform, SPD type, protection mode, and test definition matter.
Figure 4. Type 1+2 label: Uc, In, Iimp, and Up are separate declarations.
| Parameter | Buyer question | Common IEC-style waveform label | Most closely associated with | What the number alone cannot prove |
| In | What nominal discharge-current duty is declared for repeated Type 2 testing? | 8/20 μs | Type 2 classification and related product declarations | Service life, the number of real surges the product will survive, or complete system suitability |
| Imax | What maximum discharge-current value is declared under the stated Type 2 test basis? | 8/20 μs | Type 2 product comparison when the same definition and conditions are used | That the largest value is automatically the best choice, or that it is equivalent to Iimp |
| Iimp | What impulse-current duty is declared where the Type 1 classification applies? | 10/350 μs | Type 1 classification and lightning-current duty | That the product fits the project without checking Uc, Up, configuration, and coordination |
Figure 5. Type 2 label: compare Uc, In, Imax, and Up on the same test basis.
Start by confirming whether each value is given per pole, per protection mode, or for a complete assembly. Then confirm that the compared products use the same parameter and test definition. Do not rank a 10/350 μs Iimp value against an 8/20 μs Imax value as if they were the same contest.
Do not convert a laboratory declaration into a lifespan promise. Real exposure and operating conditions vary. Status indicators or remote contacts on selected models also do not replace the rating review.
The front label is the beginning of the evidence chain, not the end.
| Evidence layer | What to verify |
| On the product label | Manufacturer, full model code, SPD type, Uc, Up, applicable discharge-current ratings, protection configuration, status marking, and referenced standard where shown |
| In the manufacturer data sheet and instructions | Complete rating conditions, protection modes, short-circuit information, internal or external disconnector details, maximum permitted backup protection, coordination information, dimensions, environmental limits, and document revision |
| In the project documents | System voltage, neutral and earthing arrangement, prospective short-circuit conditions, installation point, upstream and downstream protection, equipment withstand requirements, destination-market rules, and approval requirements |
Some SPD designs contain internal disconnecting means; others require specific external backup protection under stated conditions. Never derive a universal fuse or circuit-breaker rating from another product family. Follow the instructions for the exact model and have the final coordination reviewed by qualified electrical personnel.
If a quotation lacks the current data sheet or instructions, request them before approving a substitution. Similar Type, Uc, or kA markings do not establish equivalent protection modes or backup requirements.
Once the seven checks are complete, buyers can review OOHMAGE’s AC and DC SPD product range as a commercial starting point. The category page is not a substitute for model-level documentation.
The stated system, ratings, configuration, and backup conditions appear consistent with the project information. Continue to technical, document, and commercial review; this is not local installation approval.
The marking appears relevant, but required model documents are missing. Pause and request them; do not fill gaps from a similar model or distributor listing.
A project condition is outside the product’s stated data. A larger Imax cannot compensate; return to model selection with complete system information.
A complete RFQ reduces repeated clarification.
| RFQ field | Information to provide |
| Application | Confirm that the request is for three-phase AC; use a separate DC SPD review for a DC system |
| System voltage | Nominal voltage and the actual line-to-line, line-to-neutral, or voltage-to-earth information available from the project |
| Conductors and earthing | Whether a neutral is present and the documented earthing arrangement |
| Installation point | Main incoming board, downstream distribution board, machine panel, or another defined position |
| Required SPD role | Project-specified Type 1, Type 2, Type 1+2, or a request for technical review if not yet determined |
| Required ratings | Project requirements for Uc, Up, In, Imax, and Iimp where applicable, with units and test basis |
| Configuration | Pole count, protection modes, panel space, mounting format, and terminal constraints from the project |
| Coordination | Available short-circuit information, upstream protection, and any backup-protection requirement |
| Options | Remote status contact, replaceable modules, status indication, or monitoring requirements where needed |
| Commercial details | Quantity, destination market, required label language, packing requirements, and requested delivery documents |
| Evidence | Data sheet, installation instructions, test or certification documents, and any approval format required by the project |
For a proposed product, send the full model code and a clear label photograph. Mark unknown project values as unknown rather than estimating them.
Imax is one Type 2 discharge-current parameter. Correct the comparison by checking the system, Type, Uc, Up, waveform, configuration, and documents together.
Uc concerns continuous operating voltage; Up concerns declared surge-limiting performance under specified conditions. Record both and state what each value applies to.
These waveforms are used for different test duties. Compare like with like and keep the parameter symbol attached to every kA value.
The number of modules does not fully describe the protection modes or system fit. Use the exact connection diagram and terminal designations.
Those project facts can change the configuration and coordination discussion. Collect them before requesting a final model.
Similar labels do not make products interchangeable. Use each manufacturer’s current document for the exact ordering code.
A label image without a matching data sheet and installation instructions is not a complete technical package. Request the missing documents first.
Uc is the maximum continuous operating voltage declared for an SPD or protection mode. It should be compared with the voltage continuously applied at the intended connection point, considering the system and earthing arrangement. It is not automatically identical to the nominal system voltage, and a universal Uc value should not be chosen without the exact product data.
In is the nominal discharge-current value associated with an 8/20 μs test duty. Imax is the maximum discharge-current value declared under an 8/20 μs Type 2 test basis. Compare them only within the same definitions and product conditions; neither value alone proves service life or complete project suitability.
Do not assume they are the same. The labels may represent different protected conductors, modules, terminal arrangements, or protection modes. Confirm the neutral and earthing arrangement, then check the selected model’s connection diagram, protection modes, and terminal designations. This is a document review, not a wiring instruction.
No. Imax must be considered with the SPD type, waveform, In, Uc, Up, protection configuration, installation point, short-circuit conditions, and backup-protection instructions. A higher Imax cannot correct a voltage, topology, or application mismatch.
Send the system voltage, neutral and earthing information, installation point, required SPD type, pole and protection configuration, applicable Uc, Up and discharge-current requirements, short-circuit or backup-protection information, quantity, destination market, and required documents. Include the proposed model code and label image if available.
Start with the three-phase system, then read the label against the exact product documents. Uc, Up, In, Imax, and Iimp answer different questions, while pole labels and module counts require confirmation from the model’s connection and protection-mode information. The largest kA number cannot replace those checks.
When the system data and product documents align, move to technical and commercial review. When information is missing, request it rather than estimating. To discuss an OOHMAGE option, send the three-phase system conditions and document requirements so the available SPD series can be reviewed before quotation.
Continue with the matching product range and the supporting selection guides below.
