When a project already requires a Type 1 or Type 1+2 surge protective device, the next question is usually whether its lightning impulse current rating should be Iimp 12.5kA or 25kA. The correct rating depends on the project’s lightning protection level, expected current sharing, protection modes and the way the manufacturer declares the value.
Wählen Sie Iimp 12.5kA when the calculation or accepted project rule shows that the required 10/350 μs impulse-current capability is no more than 12.5kA through the declared pole, branch or mode of protection. This is commonly associated with LPL III and IV installations.
Wählen Sie Iimp 25kA where the required duty is above 12.5kA and no more than 25kA. LPL I commonly produces a 25kA requirement in a simplified four-path calculation. LPL II produces an illustrative value of 18.75kA, so the next higher common product rating is normally 25kA.
Do not select from the largest number on the data sheet. Confirm whether Iimp applies to one pole, one mode of protection or the complete SPD arrangement, and check Itotal separately.
Inhaltsübersicht
Iimp 12.5kA or 25kA: Decision in 30 Seconds
Common for LPL III/IV installations when the current-sharing calculation supports 12.5kA or less through each declared protection path.
Common for LPL I and for LPL II projects where the illustrative 18.75kA result must be rounded upward to an available 25kA product rating.
Do not decide from the building type or local thunderstorm frequency alone. Obtain the LPS class, supply arrangement and current-distribution basis.
What Is Iimp on a Type 1 SPD?
Iimp is the peak impulse-current value used to express the lightning-current handling capability of a Type 1 SPD. It is stated in kiloamperes and associated with the 10/350 μs Wellenform.
IEC 61643-11:2025 is the current requirements and test-method standard for SPDs connected to AC low-voltage systems. IEC 61643-12:2020 provides selection, operation, location and coordination principles. [1] [2]
Iimp must not be confused with Unter oder Imax. Those values normally use the shorter 8/20 μs waveform associated with the Type 2 function.
What Does the 10/350 μs Waveform Mean?
The notation describes a standardized current impulse. The waveform rises rapidly toward its peak and then decays over a much longer period than the 8/20 μs waveform used for Type 2 testing.
What changes when Iimp rises from 12.5kA to 25kA?
When the standardized waveform shape remains the same, increasing the peak current from 12.5kA to 25kA doubles the peak current and approximately doubles the charge. Because specific energy is related to the integral of current squared, the specific-energy duty increases by approximately four times.
Iimp 12.5kA vs 25kA: Main Differences
| Comparison point | Iimp 12.5kA | Iimp 25kA | What the buyer should confirm |
|---|---|---|---|
| Test waveform | 10/350 μs | 10/350 μs | Both values must be declared using the same waveform. |
| Peak current | 12.5kA through the declared path or mode | 25kA through the declared path or mode | Check exactly where the declared value applies. |
| Typical LPL association | LPL III and IV | LPL I; also commonly selected for LPL II | LPL II has an illustrative calculated value of 18.75kA, which is normally rounded upward to 25kA. |
| Impulse-energy duty | Unter | Approximately four times the specific-energy duty for the same waveform shape | Product technology, size and follow-current performance may differ. |
| Spannungsschutzpegel Up | Product-specific | Product-specific | A higher Iimp does not guarantee a lower Up. |
| Typical selection basis | Required duty no higher than 12.5kA | Required duty above 12.5kA and no higher than 25kA | Use the next certified rating equal to or above the calculated requirement. |
| Commercial impact | Often more compact and economical | Often larger or based on a higher-energy design | Check DIN width, terminal size, backup protection and panel space. |
Swipe horizontally to view the complete table.
How Lightning Protection Level Affects Iimp
IEC 62305 defines lightning protection levels and provides the basis for lightning risk management and surge protection measures. The 2024 edition also gives greater attention to surge-current dimensioning and current-sharing considerations. [6] [7]
| Lightning protection level | Reference lightning current | Illustrative four-path calculation | Common practical selection |
|---|---|---|---|
| LPL I | 200kA | 200kA × 50% ÷ 4 = 25kA per path | Iimp 25kA |
| LPL II | 150kA | 150kA × 50% ÷ 4 = 18.75kA per path | Iimp 25kA as the next higher common rating |
| LPL III | 100kA | 100kA × 50% ÷ 4 = 12.5kA per path | Iimp 12.5kA |
| LPL IV | 100kA reference class value | Commonly evaluated on a similar 12.5kA-per-path basis | Iimp 12.5kA |
Swipe horizontally to view the complete table.
How to Choose Iimp in Real Projects
12.5kA may be suitable when
- The design is based on LPL III or IV.
- The required duty is no more than 12.5kA through each declared protection path.
- The service and bonding arrangement matches the calculation.
- The exact SPD topology is certified for the network system.
25kA may be required when
- The structure is designed to LPL I or II.
- The calculated requirement exceeds 12.5kA.
- Fewer effective current-sharing paths are available.
- The consultant, tender or national rule explicitly requires 25kA.
Does every building with an external LPS need 25kA?
No. An external lightning protection system means Type 1 protection normally needs to be considered at the origin, but it does not automatically determine the Iimp rating. An LPL III or IV project may use 12.5kA where the calculation supports it. LPL I or II commonly leads to a 25kA product selection. [4] [5]
What about high-lightning and industrial sites?
High lightning activity increases the probability of an event, but it does not by itself produce one fixed Iimp value. Industrial incomers, hospitals, telecom facilities, hazardous sites and other critical installations often justify the higher rating because of the LPL, current-distribution result or consequence of failure—not simply because they are “industrial.”
Do Not Confuse Per-Path Iimp with Itotal
Type 1 SPD quotations often place several large kA values in the same table. They may describe different protection modes and must not be treated as interchangeable.
Iimp for one path or mode
This is the impulse current through a declared protection path, such as L–N, L–PE, N–PE or L–PEN.
A product described as “12.5kA per pole” does not automatically prove that every protection mode has the same value.
Itotal for the arrangement
Itotal is a separate declaration for the complete multipole SPD or tested arrangement.
It must be taken from the manufacturer’s verified data—not created by multiplying the number of modules.
Why 3+1 products need special attention
In a 3+1 SPD, the L–N protection paths and the N–PE path may have different Iimp values. Manufacturer data may separately state Iimp for L–N, Iimp for N–PE and Itotal for the complete configuration. [8]
Practical Type 1 SPD Iimp Selection Process
Identify the external LPS, direct-lightning-current exposure, overhead supply, tender specification or local rule.
Do not ask the SPD supplier to infer the LPL from the building name or industry alone.
Consider the incoming power conductors, bonded services, cable shields, earthing arrangement and possible unequal distribution.
Confirm whether the system uses 1+1, 3+1, 4+0, L–N, L–PE, N–PE or L–PEN protection.
Use 12.5kA when the requirement is no higher than 12.5kA. Use 25kA when the requirement is above 12.5kA and no higher than 25kA.
Verify Uc, Up, In, Imax, TOV behaviour, short-circuit rating, follow current, backup protection and conductor requirements.
Confirm that the exact voltage, topology, poles and model code appear within the certificate or verified test-report scope.
Type 1+2 SPD Bulk-Purchasing Checklist
For panel projects, distributor stock and OEM orders, the purchase order should specify more than “Type 1+2, 25kA.”
- Type 1 or Type 1+2 classification
- Applicable IEC or EN standard edition
- Iimp value at 10/350 μs
- Protection path or mode covered by Iimp
- Declared Itotal for the complete SPD
- System voltage and Uc/MCOV
- Spannungsschutzpegel Up
- In and Imax at 8/20 μs
- TN-C, TN-S, TN-C-S, TT or IT system
- 1+1, 3+1, 4+0 or other topology
- L–N, L–PE, N–PE and L–PEN ratings
- Follow-current extinguishing capability
- Short-circuit current rating or withstand
- Maximum permitted backup protection
- TOV behaviour and safe failure mode
- Terminal size and tightening torque
- Pluggable or monoblock construction
- Visual indicator and remote alarm contact
- Exact model coverage in certification
- OEM label, packaging, MOQ and lead time
Parameters that need separate confirmation
Iimp does not replace Uc selection, Up selection, Nennstrom oder Imax selection. Each parameter answers a different engineering question.
| Parameter | What the supplier should state | Warning sign |
|---|---|---|
| Iimp | Value, 10/350 μs waveform and exact protection mode | Only “25kA SPD” with no waveform or mode |
| Itotal | Verified value for the complete configuration | Total created by informal multiplication |
| Type declaration | Type 1 or Type 1+2 under the applicable standard | “T1+T2” used only as marketing text |
| Zertifikat | Certificate holder, exact model family and parameter scope | Certificate belongs to another model or manufacturer |
| Network system | Approved topology for TN, TT or IT use | The same configuration is offered for every system |
| Sicherungsschutz | Maximum fuse or breaker and coordination conditions | No declared backup-protection requirement |
| Nach oben | Declared Up for the relevant modes | Higher Iimp is presented as proof of lower Up |
| OEM approval | Approved model, technical label, packaging and documents | Production begins before technical confirmation |
Swipe horizontally to view the complete table.
Copy-and-send RFQ template
Send the following information to the SPD manufacturer to reduce repeated clarification and incorrect quotations.
Häufig gestellte Fragen
Is a 25kA Type 1 SPD always better than a 12.5kA model?
It has a higher 10/350 μs impulse-current capability, but it is not automatically the better complete specification. Uc, Up, topology, short-circuit conditions, backup protection and certification must also match the project.
Can a 25kA SPD replace a 12.5kA SPD?
It may provide adequate Iimp margin, but only when the voltage rating, network system, protection modes, Up, follow-current capability, backup protection, terminals and certification are also suitable.
Why is LPL II normally matched with a 25kA SPD?
A simplified four-path calculation gives 18.75kA per path. Because 18.75kA is not a common product rating, the next higher commercial rating is normally 25kA.
Does an external lightning protection system always require 25kA?
No. LPL III or IV projects may use 12.5kA where supported by the current-distribution design. LPL I or II commonly leads to a 25kA selection.
Is Iimp 12.5kA the same as Imax 12.5kA?
No. Iimp uses a 10/350 μs waveform for Type 1 duty. Imax uses an 8/20 μs waveform for Type 2 duty. The current stresses are different.
Can I multiply per-pole Iimp by the number of poles?
Not unless the manufacturer declares the resulting value as Itotal for the tested complete configuration.
What should be checked in a 3+1 Type 1+2 SPD?
Check Iimp for the L–N paths, Iimp for the N–PE path, Itotal, Uc, Up, follow-current behaviour, backup protection and compatibility with the TT or TN-S system.
What if the required Iimp exceeds 25kA?
Review a higher-rated dedicated Type 1 solution and the complete lightning-protection architecture. Do not use a 25kA product beyond its declared and certified duty.
Need a Type 1+2 SPD Specification for an OEM Order?
Send us the system voltage, earthing arrangement, topology, LPL, required Iimp, Up, short-circuit conditions and target market. LEEYEE can review the ordering parameters before quotation and label approval.
Technical References
- IEC 61643-11:2025, Low-voltage surge protective devices connected to AC low-voltage power systems—requirements and test methods. View IEC publication
- IEC 61643-12:2020, selection, operation, location and coordination principles for SPDs connected to AC power circuits. View IEC publication
- Electrical Installation Guide, Selection of a Type 1 SPD, including the commonly referenced minimum Iimp of 12.5kA at 10/350 μs per branch where no higher rule applies. View selection guide
- Phoenix Contact, Type 1+2 surge protection portfolio and application guidance for 12.5kA and 25kA ratings. View technical reference
- Legrand, modular SPD technical documentation, including 12.5kA-per-pole guidance for LPL III/IV applications. View technical document
- IEC 62305-1:2024, Protection against lightning—general principles and lightning-current parameters. View IEC publication
- IEC 62305-4:2024, surge protection measures for electrical and electronic systems, including current-sharing considerations. View IEC publication
- DEHNshield manufacturer data showing separate Iimp values for L–N and N–PE modes and a separate Itotal declaration. View product technical data
National wiring rules, project specifications and the responsible lightning-protection engineer’s calculation take precedence over general selection examples. Verify the latest product data and certification scope before ordering.
