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Type 1 vs Type 2 Surge Protective Devices: How to Choose the Right SPD for Your Project
Compare Type 1 and Type 2 surge protective devices by installation location, Iimp and In ratings, application scenarios, and common selection mistakes, with a use-case matrix and guidance toward combined Type 1+2 SPD solutions.
When specifying surge protection for a low-voltage installation, one question tends to surface early: which surge protection device types apply to this project? The shorthand answer in many specifications is a choice between Type 1 and Type 2, but the two are not interchangeable. They are defined and tested differently under IEC 61643-11, and they address different points in a distribution system. This article lays out the practical distinctions — installation position, Iimp versus In, typical scenarios, and recurring misselection patterns — so that the choice can be made on the basis of the project's electrical architecture rather than on a generic default. The Type 1 SPD vs Type 2 SPD decision is therefore approached more usefully as a system-design question than as a catalogue default.
1. Understanding the Two SPD Types
A Type 1 surge protective device is tested to withstand a partial lightning current that may enter the installation at its origin — for example, via the service entrance or via conductive parts bonded to an external lightning protection system. Its defining test is the Class I impulse-current test, which uses a 10/350µs waveform and rates the device by an impulse current value, Iimp, that includes charge (Q) and specific energy (W/R).
A Type 2 surge protective device is tested for surges that have already been attenuated by upstream protection or that arise from indirect lightning and switching events. Its defining test is the Class II nominal-discharge-current test, which uses an 8/20µs waveform and rates the device by a nominal discharge current, In, and a maximum discharge current, Imax. In everyday procurement language, both are sometimes called a surge arrester, though "SPD" is the term used in the IEC standards and is the one applied throughout this article.
2. Test Basis: Iimp vs In
The difference between the two devices is most clearly seen in what their test current represents. A Type 1 SPD vs Type 2 SPD comparison that considers peak current values in isolation can be misleading, because the energy carried by a 10/350µs impulse is considerably higher than that of an 8/20µs pulse of a similar peak.
Iimp applies to Type 1 devices and expresses the ability to divert a high-energy partial lightning current. In applies to Type 2 devices and expresses tolerance to repeated lower-energy surges. Imax, the maximum 8/20µs current the device can withstand once, is also declared for Type 2 (and for combined) devices. When reviewing a datasheet, the test waveform behind each number is worth noting, since it determines the surge environment the device was qualified for.
Protection level, Up, is declared for both types and indicates the residual voltage presented to the protected equipment. A lower Up generally allows better protection of sensitive loads, provided it is reviewed at the current at which it was measured.
3. Installation Location and Lightning Protection Zones
The Lightning Protection Zone (LPZ) concept from IEC 62305 gives a useful framework for placement. A Type 1 device is positioned at the boundary between the external environment (LPZ 0) and the interior (LPZ 1) — commonly the main incoming panel. A Type 2 device is positioned further in, at the boundary between LPZ 1 and LPZ 2, such as a sub-distribution board feeding a specific load. In many installations the two are applied in sequence: Type 1 at the origin, Type 2 closer to the equipment, with each stage handling the surge energy appropriate to its location.
4. Application Scenarios
Whether a project needs Type 1, Type 2, or both is usually informed by a lightning-protection risk assessment and by the presence of an external lightning protection system. A facility with an external LPS, or one located in an area with notable lightning activity, is commonly specified with a Type 1 stage at the origin. Installations where the dominant threat is indirect lightning or switching transients, and where no external LPS is present, may be addressed primarily with Type 2 devices at sub-distribution. Sensitive environments such as server rooms or process-control areas often use a cascade so that a lower Up is reached at the equipment terminals.
5. Type 1 vs Type 2 Comparison Table
The Type 1 SPD vs Type 2 SPD comparison table below summarises the parameters most often reviewed during specification.
| Parameter | Type 1 SPD | Type 2 SPD |
|---|---|---|
| Test class (IEC 61643-11) | Class I | Class II |
| Test waveform | 10/350µs impulse current | 8/20µs nominal current |
| Key rated value | Iimp (with Q and W/R) | In / Imax |
| Typical installation point | Main distribution / service entrance | Sub-distribution boards |
| Lightning Protection Zone | LPZ 0 → LPZ 1 boundary | LPZ 1 → LPZ 2 boundary |
| Surge source addressed | Partial direct lightning current | Indirect / switching surges |
| Protection level | Up declared | Up declared |
| Common cascade role | Upstream, high-energy stage | Downstream, voltage-limiting stage |
6. Use-Case Matrix
The matrix below maps common project types to a typical SPD arrangement. It is a starting point for discussion rather than a substitute for a site-specific assessment.
| Project type | Typical SPD arrangement | Notes |
|---|---|---|
| Residential, no external LPS | Type 2 at main board | Indirect-lightning and switching dominant |
| Commercial building with external LPS | Type 1 at origin + Type 2 downstream | Partial lightning current at service entrance |
| Industrial plant, mixed exposure | Type 1 + Type 2 cascade | Coordinate stages by Up, Uc, cable length |
| Data center / server room | Type 1 + Type 2 (+ Type 3 near loads) | Low equipment withstand voltage |
| Photovoltaic plant (DC side) | PV DC SPD per IEC 61643-31 | See Solar PV SPD |
| Retrofit of a single panel | Type 2, or combined Type 1+2 | Depends on incoming exposure |
7. Common Selection Mistakes
Several patterns of misselection appear frequently enough to be worth flagging before a specification is finalised:
- Using a Type 2 device at the service entrance where a Type 1 stage is indicated. A Type 2 device is not tested for the 10/350µs energy of a partial lightning current, so it may be assigned a role beyond its qualification.
- Comparing peak currents without the waveform. An Imax figure and an Iimp figure describe different tests; reading them as directly comparable can lead to an under-specified origin protector.
- Ignoring coordination between stages. Without adequate decoupling or cable length, a downstream device may conduct more current than intended.
- Selecting Uc without margin. For AC systems, a Uc matched too closely to nominal voltage can leave little allowance for temporary overvoltages.
- Assuming one SPD protects all equipment. A single device at the origin rarely delivers the low Up required at distant sensitive loads.
8. Combined Type 1+2 SPD: When to Consider
A combined Type 1+2 SPD is tested to both Class I and Class II requirements, carrying an Iimp rating and an In/Imax rating in one module. It is often considered where a single location is expected to address both a partial lightning current and subsequent induced surges — for example, at the origin of a smaller installation, or where panel space and bill-of-materials simplicity are priorities. Whether a combined device or separate stages are preferred is generally determined by the project's risk assessment, available space, and coordination requirements.
9. How to Choose the Right SPD — A Checklist
A practical approach to how to choose an SPD can be summarised as a short sequence:
- Confirm the installation's exposure — external LPS present? local lightning density? — via a risk assessment referencing IEC 62305.
- Map SPD stages onto the single-line diagram: origin (Type 1) and sub-distribution (Type 2) at minimum.
- Derive Uc from the system's nominal voltage and expected temporary overvoltages.
- Assign Iimp (Type 1) and In/Imax (Type 2) from the assessed protection level.
- Verify Up against the impulse withstand voltage of the equipment being protected.
- Confirm coordination between stages (decoupling or cable length).
- Decide between separate stages and a combined Type 1+2 device.
For AC low-voltage distribution, the AC Power SPD product range from Cresin covers Type 1, Type 2, and combined configurations rated to IEC 61643-11, which can be matched to the steps above.
Frequently Asked Questions
Q1: What is the main difference between Type 1 and Type 2 SPD?
The primary difference is the test waveform and the corresponding energy: Type 1 is tested with a 10/350µs impulse current (Iimp) for partial lightning current, while Type 2 is tested with an 8/20µs nominal current (In/Imax) for induced and switching surges. This informs where each is installed.
Q2: Can a Type 2 SPD replace a Type 1 SPD at the service entrance?
In installations where partial lightning current may enter at the origin, a Type 2 device is generally not assigned that role, because it is not tested to the 10/350µs energy level. A combined Type 1+2 device may be used where both functions are required at one location.
Q3: What does Iimp mean and why does it matter?
Iimp is the impulse-current rating of a Type 1 SPD, characterised by peak current, charge (Q), and specific energy (W/R). It indicates the device's ability to divert a high-energy partial lightning current at the installation origin. It is distinct from the In/Imax values used for Type 2 devices.
Q4: How do I know if my project needs a Type 1 stage?
A lightning-protection risk assessment referencing IEC 62305 typically informs this. Factors include the presence of an external lightning protection system, building height, local lightning density, and the consequence of equipment failure. In many buildings with an external LPS, a Type 1 stage at the origin is included.
Q5: What is a combined Type 1+2 SPD?
It is a device tested to both Class I and Class II requirements, carrying an Iimp rating and an In/Imax rating in a single module. It is considered where one location is expected to address both a partial lightning current and subsequent induced surges, such as the origin of a smaller installation.
Q6: How are cascaded SPDs coordinated?
Coordination relies on the upstream device handling the higher-energy portion while the downstream device limits residual voltage, without being exposed to more current than its rating allows. Up, Uc, cable impedance/length, and decoupling elements are the factors typically reviewed.
Q7: Is a "surge arrester" the same as an SPD?
The terms are used interchangeably in some regions, but "surge protective device (SPD)" is the term used in IEC 61643. A device marketed as a surge arrester should still be checked against the relevant IEC 61643 part and its declared test class to confirm suitability.
Not sure which SPD type your project needs?
Our engineering team can review your single-line diagram and risk assessment to confirm the appropriate Type 1, Type 2, or combined arrangement.
In summary, the choice in a Type 1 SPD vs Type 2 SPD comparison is shaped by where the device sits in the distribution system and what surge energy it is expected to handle, building on the distinction between a Type 1 surge protective device and a Type 2 surge protective device. Reviewing test class, Iimp versus In, installation point, and coordination together — rather than any single parameter — supports a specification that aligns with IEC 61643-11 and with the protection needs of the installation. For projects where both functions are required at one location, a combined Type 1+2 device is an option worth evaluating.
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