AC Power SPD Type 1 vs Type 2: Selection Guide for Industrial Applications

• Why Type 1 vs Type 2 Comparison Matters
• IEC 61643-11 Classification: Type 1 and Type 2 Defined
• Test Waveforms: 10/350μs vs 8/20μs Explained
• Installation Locations in Low-Voltage Distribution Systems
• Protection Level (Up): What It Means and How to Compare
• Type 1 vs Type 2 SPD Parameter Comparison Table
• Cascaded Protection: Coordinating Type 1 and Type 2
• Application Scenarios: When to Choose Which
• Selection Considerations to Review
• Frequently Asked Questions
• Summary

Selecting the appropriate AC power surge protective device (SPD) is a key consideration for protecting low-voltage distribution systems in industrial environments. This guide compares Type 1 and Type 2 SPDs defined by IEC 61643-11, examining their test waveforms, installation locations, protection levels, and application scenarios to help engineers make informed selection decisions.

Why Type 1 vs Type 2 Comparison Matters

In industrial facilities, transient overvoltages from lightning strikes and switching operations can propagate through low-voltage distribution networks and affect sensitive equipment. Surge protective devices are designed to limit these transient voltages and divert surge current away from protected loads. However, not all AC SPDs are tested the same way, and selecting an inappropriate type can lead to inadequate protection or unnecessary cost.

The IEC 61643-11 standard classifies AC power SPDs into test classes, among which Type 1 and Type 2 are most commonly applied in industrial low-voltage installations. Understanding the distinction between them—particularly in terms of test waveform energy, installation placement, and protection level—is a foundation for sound surge protector selection. This article references the AC power SPD product range that Cresin manufactures in compliance with IEC 61643-11.

IEC 61643-11 Classification: Type 1 and Type 2 Defined

Under IEC 61643-11, an SPD's test class describes the standardized surge current waveform it has been evaluated against, rather than simply its rated current value. This classification guides where the device is intended to be installed within a lightning protection zone (LPZ) structure.

Type 1 SPD (Class I test) is tested with an impulse current waveform of 10/350μs, simulating the partial lightning current that may enter a building's electrical system following a direct lightning strike to an external lightning protection system. Type 1 devices are generally intended for installation at the origin of the installation—typically at the main distribution board—where they divert a portion of the lightning current to the earthing system.

Type 2 SPD (Class II test) is tested with a nominal discharge current (In) and a maximum discharge current (Imax) using an 8/20μs waveform. This waveform represents the shorter-duration surges associated with indirect lightning effects and switching transients. Type 2 SPDs are typically installed at sub-distribution boards or closer to end equipment, where residual surge energy from upstream protection is further limited.

Note: A third category, Type 3 (Class III test), is tested with a combination wave at lower energy and is intended for outlet-level protection. It may complement a Type 1/Type 2 cascade but is outside the scope of this comparison.

Test Waveforms: 10/350μs vs 8/20μs Explained

The distinction between the 10/350μs and 8/20μs test waveforms is frequently discussed in Type 1 vs Type 2 SPD comparison, because it relates to the energy the device is expected to handle.

A 10/350μs waveform rises to peak in approximately 10 microseconds and decays to half-value in around 350 microseconds. The long duration means the total charge and specific energy (action integral) transferred is substantially greater than that of a shorter waveform of the same peak amplitude. This is why Type 1 testing uses an impulse current (Iimp) characterized by charge transfer (Q) and specific energy (W/R), rather than the peak current value used in Type 2 testing.

An 8/20μs waveform reaches peak in about 8 microseconds and decays to half-value in approximately 20 microseconds. Although the peak current of a Type 2 test may be numerically similar to or higher than a Type 1 Iimp value, the total energy is considerably lower because the duration is much shorter. Engineers comparing Type 1 vs Type 2 SPD specifications should consider the underlying waveform rather than comparing peak current figures in isolation.

This energy difference has implications for component selection. A Type 1 SPD typically employs components capable of absorbing higher energy—such as spark gaps or high-energy MOV combinations—while a Type 2 SPD generally relies on metal oxide varistors (MOVs) optimized for a more compact footprint suitable for sub-distribution panels.

Installation Locations in Low-Voltage Distribution Systems

Where a Type 1 or Type 2 SPD is installed is closely related to its test class. The Lightning Protection Zone (LPZ) concept, defined in IEC 62305, provides a framework for placing SPDs at zone boundaries to progressively reduce surge threat levels closer to sensitive equipment.

A Type 1 SPD is typically installed at the boundary between the external environment (LPZ 0) and the internal installation (LPZ 1)—that is, at the main distribution board or incoming supply point. This is where partial lightning current may enter the system, and the SPD diverts the majority of this energy to the earthing system.

A Type 2 SPD is usually installed at the boundary between LPZ 1 and LPZ 2, such as at sub-distribution boards feeding load circuits. At this stage, the surge has been partially attenuated by upstream protection, and the Type 2 device further limits the residual voltage. In many industrial installations, a combined approach—Type 1 at the origin followed by Type 2 at sub-distribution—provides a coordinated multi-stage scheme.

An SPD's effectiveness also depends on connecting conductor length and coordination with adjacent devices. Excessive lead length can introduce additional voltage drop during a surge event, reducing the protection level delivered to the load—a consideration applicable to both Type 1 and Type 2 installations.

Protection Level (Up): What It Means and How to Compare

The protection level, designated Up, represents the residual voltage across the SPD terminals when the device conducts a nominal discharge current. A lower Up generally indicates less voltage is let through to the protected equipment. For effective protection, the Up should be below the rated impulse withstand voltage (Uw) of the equipment, with a suitable safety margin.

Type 1 SPDs, because they handle higher-energy 10/350μs impulses, may exhibit a relatively higher Up at rated impulse current compared with a Type 2 device tested at 8/20μs. This is one reason a Type 1 device alone may not provide sufficient protection for sensitive equipment, and why a downstream Type 2 SPD is often applied to achieve a lower residual voltage. When comparing datasheets, the Up value should be reviewed together with the corresponding test current and waveform.

Another parameter worth examining is the maximum continuous operating voltage (Uc), the steady-state voltage the SPD can withstand without conducting. Selecting a Uc appropriate for the system's nominal voltage helps ensure long-term reliability. For example, in a 230/400 V TN-S system, a Uc of 385 V or higher is commonly selected for Type 2 AC SPDs such as the CSMS-B40 series, rated for AC 385 V / 420 V / 440 V systems.

Type 1 vs Type 2 SPD Parameter Comparison Table

The table below summarizes the principal differences between Type 1 and Type 2 AC power SPDs as defined by IEC 61643-11, providing a reference for surge protector selection.

Cascaded Protection: Coordinating Type 1 and Type 2

In many industrial installations, a single SPD stage does not address all threat levels effectively. A cascaded approach—Type 1 at the main distribution board and one or more Type 2 SPDs at sub-distribution boards—allows each stage to handle surge energy appropriate to its location, while progressively reducing the residual voltage delivered to end equipment.

For this cascade to function as intended, coordination between stages is important. If upstream and downstream SPDs are not coordinated, the downstream device may conduct more current than expected, potentially leading to premature wear. Coordination depends on the Up and Uc of each device, the cable length between stages, and the surge characteristics. Decoupling elements or sufficient cable length (commonly on the order of 10 metres, depending on device types) are often applied to support proper energy sharing between stages.

When both protection stages are required, devices classified as combined Type 1+2 (tested to both Class I and Class II requirements) are available and can simplify specification. These devices carry both an Iimp rating and an In/Imax rating, allowing them to serve either as the origin-of-installation protector or as a high-capacity sub-distribution stage depending on the system design.

Application Scenarios: When to Choose Which

The choice between Type 1, Type 2, or a combination depends on the specific characteristics of the installation. Several common industrial scenarios illustrate how the selection decision is approached:

Scenario A — Facility with external lightning protection system. When a building is equipped with an external LPS in a region with notable lightning activity, a Type 1 SPD at the main distribution board is generally recommended to manage partial lightning current entering via the service entrance. A downstream Type 2 stage at sub-distribution boards further protects sensitive loads.

Scenario B — Industrial plant without external LPS. Where the primary surge sources are indirect lightning and switching transients, Type 2 SPDs at sub-distribution boards often form the core of the protection scheme. Whether the incoming supply also requires a Type 1 stage depends on a risk assessment per IEC 62305.

Scenario C — Sensitive equipment room. For installations housing equipment with low impulse withstand voltage, such as servers or precision controllers, a multi-stage cascade (Type 1 at origin, Type 2 at sub-distribution, and in some cases Type 3 near the equipment) is typically applied to achieve a low Up at the equipment terminals.

For projects requiring a combined assessment of SPD type, Uc, Up, and pole configuration for a given earthing arrangement, the Cresin technical team provides selection support. Readers are welcome to contact us for guidance tailored to a specific installation, or refer to the power distribution surge protection solutions for reference configurations.

Selection Considerations to Review

When evaluating Type 1 vs Type 2 SPD options, several considerations are worth reviewing:

• Comparing peak current without considering waveform. An Imax of 40 kA (8/20μs) and an Iimp of 25 kA (10/350μs) are not directly equivalent; the energy content differs considerably. Selection should reference the relevant test class rather than peak current alone.
• Overlooking Up vs. equipment Uw. The Up should be evaluated at the current level relevant to the installation's exposure and coordinated with the equipment's impulse withstand voltage.
• Lead length and wiring practices. Excessive connecting lead length can add inductive voltage drop during a surge, raising the effective protection level at the load terminals.
• Backup protection coordination. SPDs require suitable upstream overcurrent protection (a dedicated backup disconnector or appropriately selected fuse/breaker) to disconnect the device safely at end of life. Reference the IEEE surge protection guidelines and the relevant sections of IEC 61643-11.

Summary

Type 1 and Type 2 AC power SPDs serve complementary roles in industrial low-voltage distribution systems. Type 1 devices, tested with the 10/350μs impulse current defined in IEC 61643-11, are intended for the origin of an installation where partial lightning current may enter, while Type 2 devices, tested with the 8/20μs waveform, are applied at sub-distribution boards to limit residual voltage reaching equipment. The choice between them—and whether a cascaded arrangement is appropriate—depends on the installation's lightning exposure, the presence of an external lightning protection system, the impulse withstand voltage of protected equipment, and a site-specific risk assessment.

By evaluating test waveform, installation location, protection level Up, and application scenario together—rather than relying on any single parameter—engineers can approach AC SPD selection in a manner consistent with IEC 61643-11 and suited to the protection requirements of their installation.