Type 1+2 SPD Explained: When to Use Combined Lightning and Surge Protection

A Type 1+2 SPD — also referred to as a Class B+C SPD — combines partial lightning current and induced surge diversion in a single module. This guide examines when a combined SPD is the appropriate choice over separate protection stages, where it fits in a cascaded architecture, and which parameters govern the selection.

Type 1 and Type 2 SPD: Classification Recap

The IEC 61643-11 standard classifies low-voltage AC surge protective devices into test classes based on the type of surge they are designed to handle. Type 1 SPDs are tested with an impulse current (Iimp) waveform of 10/350 μs, which simulates a partial lightning current. The 10/350 μs waveform has a relatively slow rise time (10 μs to peak) and a long tail (350 μs to half-value), reflecting the duration of a direct lightning stroke. Type 2 SPDs are tested with a nominal discharge current (In) and maximum discharge current (Imax) using an 8/20 μs waveform, which represents induced switching and lightning surges with a faster rise and shorter duration.

The energy content of the two waveforms differs by roughly an order of magnitude. A 10/350 μs impulse at 12.5 kA delivers substantially more charge and specific energy than an 8/20 μs impulse at the same peak current, which is why Type 1 protection elements are typically larger or use spark-gap technology that can tolerate the longer duration. This energy difference is the technical reason that a combined device is not simply a Type 2 module with a higher rating — the internal components are selected and arranged to withstand both waveform profiles without degradation.

The distinction matters for installation location. A Type 1 SPD is installed at the main entrance of a building, where it can divert a portion of the direct lightning current that enters through the supply or the earthing system. A Type 2 SPD is installed downstream at sub-distribution boards or closer to sensitive equipment, where the surge has already been partially attenuated and the residual overvoltage is lower. Together, these two stages form a cascaded protection architecture.

The older VDE classification used the terms Class B and Class C for what IEC 61643-11 now calls Type 1 and Type 2 respectively. The term Class B+C therefore refers to a combined Type 1+2 device. Both naming conventions are still encountered in product literature, and they describe the same category of device. A detailed comparison of the two types is available in the Type 1 vs Type 2 SPD selection guide.

What Is a Type 1+2 SPD (Class B+C)?

A Type 1+2 SPD is a combined device that satisfies both the Type 1 impulse current test (Iimp, 10/350 μs) and the Type 2 nominal and maximum discharge current tests (In and Imax, 8/20 μs) within a single module. In practical terms, the device contains protection elements rated for both high-energy partial lightning current and the repetitive lower-energy induced surges that follow.

The value of this combination lies in installation simplification. In a conventional two-stage cascade, a Type 1 SPD is mounted at the main distribution board and a separate Type 2 SPD is installed at a downstream sub-board. The Type 1+2 combined SPD consolidates both functions into one physical unit at the main board, reducing wiring length, panel space, and the coordination effort between stages.

The internal design of a combined SPD typically uses high-energy spark gaps or varistors configured to handle the 10/350 μs impulse without degradation, while also providing the lower protection level (Up) expected of a Type 2 device for downstream equipment. The thermal disconnect mechanism and status indicator operate in the same manner as a standard Type 2 module, providing end-of-life signaling for maintenance.

A common internal architecture uses a spark gap or a high-energy varistor stage for the Type 1 function, coupled with a varistor stage for the Type 2 function. The two stages are integrated so that the high-energy component absorbs the bulk of the lightning impulse, and the finer varistor stage clamps the residual voltage to the lower Up value. In modules that use varistor-only technology, the components are sized to meet both test waveforms through their combined surge energy rating, and the thermal disconnect is calibrated to operate under the repetitive 8/20 μs duty that characterizes long-term service.

Note: "Class B+C" and "Type 1+2" describe the same device category. The VDE naming (Class B, C) predates the IEC 61643-11 reclassification (Type 1, Type 2), and both conventions appear in manufacturer datasheets and project specifications.

When to Choose a Combined SPD Over Separate Stages

The decision between a combined Type 1+2 SPD and separate Type 1 plus Type 2 stages depends on several factors that should be evaluated for each installation.

Space constraints. A combined SPD occupies one DIN-rail slot block instead of two separate modules at different locations. In compact main distribution boards or prefabricated outdoor cabinets where panel space is limited, the combined device is often the practical choice. This is particularly relevant for retrofit projects where adding a second stage at a sub-board is not feasible.

Single-point surge entry. Where the building has a single point of supply entry and the lightning protection system (LPS) is bonded to the main earthing terminal at that point, a combined SPD at the incoming section can address both the direct partial lightning current and the induced surges that would otherwise propagate downstream. In this topology, the need for a separate upstream Type 1 stage is reduced.

Wiring length and coordination. In a two-stage cascade, the decoupling impedance between the Type 1 and Type 2 SPDs depends on the wiring length between them. IEC 61643-11 requires a minimum conductor length (typically 10 m, or a validated decoupling inductor) between stages to ensure proper energy sharing. Where this length is not achievable, a combined SPD eliminates the coordination concern by integrating both functions into one module.

Cost and complexity. A single combined module reduces the number of devices, backup protectors, and wiring connections, which can lower installation cost and simplify maintenance. For installations where a downstream Type 2 stage is still planned for sensitive loads, the combined device at the main board serves as the first stage, and the sub-board Type 2 provides a second level of coordination with a lower Up.

Retrofit and upgrade projects. In existing installations where a Type 2 SPD is already present at the main board but lightning exposure has increased — for example, after a structural lightning protection system is added or a taller adjacent building is constructed — replacing the Type 2 module with a Type 1+2 combined SPD is a practical upgrade path that does not require additional panel space or a second installation location. The existing backup protector and wiring are typically retained, provided the combined module's terminal format and backup protection requirements are compatible.

Application Examples

The following scenarios illustrate where a combined SPD is commonly selected.

Commercial Building Main Distribution Board

In a mid-size commercial building with a single transformer and a main low-voltage distribution board, a Type 1+2 SPD is installed at the incoming section. The building's lightning protection system is bonded to the main earthing terminal, so partial lightning current can enter through the earthing conductor. The combined SPD diverts this current at the entry point. Downstream, Type 2 SPDs at sub-distribution boards protect sensitive loads such as IT equipment, HVAC controls, and LED drivers.

Outdoor Cabinet (Telecom or EV Charging)

Outdoor cabinets for telecom base stations, EV charging stations, or traffic control systems are exposed to direct lightning effects because they are typically installed in open areas. The cabinet's compact internal layout leaves little room for a two-stage cascade. A Class B+C SPD at the cabinet's incoming section provides both the partial lightning current rating and the induced surge protection in one module, protecting the power supply, control board, and communication interface within the enclosure.

Industrial Facility Main Panel

In an industrial facility with a dedicated substation, the main panel may receive partial lightning current through the supply cable or through the building's structural lightning protection. A combined SPD at the main panel handles this entry point, while separate Type 2 SPDs at motor control centers (MCCs) and automation panels provide coordinated downstream protection. The combined device simplifies the main panel layout and reduces the coordination study needed between the substation and the first protection stage.

Cascaded Protection Coordination

A Type 1+2 SPD does not replace the entire cascaded architecture — it replaces the first one or two stages at the main board. The concept of cascaded protection is defined by IEC 62305 through lightning protection zones (LPZ), which describe the progressive reduction of surge threat as one moves deeper into the installation.

In the zone model, LPZ 0 represents the exterior environment with direct lightning exposure. The boundary from LPZ 0 to LPZ 1 is where a Type 1 or Type 1+2 SPD is installed, reducing the threat from partial lightning current to induced surge levels. The boundary from LPZ 1 to LPZ 2 is where a Type 2 SPD further reduces the residual overvoltage to a level that sensitive equipment can tolerate.

A combined SPD positioned at the LPZ 0→1 boundary performs the energy reduction of the first two stages in one module. Where sensitive equipment is located far downstream, an additional Type 2 SPD at the LPZ 1→2 boundary is still recommended to achieve the lowest possible Up at the equipment terminals. The coordination between the combined device and the downstream Type 2 SPD follows the same decoupling principles as a conventional cascade: the upstream device handles the high-energy portion, and the downstream device limits the residual voltage.

The energy sharing between stages depends on the dynamic impedance characteristics of the two SPDs during conduction. When a surge arrives, the upstream combined device begins conducting at its breakdown voltage and diverts the bulk of the surge current to earth. The residual voltage wave continues toward the downstream Type 2 SPD, which clamps it to a lower level. The 10-meter conductor length between stages provides inductive decoupling that allows the two devices to share the surge energy without one stage absorbing an excessive portion. Where this length is available, the cascade performs as designed; where it is not, a decoupling inductor can substitute for the missing conductor length.

Key Selection Parameters

When specifying a Type 1+2 SPD, the parameters below should be reviewed alongside the standard Type 2 selection criteria.

The maximum continuous operating voltage (Uc) should be selected with the same margin as for a Type 2 device, accounting for the system's nominal voltage and temporary overvoltage exposure. The protection level (Up) of a combined SPD is typically comparable to that of a Type 2 module, which is an advantage over a pure Type 1 device that tends to have a higher Up due to the spark gap technology. This lower Up at the main board can reduce or eliminate the need for a second coordination stage in installations where the downstream equipment has a moderate impulse withstand rating.

The response time of a combined SPD is another parameter worth noting. Spark-gap-based Type 1 devices exhibit a finite ignition delay before conduction begins, whereas varistor-based Type 2 devices begin clamping almost instantaneously. A combined SPD that uses varistor technology for the Type 2 portion provides the fast response associated with Type 2 protection, while the high-energy components handle the longer-duration 10/350 μs impulse. The overall response is generally faster than a standalone spark-gap Type 1 device, which benefits the protection of sensitive downstream electronics.

The impulse current rating (Iimp) is the parameter that distinguishes a combined SPD from a standard Type 2 device. A typical value of 12.5 kA per pole is the minimum specified by IEC 61643-11 for Type 1 testing, and combined devices commonly meet or exceed this value. For installations with a high lightning exposure level, a higher Iimp may be selected based on the lightning protection level (LPL) defined in IEC 62305.

Frequently Asked Questions