Electronic information equipment is commonly installed in buildings. During the summer, lightning storms become frequent, requiring not only external lightning protection devices like lightning rods but also internal surge protection devices (SPDs) to safeguard electrical equipment within the building. Due to a general lack of awareness about electrical protection, external devices play a crucial role. Selecting the right SPD in electrical design for civilian buildings is especially important, as ensuring its performance meets requirements has become a focal point of attention. In electrical design for buildings, SPDs are essential for preventing overvoltage and diverting lightning currents. Let’s dive into what an SPD is, its parameters, and its related tests.

What is an SPD, and What Does it Do?

A Surge Protection Device (SPD) is a component designed to limit transient overvoltages and divert surge currents.
SPDs are mainly used in low-voltage distribution systems and information systems to protect against lightning overvoltage, switching overvoltage, lightning electromagnetic pulses, and electromagnetic interference pulses. For overvoltage protection on the high-voltage side, lightning arresters are used.

Key Parameters of SPD

Maximum Continuous Operating Voltage (Uc)

The maximum RMS or DC voltage that can continuously be applied to the SPD in its protection mode. This is effectively the SPD’s rated voltage.

The Uc value affects the lifespan and voltage protection level of the SPD. Selecting a higher Uc can extend the SPD’s lifespan but increases residual voltage, which may not be ideal for the protected equipment.

Nominal Discharge Current (In)

The peak current of an 8/20 μs current wave passing through the SPD.

This parameter is used in type II testing and also for preconditioning SPDs during type I and II tests. In standards, specific In values are defined, and an SPD must meet its targeted In value during testing to pass.

Maximum Discharge Current (Imax)

The peak current of an 8/20 μs current wave passing through the SPD during type II testing.

Although similar to In in definition, Imax is typically higher than In for a given SPD level. Passing an In-level test does not guarantee the SPD will pass at the Imax level, as the waveforms and test cycles differ.

Impulse Current (Iimp)

The ability to withstand a 10/350 μs lightning current wave, including two parameters: peak current (Ipeak) and charge (Q). This parameter is used for type I testing.

Voltage Protection Level (Up)

Also known as residual voltage, this is the voltage across the SPD terminals when a specific current waveform is applied.

Up is critical for protecting equipment’s impulse withstand voltage and must be lower than the equipment’s impulse withstand rating.

The standard GB 50343-2012 specifies the impulse withstand voltage levels for different low-voltage systems as follows:

For SPD selection, Up must satisfy Up < Uw.

SPD Testing Procedures

SPD testing includes three main types based on the intended protection level:

Type I Test

Conducted using nominal discharge current (In), 1.2/50 μs impulse voltage, and maximum impulse current (Iimp) with a 10/350 μs lightning current wave.

If the total charge passed within 10ms is Q, then the peak current Ipeak = 0.5Q.

Type II Test

Conducted using nominal discharge current (In), 1.2/50 μs impulse voltage, and maximum discharge current (Imax) with an 8/20 μs current wave.

Type III Test

Conducted using a combined wave test. A combination wave is defined as an open-circuit voltage (Uoc, 1.2/50 μs) and a short-circuit current (Isc, 8/20 μs) generated by a 2Ω combination wave generator.

Test Standards and Selection Criteria

While different tests have distinct purposes and are not ranked hierarchically, the appropriate test should be selected based on design specifications. Standards such as GB 50057-2010 (Code for Design Protection Against Lightning) and GB 50343-2012 (Technical Code for Protection of Electronic Information Systems in Buildings Against Lightning) outline whether type I or type II SPDs should be used.

In building electrical design, aligning SPD selection and testing with these codes ensures reliable protection against overvoltage and lightning-induced surges.