Key Parameters and test standards of Surge Protective Devices (SPDs)

Surge Protective Device (SPD): A device used to limit transient overvoltages and divert surge currents. SPDs are primarily employed in low-voltage distribution systems and information systems to protect against lightning overvoltages, switching overvoltages, lightning electromagnetic pulses (LEMP), and electromagnetic interference (EMI) pulses. For high-voltage side protection against such overvoltages, surge arresters are used instead.  

I. Key Parameters of Surge Protective Devices (SPDs)  

1. Maximum Continuous Operating Voltage (Uc): The maximum root-mean-square (RMS) or DC voltage that can be continuously applied to the SPD’s protection mode. This is essentially the rated voltage of the SPD. The Uc value affects both the lifespan of the SPD and its voltage protection level. Selecting a higher Uc value may extend the product’s service life but will also increase the residual voltage, which is detrimental to the protected equipment.  

2. Nominal Discharge Current (In): The peak current of an 8/20μs current wave that the SPD can withstand. This parameter is used for Class II tests and also serves as a preconditioning criterion for Class I and II tests. SPD standards define a series of In values, and manufacturers select an appropriate In level for testing. Once the SPD passes the test, its In value is officially designated as the selected level.  

3. Maximum Discharge Current (Imax) in Class II Tests: The peak current of an 8/20μs current wave that the SPD can handle. While the definition is similar to In, SPD standards provide separate series for In and Imax, with Imax > In for the same class. Passing a test at a given In level does not guarantee that the SPD will pass at the corresponding Imax level, as the test conditions (current magnitude and number of impulses) differ.  

4. Impulse Current (Iimp): The SPD’s capability to withstand a 10/350μs lightning current wave, characterized by two parameters: peak current (Ipeak) and charge (Q). This parameter is used for Class I tests.  

5. Voltage Protection Level (Up): Also known as residual voltage, this refers to the voltage across the SPD when a specified waveform of peak current flows through it after activation. Up must be lower than the withstand voltage of the protected equipment. GB50343-2012 *Technical Code for Lightning Protection of Building Electronic Information Systems* specifies the impulse withstand voltage levels for different stages of low-voltage distribution systems. The equipment’s rated impulse withstand voltage (Uw), provided by the manufacturer, indicates its insulation capability against overvoltages. Therefore, SPD selection must ensure Up < Uw.  

II. Relevant Tests for Surge Protective Devices (SPDs)

The above sections mention Class I and Class II tests. Below are the test methodologies:  
1. Class I Test: Conducted using nominal discharge current (In), a 1.2/50μs voltage impulse, and maximum impulse current Iimp (10/350μs lightning current wave). If the charge transferred within 10 ms is Q, the peak current Ipeak = 0.5Q.  
2. Class II Test: Conducted using nominal discharge current (In), a 1.2/50μs voltage impulse, and maximum discharge current Imax (8/20μs current wave).  
3. Class III Test: Conducted using a combination wave, defined as a 1.2/50μs open-circuit voltage (Uoc) and an 8/20μs short-circuit current (Isc) generated by a 2Ω combination wave generator.  

These test categories are not hierarchical and thus cannot be directly compared. Manufacturers may choose any test class for validation. However, in building electrical design, the selection of SPDs tested under Class I or Class II must comply with:  

IEC 61024: Pertains to the safety protection of persons against lightning strikes in buildings, covering the selection of lightning protection system (LPS) protection levels.  

IEC 61312: Addresses protection against lightning electromagnetic pulses (LEMP), including requirements for building shielding, grounding, and equipotential bonding.  

IEC 62305: Provides guidelines for the design of lightning protection systems, consolidating general principles and requirements for lightning protection.