The UL 1449 standard for surge protective devices (SPDs) in North America differs significantly from the internationally accepted IEC 61643-11 standard in terms of parameter requirements and testing concepts. This profoundly reflects the different emphases on electrical safety and surge protection in different regions. These differences are primarily reflected in core test waveforms, key performance parameters, classification methods, and safety requirements.

In terms of core test waveforms, the UL 1449 standard primarily uses a combination waveform, which combines specific open-circuit voltage (1.2/50μs) and short-circuit current (8/20μs) characteristics. While the open-circuit voltage is applied during testing, the actual current the SPD withstands depends on its own characteristics. In contrast, the IEC 61643-11 standard clearly distinguishes between current waves (8/20μs) and voltage waves (1.2/50μs), testing the SPD's current discharge capacity and voltage limiting characteristics, respectively. For Class I SPDs designed for direct lightning protection, the more energetic 10/350μs impulse current wave (Iimp) is also introduced.

The definition and emphasis of key performance parameters also differ significantly. The core voltage parameter of UL 1449 is the voltage protection rating (VPR), which is the peak clamping voltage measured under specified combination wave test conditions (e.g., the prospective current corresponding to a 6kV open-circuit voltage) and is a key factor in selection. Regarding current capability, UL focuses more on the maximum discharge current (Imax), which is the peak current that the SPD can withstand a single maximum impulse without failure under an 8/20μs waveform, representing its ultimate handling capability. The concept of the nominal discharge current (In) was introduced later and is generally smaller, representing its multiple withstand capability. The core voltage parameter of IEC 61643-11 is the voltage protection level (Up), which is the maximum clamping voltage measured at the nominal discharge current (In) or a specific measurement current and must be lower than the insulation withstand voltage (Uw) of the protected equipment. In terms of current capability, the core of the IEC standard is the nominal discharge current (In), defined as the peak current at which the SPD can withstand 15 8/20μs current surges without degradation. This represents its ability to withstand multiple typical surges. The maximum discharge current (Imax) represents its ultimate withstand capability for a single or very few surges. It must be emphasized that due to fundamental differences in test waveforms and conditions, UL's VPR and IEC's Up, as well as UL's Imax/In and IEC's Imax/In, even if the values are the same, do not represent equivalent actual performance and cannot be directly compared.

Classification logic also reflects differences in application scenarios. UL 1449 categorizes SPDs into Types 1, 2, 3, and 4 based on mounting location and built-in disconnect mechanism. IEC 61643-11 categorizes SPDs into Class (I, II, and III) based on test waveforms and application locations: Class I for direct lightning protection (test waveform 10/350μs), Class II for typical surge protection (test waveform 8/20μs), and Class III for protection near equipment, using a combination wave test.

Safety requirements are one of the most fundamental differences between the two standards. The core safety requirement of UL 1449 is the short-circuit current rating (SCCR). This mandates that the SPD clearly indicate the maximum prospective fault current (AC RMS) that its associated external overcurrent protection device (such as a fuse or circuit breaker) can safely interrupt in the event of an internal short-circuit fault (such as an MOV failure). The UL standard places great emphasis on fail-safe operation, particularly for MOV-type SPDs, and mandates the use of a thermal trip device to ensure timely disconnection in the event of degradation or overload, preventing the risk of fire.And italso includes rigorous withstand testing for abnormal overvoltages (such as TOVs). The core safety requirement of IEC 61643-11 focuses on the follow-through current interrupting capability (Ifi), particularly for switching SPDs (such as spark gaps), anddefines the SPD's ability to safely interrupt the subsequent power-frequency follow-through current of the power grid after conducting a large surge. This capability must be compatible with the grid parameters (voltage and prospective short-circuit current) at the installation point. The IEC standard also addresses the failure mode (requiring a statement of short circuit or open circuit), but does not mandate that all MOV SPDs have a built-in thermal trip as UL does.

In summary, the UL 1449 standard prioritizes system electrical safety. Its parameters (such as VPR, SCCR) and tests (failure modes, TOV) are closely designed to ensure that SPD failure does not pose a fire or electric shock risk, and rely on the coordination of external protective devices. The IEC 61643-11 standard, on the other hand, focuses more on performance grading and application adaptation, defining the SPD's protection level and applicable scope through Class classification and key parameters (In, Up, Iimp). Its core safety requirements (Ifi) primarily address the risk of power frequency continuous current after operation for specific technologies (switching SPDs). These two systems are rooted in different engineering practices and safety regulations. Therefore, products entering different markets must meet corresponding mandatory certification requirements (e.g., UL certification is required in North America, and CE certification, including IEC standards, is required in Europe). Understanding these differences is crucial for the correct selection, installation, and compliance of SPDs.