Understanding the Differences Between CLASS I, II, III, and CLASS 2 Power Supplies

Different electrical engineering and safety standards classifications help identify and categorize power supply systems based on their insulation and protective measures. The distinctions between CLASS I, II, III, and, specifically, CLASS 2 power supplies play a crucial role in ensuring the safety and functionality of various electronic products.

CLASS I, II, III Overview:

The CLASS I, II, and III classifications primarily revolve around the insulation systems of products, following the IEC (International Electrotechnical Commission) framework. Here’s a brief breakdown:

CLASS I: This classification indicates that a product’s protection against electric shock relies on essential insulation and a grounding method. CLASS I power supplies typically use three-prong plugs.

CLASS II: In this case, protection against electric shock relies on basic insulation, with additional safety measures such as double or reinforced insulation. Notably, there needs to be grounding, and protection depends on installation conditions. CLASS II power supplies typically use two-prong plugs.

CLASS III: Products falling under CLASS III rely on a safety extra-low voltage (SELV) to protect against electric shock, ensuring no hazardous voltage is present.

IEC Insulation Levels:

The CLASS 2 classification considers current and energy levels from the Canadian Electrical Code Part I and U.S. regulations. The circuit voltage is divided into four ranges: 0-20 V, 20-30 V, 30-60 V, and 60-150 V. Each range has specific requirements for current and energy. CLASS 2 circuits, once identified, may have reduced accessory requirements, contributing to cost savings. Standards like CAN/CSA C22.2 No.223 (ELV, CLASS 2 Output Power Adapters), UL1310, CAN/CSA C22.2 No.950 (Computer Information Technology Equipment), and UL1950 apply to CLASS 2 circuits.

To meet CLASS 2 circuit requirements, limiting the circuit’s current can be achieved by using the transformer’s impedance, employing overcurrent protection devices like fuses, or utilizing electronic circuit protection to limit the current automatically.

Differences Between CLASS 1 and CLASS 2 Power Supplies:


CLASS 1 power supplies feature a 3-pin AC plug.

CLASS 2 power supplies feature a 2-pin AC plug.

Leakage Current:

CLASS 1: 0.75mA (Max)

CLASS 2: 0.25mA (Max)

HI-POT Testing:

CLASS 1: 1500VAC 60SEC 10mA

CLASS 2: 3000VAC 60SEC 10mA


CLASS 1 power supplies lack the “double insulation” symbol.

CLASS 2 power supplies feature the “double insulation” symbol.

Protective Ground Resistance Testing:

CLASS 1 power supplies have requirements for protective ground resistance testing.

CLASS 2 power supplies have no such requirements.

In conclusion, understanding the distinctions between CLASS I, II, III, and CLASS 2 power supplies is essential for engineers and manufacturers to ensure compliance with safety standards and create products prioritizing user safety. These classifications provide a framework for designing and evaluating electrical systems’ insulation and protective measures.


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