Transformers classified as Class II or Class III are used for circuits classified as Class II or Class III, respectively, according to the National Electrical Code, ANSI/NFPA 70. The main purpose of these connectors is to connect sinusoidal power supplies. A transformer that is permanently connected is rated at 600 volts or less, and a transformer that is corded or plugged in is rated at 120 volts or less. The output current of an inherently limited Class II or Class III transformer is limited by an impedance within the transformer. A thermostat or other temperature-sensitive device may or may not be provided to limit its maximum temperature.
Transformers of Class II or Class III without inherent limitations do not have impedances that limit the maximum output current. However, the overcurrent protection device limits the maximum power output. Class II and Class III transformers with independent current-limiting impedances, such as resistors or devices with positive temperature coefficients, are subject to these provisions. Class II transformers have a maximum secondary potential of 30 volts rms, regardless of whether they are loaded or open circuited. These requirements apply to transformers of Class II or Class III that include resonant regulating circuits. There are no requirements for power supplies, toy transformers, direct plug-in transformers, or transformers for audio, radio, or television appliances.
In CLASSI, ll, the main focus is on the insulation system of the product, which is derived from the IEC system. The system is briefly explained as follows:
- CLASSI means that a product’s protection against electric shock is not only based on basic insulation, but also includes grounding.
- In CLASS I, electric shock protection is not solely based on basic insulation, but includes additional safety measures, such as double insulation or reinforced insulation, but no grounding or protection measures are dependent on installation conditions.
- As a CLASS III product, electric shock protection relies on the power supply voltage as a safety extra-low voltage (SELV), and dangerous voltages will not be generated.
Based on Canadian Electrical Code Part l/US Code, Class II considers current and energy. Circuit voltages can be divided into four ranges: 0-20 volts, 20-30 volts, 30-60 volts, and 60-150 volts. Each range has specific current and energy requirements. In product standards, CAN/CSA C22.2No.223 (ELV, Class II output power adapter), UL1310 and CAN/CSA C22.2No.950 (computer information products) and UL1950 have applications.
The following methods can be used to limit the circuit’s current to meet the requirements of Class II circuits:
- The transformer’s impedance is used to limit the current (after a short circuit of one minute, the current drops below the required value).
- Over current protection (using a fuse of the appropriate size) should be used to limit the current.
- Electronic circuit protection can be used to limit the current (both under normal and abnormal conditions)
