725.121 Power Sources for Class 2 and Class 3 Circuits
Informational Note No. 1: Informational Note Figure 725.121 illustrates the relationships between Class 2 or Class 3 power sources, their supply, and the Class 2 or Class 3 circuits.
Informational Note Figure 725.121 Class 2 and Class 3 Circuits.
- A listed Class 2 or Class 3 transformer
- A listed Class 2 or Class 3 power supply
- Other listed equipment marked to identify the Class 2 or Class 3 power source
Exception No. 1 to (3): Thermocouples shall not require listing as a Class 2 power source.
Informational Note: Examples of other listed equipment are as follows:
- A circuit card listed for use as a Class 2 or Class 3 power source where used as part of a listed assembly
- A current-limiting impedance, listed for the purpose, or part of a listed product, used in conjunction with a nonpower-limited transformer or a stored energy source, for example, storage battery, to limit the output current
- A thermocouple
- Limited voltage/current or limited impedance secondary communications circuits of listed industrial control equipment
(4) Listed audio/video, information technology (computer), communications, and industrial equipment limited-power circuits
Informational Note: One way to determine applicable requirements for listing of information technology (computer) equipment is to refer to UL 60950-1-2011, Standard for Safety of Information Technology Equipment. Another way to determine applicable requirements for listing of audio/video, information technology, and communications equipment is to refer to UL 62368-1-2014, Safety of audio/video, information and communication technology equipment. Typically such circuits are used to interconnect data circuits for the purpose of exchanging information data. One way to determine applicable requirements for listing of industrial equipment is to refer to UL 61010-2-201, Safety requirements for electrical equipment for measurement, control, and laboratory use — Part 2—201: Particular requirements for control equipment, and/or UL 61800-5-1, Adjustable speed electrical power drive systems — Part 5—1: Safety requirements — Electrical, thermal and energy.
(5) A battery source or battery source system that is listed and identified as Class 2
Informational Note No. 1: Rated current for power sources covered in 725.144 is the output current per conductor the power source is designed to deliver to an operational load at normal operating conditions, as declared by the manufacturer.
Informational Note No. 2: An example of a label is "52V @ 0.433A, 57V MAX" for an IEEE 802.3 compliant Class 8 power source.