Power supplies class 2, class 3

[ryan_618]

I am having a difficult time understanding section 725.41

725.41 Power Sources for Class 2 and Class 3 Circuits.
(A) Power Source. The power source for a Class 2 or a Class 3 circuit shall be as specified in 725.41(A)(1), (A)(2), (A)(3), (A)(4), or (A)(5):

(1) A listed Class 2 or Class 3 transformer
(2) A listed Class 2 or Class 3 power supply
(3) Other listed equipment marked to identify the Class 2 or Class 3 power source
Exception No. 1: Thermocouples shall not require listing as a Class 2 power source.
Exception No. 2: Limited power circuits of listed equipment where these circuits have energy levels rated at or below the limits established in Chapter 9, Table 11(A) and Table 11(B).
(4) Listed information technology (computer) equipment limited power circuits.
FPN: One way to determine applicable requirements for listing of information technology (computer) equipment is to refer to UL 1950-1995, Standard for Safety of Information Technology Equipment, Including Electrical Business Equipment. Typically such circuits are used to interconnect information technology equipment for the purpose of exchanging information (data).
(5) A dry cell battery shall be considered an inherently limited Class 2 power source, provided the voltage is 30 volts or less and the capacity is equal to or less than that available from series connected No. 6 carbon zinc cells.

I always figured you could supply a class 3 circuit with a class 2 power supply (and call it a class 2 circuit), but I don't think you can supply a class 2 circuit with a class 3 power supply. Class 3 circuits are considered to be safe from a fire perspective, but not from an electric shock perspective. Class 2 circuits, however, are safe from both.
What this section seems to imply is that I could use class 2 cables and a class 3 power supply, which in my opinion is not safe.

Does this section confuse anyone else? I still think you must have class 3 cable if you are using a class 3 power supply, not class 2 calbe and a class 3 power supply.

Any thoughts?
Thanks,

 

[hbiss]

Re: Power supplies class 2, class 3

What this section seems to imply is that I could use class 2 cables and a class 3 power supply...

I'm not reading that at all, and of course you are right. They are lumping class 2 and class 3 into one section and that's what is confusing.

How about:

The power source for a Class 2 circuit shall be as specified in 725.41(A)(1),(A)(2),(A)(3),(A)(4), or (A)(5):

(1) A listed Class 2 transformer
(2) A listed Class 2 power supply
(3) Other listed equipment marked to identify the Class 2 power source.

The power source for a Class 3 circuit shall be as specified in 725.41(A)(1), (A)(2), (A)(3) or (A)(4):

(1) A listed Class 3 transformer
(2) A listed Class 3 power supply
(3) Other listed equipment marked to identify the Class 3 power source

Oh, and yes you can use CL3 wiring with a CL2 source but not the other way around. A CL2 source with CL3 wiring is still a CL2 circuit. A CL3 source with CL2 wiring is a violation.

-Hal

[ July 04, 2005, 09:19 PM: Message edited by: hbiss ]

 

[ryan_618]

Re: Power supplies class 2, class 3

Thanks Hal, I'm glad you see the way I am reading it. I agree with you on what it should say and what it is supposed to say, I just wish it were more clear. i think a proposal is in order for this