Electrical Engineering - Beta field trials with 120VAC non listed power supplies

Author

Beta field trials with 120VAC non listed power supplies

taistoi

2008-04-02, 8:25 pm

Any provision in NFPA 70 to allow for use of non-listed/recognized/
labelled 24dc power supply? presumably, it will be class 2 type.

We have a computer based product that will be powered by a 120vac-24dc
power supply that is currently being listed, however, our project
cannot wait the months it will take in order to begin deploying in the
field.

Thanks,
Jason

Gerald Newton

2008-04-03, 3:25 am

On Apr 2, 3:46 pm, taistoi <jasoncardi...@gmail.com> wrote:
> Any provision in NFPA 70 to allow for use of non-listed/recognized/
> labelled 24dc power supply? presumably, it will be class 2 type.
>
> We have a computer based product that will be powered by a 120vac-24dc
> power supply that is currently being listed, however, our project
> cannot wait the months it will take in order to begin deploying in the
> field.
>
> Thanks,
> Jason

Class 2 power supplies shall be listed and are readily available on
the market.
However, listed computers are given slack see (4) below.

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):

(A)(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).

Here is the whole section:
III. Class 2 and Class 3 Circuits
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):
FPN No. 1: Figure 725.41 illustrates the relationships between
Class 2 or Class 3 power sources, their supply, and
the Class 2 or Class 3 circuits.
FPN No. 2: Table 11(A) and Table 11(B) in Chapter 9
provide the requirements for listed Class 2 and Class 3
power sources.
(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).
FPN: Examples of other listed equipment are as follows:
(1) A circuit card listed for use as a Class 2 or Class 3
power source where used as part of a listed assembly
(2) A current-limiting impedance, listed for the purpose,
or part of a listed product, used in conjunction with a
non-power-limited transformer or a stored energy source,
for example, storage battery, to limit the output current
(3) A thermocouple
(4) Limited voltage/current or limited impedance secondary
communications circuits of listed industrial control
equipment
(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.
(B) Interconnection of Power Sources. Class 2 or Class 3
power sources shall not have the output connections paralleled
or otherwise interconnected unless listed for such interconnection.

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