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The class 2 requirements used to be specified by the National Electrical Code. The situation is slightly different now in that Class 2 circuits are now defined in the Listing requirements, e.g. UL 1310, UL 60950, etc, with direction to the standards committees provided by the code. The tables, from Chapter 9 of the code are shown below, but their implementation in the product standards may vary slightly (please excuse the html format).
Tables 11(A) and 11(B)
For listing purposes, <javascript:pare
t.loadDoc('/nfpa0050-0099/0070/codes-55903-3',%20'',%20'codes-id00007017986')>
Table 11(A) and <javascript:parent.loa
Doc('/nfpa0050-0099/0070/codes-55903-3',%20'',%20'codes-id00007017987')> Table
11(B) provide the required power source limitations for Class 2 and Class 3
power sources. <javascript:parent.load
oc('/nfpa0050-0099/0070/codes-55903-3',%20'',%20'codes-id00007017986')> Table
11(A) applies for alternating-current sources, and
<javascript:parent.loadDoc('/nfpa0050-0
99/0070/codes-55903-3',%20'',%20'codes-id00007017987')> Table 11(B) applies
for direct-current sources.
The power for Class 2 and Class 3 circuits shall be either (1) inherently
limited, requiring no overcurrent protection, or (2) not inherently limited,
requiring a combination of power source and overcurrent protection. Power
sources designed for interconnection shall be listed for the purpose.
As part of the listing, the Class 2 or Class 3 power source shall be durably
marked where plainly visible to indicate the class of supply and its
electrical rating. A Class 2 power source not suitable for wet location use
shall be so marked.
Exception: Limited power circuits used by listed information technology
equipment.
Overcurrent devices, where required, shall be located at the point where the
conductor to be protected receives its supply and shall not be interchangeable
with devices of higher ratings. The overcurrent device shall be permitted as
an integral part of the power source.
Table 11(A) Class 2 and Class 3 Alternating-Current Power Source Limitations
Inherently Limited Power Source (Overcurrent
Protection Not Required)
Not Inherently Limited Power Source
(Overcurrent Protection Required)
Power Source
Class 2
Class 3
Class 2
Class 3
Source voltage
Vmax (volts)
(see Note 1)
0 through 20*
Over 20 and through 30*
Over 30 and through 150
Over 30 and through 100
0 through 20*
Over 20 and through 30*
Over 30 and through 100
Over 100 and through 150
Power limitations
VAmax (volt-amperes)
(see Note 1)
—
—
—
—
250 (see Note 3)
250
250
N.A.
Current limitations
Imax (amperes)
(see Note 1)
8.0
8.0
0.005
150/Vmax
1000/Vmax
1000/Vmax
1000/Vmax
1.0
Maximum overcurrent protection (amperes)
—
—
—
—
5.0
100/Vmax
100/ Vmax
1.0
Power source maximum nameplate rating
VA (volt- amperes)
5.0 Vmax
100
0.005 Vmax
100
5.0 Vmax
100
100
100
Current (amperes)
5.0
100/Vmax
0.005
100/Vmax
5.0
100/Vmax
100/Vmax
100/Vmax
*Voltage ranges shown are for sinusoidal ac in indoor locations or where wet
contact is not likely to occur.
For nonsinusoidal or wet contact conditions, see Note 2.
Table 11(B) Class 2 and Class 3 Direct-Current Power Source Limitations
Inherently Limited Power Source (Overcurrent
Protection Not Required)
Not Inherently Limited Power Source (Overcurrent
Protection Required)
Power Source
Class 2
Class 3
Class 2
Class 3
Source voltage
Vmax (volts)
(see Note 1)
0 through 20*
Over 20 and through 30*
Over 30 and through 60*
Over 60 and through 150
Over 60 and through 100
0 through 20*
Over 20 and through 60*
Over 60 and through 100
Over 100 and through 150
Power limitations
VAmax (volt-amperes)
(see Note 1)
—
—
—
—
—
250 (see Note 3)
250
250
N.A.
Current limitations
Imax (amperes)
(see Note 1)
8.0
8.0
150/Vmax
0.005
150/Vmax
1000/Vmax
1000/Vmax
1000/Vmax
1.0
Maximum overcurrent protection (amperes)
—
—
—
—
—
5.0
100/Vmax
100/Vmax
1.0
Power source maximum nameplate rating
VA (volt-amperes)
5.0 Vmax
100
100
0.005 Vmax
100
5.0 Vmax
100
100
100
Current (amperes)
5.0
100/Vmax
100/Vmax
0.005
100/Vmax
5.0
100/Vmax
100/Vmax
100/Vmax
*Voltage ranges shown are for continuous dc in indoor locations or where wet
contact is not likely to occur.
For interrupted dc or wet contact conditions, see Note 4.
Notes for Tables 11(A) and 11(B)
1.Vmax, Imax, and VAmax are determined with the current-limiting impedance in
the circuit (not bypassed) as follows:
Vmax: Maximum output voltage regardless of load with rated input applied.
Imax: Maximum output current under any noncapacitive load, including short
circuit, and with overcurrent protection bypassed if used. Where a transformer
limits the output current, Imax limits apply after 1 minute of operation.
Where a current-limiting impedance, listed for the purpose, or as part of a
listed product, is used in combination with a nonpower-limited transformer or
a stored energy source, e.g., storage battery, to limit the output current,
Imaxlimits apply after 5 seconds.
VAmax: Maximum volt-ampere output after 1 minute of operation regardless of
load and overcurrent protection bypassed if used.
2. For nonsinusoidal ac, Vmax shall not be greater than 42.4 volts peak. Where
wet contact (immersion not included) is likely to occur, Class 3 wiring
methods shall be used or Vmax shall not be greater than 15 volts for
sinusoidal ac and 21.2 volts peak for nonsinusoidal ac.
3. If the power source is a transformer, VAmax is 350 or less when Vmax is 15
or less.
4. For dc interrupted at a rate of 10 to 200 Hz, Vmax shall not be greater
than 24.8 volts peak. Where wet contact (immersion not included) is likely to
occur, Class 3 wiring methods shall be used, or Vmax shall not be greater than
30 volts for continuous dc; 12.4 volts peak for dc that is interrupted at a
rate of 10 to 200 Hz.
From: [email protected]
[mailto:[email protected]] On Behalf Of Pierre SELVA
Sent: Tuesday, February 11, 2003 5:25 AM
To: [email protected]
Subject: Class 2 circuit requirements - UL1310
Hello,
As I don't have the UL1310, I need some help.
Can somebody give me, very shortly, the main requirements wich define a class
2 circuit, knowing this should apply to a 24VDC power supply output ?
Many thanks,
Pierre Selva
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