UL Class 2 Circuits

Also - depending on your load size (and how many), it is sometimes cheaper to just get an individual power supply for a couple of units.

Even Allen Bradley & Siemens 1/2A power supplies are usually $50 or less. (usually instruments, HMIs, ethernet switches, etc. fall in this category)
 
OK super dumb question that I should have asked to start with.

When is a Class 2 Circuit required?




Side Note: How is a fuse not "Power Limiting" :unsure: I feel like I am missing something.


Edit:

Because it doesn't also limit Voltage? meaning it won't protect against voltage spikes?
 
Last edited:
OK super dumb question that I should have asked to start with.

When is a Class 2 Circuit required?




Side Note: How is a fuse not "Power Limiting" :unsure: I feel like I am missing something.


Edit:

Because it doesn't also limit Voltage? meaning it won't protect against voltage spikes?

Forgive me if I'm not getting everything right - someone else can correct me.

I believe a fuse does not count because it doesn't actually limit the current. It simply opens the circuit if the current is over a certain point - which it will be for several hundredths of a second (or seconds) before the element melts. A Class 2 circuit won't go over 100W. It is a form of fire protection.

Typically, class 2 circuits are stated as required by the device's installation instructions. I've most commonly seen them on PLCs, HMIs, and most instruments (flow meters, pressure transmitters, etc.)
 
?
Side Note: How is a fuse not "Power Limiting" :unsure: I feel like I am missing something.

In this case the answer is simple but not satisfying. If the code doesn’t let you use a fuse to make a class 2 circuit then you can’t do it.

Here’s another reason someone would want a class 2 circuit: “ Components and internal wiring located entirely within a Class 2 circuit are not required to be
investigated”

That means you can put anything in there whether or not it has a listing.
 
In this case the answer is simple but not satisfying. If the code doesn’t let you use a fuse to make a class 2 circuit then you can’t do it.

Here’s another reason someone would want a class 2 circuit: “ Components and internal wiring located entirely within a Class 2 circuit are not required to be
investigated”

That means you can put anything in there whether or not it has a listing.

Yeah - forgot this is a great one when you need UL / CSA certification. Just use a Class 2 power supply and life is good.
 
And the above is wrong. Class 2 is limited to 100 ma. Either a class 2 power supply, or class 2 converter. Something like a 1692 ELP.

Fusing doesn’t cut it per UL.

And yes, I often have a class 2 power supply in addition to say a 240W power supply.

Cheers!
Yeah I'll admit it didn't sound like the correct answer to me, but in situations like that, what the UL inspector says is always "right". 🍺
 
The real issue is in the proof. The section of the electrical code that talks about what can be a Class 2 circuit leads with what can act as a Class 2 power source.
Most of those options are listed devices. If you choose to use a fuse to create a Class 2 circuit the burden of proof is on you to make sure your design fulfills the requirements of a Class 2 circuit. Since you aren't using devices that are already tested to Class 2 requirements that means YOU need to have the testing done, likely by an NRTL.
This is a general truth with UL. You can choose to go off on your own and you may well have correctly engineered a solution that meets the standards. However, the burden of proof at that point shifts to you, not the component manufacturer. That will typically turn into a dollar sign and lots of zeros pretty quickly.

Keith
 

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