Non-Isolated Electronic tranformers w/UL approval? 2

[taharvey]

I’d like to get UL approval on the device. Electronic transformers typically empoly a small transformer for isolation (smaller than typical transformer due to high freq operation). However in this application where 120VAC is an output option, the transformer would be 1:1… and thus be seemingly of dubious value for isolation.

It is not clear when UL requires isolation. There are lots of non-isolated AC-DC converters that are UL approved. I also noticed that there are new rules that allow non-isolated PV inverters that are high-voltage DC-AC connected the mains (high stakes consqeunces).

So the basic questions are:
1. Under what circumstances is non-isolation acceptable to UL?
2. What saftey measures would be required for this class of control device?

 

[VEBill]

...However in this application where 120VAC is an output option, the transformer would be 1:1... and thus be seemingly of dubious value for isolation. ...

Turns ratio is not the same thing as 'isolation'.

A transformer might have a secondary output voltage of 12 volts, or 120 volts, or even 1200 volts, and will still provide "isolation" (in the sense of neutral/ground isolation, depending how it's connected); and it (and the product incorporating it) could also be approved - provided they met the applicable requirements.

I'm not a subject matter expert on UL, but I assume that the requirement will vary with voltage level.

 

[LionelHutz]

UL is for safety. No-one should get shocked by this device. If isolation is required to protect against shocks then you need it. If you could ground the device case or "double insulate" then you could get away without it.

 

[DRWeig]

What Lionel said is correct. We can't be specific since we don't know what your device is, and therefore which UL standard you'll be designing/testing to.



Good on ya,

Goober Dave

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[taharvey]

Show how part of my question got cut off, here it is in full:

I’m looking at a project to make a electronic tranformer for use within a product, and driving its teathered external devices for a control system. I want to make the output AC voltage configurable between 24VAC and 120VAC. This can easily be done with PWM of the AC signal and a feedback path. In its simplest form it is just back-to-back MOSFETs in a AC switch with a little filtering, and the PWM controls.

I’d like to get UL approval on the device. Electronic transformers typically empoly a small transformer for isolation (smaller than typical transformer due to high freq operation). However in this application where 120VAC is an output option, the transformer would be 1:1… and thus be seemingly of dubious value for isolation.

It is not clear when UL requires isolation. There are lots of non-isolated AC-DC converters that are UL approved. I also noticed that there are new rules that allow non-isolated PV inverters that are high-voltage DC-AC connected the mains (high stakes consqeunces).

So the basic questions are:
1. Under what circumstances is non-isolation acceptable to UL?
2. What safty measure would be required for this class of control device?

 

[DRWeig]

Again, I'll refer you to Lionel's answer. The circuit leaving your device through a cord will have to have overcurrent and short circuit protection (fire safety). It will also have to be designed for shock safety, either through grounding, isolation, or insulation.

If you're subimitting to UL, get a copy of the standard against which it will be tested and evaluated. The rules will be in there. You'll often find that you get to choose which method to employ, it can be an either/or proposition.

Good on ya,

Goober Dave

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[waross]

However in this application where 120VAC is an output option, the transformer would be 1:1... and thus be seemingly of dubious value for isolation.

Scary statement from a designer..
If you don't understand this then you may be dangerous designing such devices. A basic understanding of the reasons for and the methods employed to effect isolation would seem to be lacking here to an alarming degree.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

UL requires an isolation (Class 2) transformer when you are building something and are NOT going to test the entire assembly as a unit. If you are going to test the entire assembly as a unit, and take on ALL of the protection requirements UL will insist upon, then the transformer component may or may not need to be Class 2, it's up to you. What you will likely find however is that unless you are planning on making a bazillion of these things and can amoritize the higher expense of meeting UL's safety requirements over a large production run, it will likely be less expensive to just use an already approved Class 2 transformer. UL isn't going to tell you how to design it, but there is a reason why most electronic devices use a separate Class 2 transformer in their power supplies.

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[taharvey]

@jraef Thanks for a thoughtful post.

So I understand your post, but what about when the external devices are already UL approved. For example no isolation is required when powering a 120 VAC UL approved motor through a mechanical relay - it is as if the motor was plugged into the wall, which it is already approved for. True?

So what about the 24VAC mode? If it is again plugged into a UL approved 24VAC motor (for example) is there still a need for isolation?

 

[LionelHutz]

It doesn't sound like isolation is required but no-one can give a complete answer to an incomplete question.

Power electronics are often not isolated.

 

[MiketheEngineer]

If you want UL - be prepared to spend a ton of money!!