AC mains circuit : safety related question

[Kepco]

Hi Folks,
In the attached application note figure 12 - Page 7, is there a need to select a safety approved (UL) resistor Rx/2 for any type of safety certification purposes? The IC HCPL-3700 is already providing safety rated optical isolation between 110/220V mains input and 5V logic output. Any thoughts?
Thanks

 

[DRWeig]

Are you building a power monitor? More of your specific application, please. I can't tell which UL standard might be right for this product.

In general though, UL is about fire and personnel safety, not just protection of the low voltage side from the mains side. I will bet that you are going to need fuses (of type specified in the UL standard) on the mains side of those resistors and possibly resistors with a short circuit current rating.

Best to you,

Goober Dave

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[Kepco]

Thanks for the response.
Yes, this is a power monitoring circuit interfaced to a micro to monitor the AC input. You are correct that UL is about fire and safety and therefore thought of isolation between primary and secondary circuits might be important. A user might come in contact with secondary circuit in a computing (VME,CPCI etc) chassis type environment. The UL rated short circuit current resistors are not available in all values (e.g. 19.1K,1%,1/2W and 36K,1%,1W). This is the reason this question came up. It is great you pointed out, the UL rated fuses before the resistors is already selected in the actual circuit. UL-60950 is the primary target for this application.

 

[Kepco]

The attached example circuit from UL safety rated fuse resistor app note shows UL rated fuse and a regular resistor ( I believe) and replaced with one UL rated fuse/resistor as shown. Is this assumption correct that a regular resistor with UL fuse will be suffice.

 

[DRWeig]

Sounds fun!

Unfortunately, 60950 is not one of the UL standards with which I am familiar. The interrupting rating may not be applicable.

There should be some table in the standard that discusses mains-voltage equipment and category code that must be satisfied for each piece. You have a copy of the standard, correct?

In general, safety-rated resistors may either eliminate the need for upstream protection or reduce the requirements of upstream components. You may not need them if the upstream protection is adequate. Hang on and see who else answers.

You might also consider posting a link to this thread in the UL forum (

http://www.eng-tips.com/threadminder.cfm?pid=244).

Good luck with it!



Best to you,

Goober Dave

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[DRWeig]

We were typing at the same time.

The TT application note goes along with part of my last post:

"In general, safety-rated resistors may either eliminate the need for upstream protection or reduce the requirements of upstream components. You may not need them if the upstream protection is adequate."

Again, I'm not sure if 60950 would have any different requirements. Give it a look-see.

Best to you,

Goober Dave

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[Kepco]

Thanks Dave.
I will try going through 60950 again to see I find something directly related. I have posted this thread link on UL forum also.

 

[Noway2]

Coming from the experience of having been a design engineer on UL-60950 equipment, predominantly static transfer switches, I can't say that I've ever encountered a requirement to use UL safety resistors on the line voltage sensing circuits. The UL spec does state insulation requirements to live parts. I can't recall the exact requirement but the term "double insulation" does come to mind from the last training class I attended on this subject, along with the qualification that sufficient air gap is equal to a layer of insulation.

 

[Reeq]

I agreed with Noway2