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June 2014

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From:
Robert Kondner <[log in to unmask]>
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Date:
Mon, 23 Jun 2014 19:21:37 -0400
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Curt,

 Thanks, yes I have been through these issues but a second read is always a good idea. Some of my docs are real old.

 The hazardous locations I am working in is a room filled with explosive vapors. One spark and it is a real problem!

 I need an isolation transformer that feeds power from one side to an isolated secondary. The coil needs to be physically small as there are limits to the power I can allow through during fault conditions. But small transformers make insulating difficult.  I am working with an EFD10 which is rather tiny, only 2 or 3 watts can ever go through that tiny magnetic circuit.

 Normally both transformers sides are at a common earth ground. But with a power system faults GND ain't GND anymore. I have not been able to locate docs on how to define this event so I am looking for as much side to side isolation as I can. The optos in the system are rated at 3750VAC so I figure I want to be a little better than the optos. 

 Using Quad Build wire I get over 6KV on each wire and a single layer of tape gets me 6KV. But I would like to establish a safety margin but I have no idea what is a reasonable safety margin number. Is it 10%, 100% or 500%? 

 I can measure to 6KV here, and I think that is plenty, but with a reasonable margin what should I spec?

Thanks,
Bob K.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Curt McNamara
Sent: Monday, June 23, 2014 5:35 PM
To: [log in to unmask]
Subject: Re: [TN] z-axis voltage separation

Maybe this will help, apologies if you know all of it!

A hazardous design (i.e. one that has high voltages in it) needs to meet the relevant safety standard. The appropriate standard is determined by the market and the end use of the design. For example, IT equipment typically has to meet 60950, while laboratory equipment may need to meet 61010. There are many many standards.

To get an approval, the safety agency will do an analysis of the design by examining the components, including the PCB. 

If a component (like an insulator, transformer, or fuse) has UL Recognition then that qualification can be used in the final design.

If a component (like a PCB) does not have UL recognition (and PCBs don't) then it needs to meet Creepage and clearance as defined in that standard.

If the component (like the PCB) does not meet creepage and clearance, then each individual instance will need to be hi-pot tested.

This is a reasonable overview of the material:
http://www.springer.com/cda/content/document/cda_downloaddocument/9781461410522-c1.pdf?SGWID=0-0-45-1298737-p174268563

		Curt

Curt McNamara, M. Eng. P.E.  //  principal electrical engineer | electrical engineering

Logic PD
411 Washington Ave. N. Suite 400
Minneapolis, MN 55401
T // 612.436-5178
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-----Original Message-----
From: Robert Kondner [mailto:[log in to unmask]]
Sent: Monday, June 23, 2014 4:06 PM
To: 'TechNet E-Mail Forum'; Curt McNamara
Subject: RE: [TN] z-axis voltage separation

Curt,

 But thickness matters too!  I recall the UL double insulated requirement is only about 7 mil thick insulation. Do I remember that correctly?

 I think they call this a dielectric withstanding rating.

 A quick look at my old UL 478 spec shows a 250 VAC must withstand 1250V. Now that you can propably do with a single layer of Scotch Tape. I would not consider that very safe. Even a single layer of transformer tape (polyester) does not strike me as "Safe". 

There are "Resistance Measurements" for items, they soak at specific humidity levels and then the resistance is checked. But this highest withstanding voltage I see at any level is only 3000V. That is all Class II, Class I is much less.

Summary: There really is no "Safety Margin" to speak of, we need only meet insulation tests and fairly low (3000V worse case) for UL 478 type items.  

I have been looking at some hazardous location specs which have their own clearances but withstanding voltages is all I have to go on for HV isolation in small power supplies. 

Anyone out there really know this subject?

Thanks,
Bob K

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Curt McNamara
Sent: Monday, June 23, 2014 4:08 PM
To: [log in to unmask]
Subject: Re: [TN] z-axis voltage separation

As others have noted, voltage isolation spacings are defined by Creepage and clearance requirements in the relevant safety standard. 3.5mm seems about right. If you move the traces far enough away in X or Y you should be able to meet the spacing requirements.

As Wayne notes, FR4 is not a consistent insulator, as its' properties vary from build to build (plus there could be copper in there!). 

A PCB could be designed, built, and pass a safety examination with spacing violations (since FR4 will insulate), however each PCB will then need to be hi-pot tested to meet the standard.

		Curt

Curt McNamara, M. Eng. P.E.  //  principal electrical engineer | electrical engineering

Logic PD
411 Washington Ave. N. Suite 400
Minneapolis, MN 55401
T // 612.436-5178
NOTICE: Important disclaimers and limitations apply to this email.
Please see this web page for our disclaimers and limitations:
http://logicpd.com/email-disclaimer/


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Monday, June 23, 2014 2:44 PM
To: [log in to unmask]
Subject: Re: [TN] z-axis voltage separation

Surface flash and arcing in air are pretty well understood, but there are large fudge factors for shape and humidity and contamination.

Breakover inside a dielectric which isn't fundamentally designed as a high voltage material and consists of a bunch of fibers with resin filler has a bunch of interesting failure modes, some of which take a long time to happen.

As has been suggested, you could get insignificant tiny arcs from fiber to fiber which would eventually result in a "tree-like" failure in the material.

If you need high voltage high reliability in a dielectric, the material has to be qualified via HAST in the usage condition.  Manufacturers do sell pre-qualified materials--FR4 isn't one of them.

So starting with surface flash criteria is one way to look at it, but I think that is pretty darn conservative on it's own.  I wouldn't add extra factors, and you could probably go higher stress than that safely.

But as I previously indicated, go with CAF resistant material and go extremely conservative if there's a plated through-hole near by.

And that's all you get for a nickel!

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Monday, June 23, 2014 1:49 PM
To: [log in to unmask]
Subject: Re: [TN] z-axis voltage separation

Ahne,

 You mentioned "I would put in a generous safety factor"

 I have been looking for a realistic "Safety Factor" for some small high isolation voltage coils. Any idea how "Safety Factors" are computed? Are safety factors just wild guesses for engineers to make up themselves?

Thanks,
Bob K.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ahne Oosterhof
Sent: Monday, June 23, 2014 12:54 PM
To: [log in to unmask]
Subject: Re: [TN] z-axis voltage separation

After a long search,
http://advancedmaterialscience.com/products/NEMA%20FR4.pdf
Dielectric strength:  550 volts/mil

An example (from a vendor).
Semi-Flex FR4, 2 Layers Prototype PCB FR4, 0.3mm Thick Board thickness: 0.3mm Electric strength: >1.3kv/mm

Dupont Pyralux: Dielectric Strength,  ASTM-D-149,  6-7 kV/mil

FR4 is not a homogenous material, so I expect that the dielectric strength for thinner layers will be lower than for thicker layers. And if you measure it, I would put in a generous safety factor.
Many vendors provide some kind of number for dielectric strength, but rarely add the complete description of and conditions under which the material was tested.

If your concern is rooted in safety you will end up testing it yourself for UL (or something like that) approval.

Have fun,
Ahne.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Olson
Sent: Friday, June 20, 2014 8:38 AM
To: [log in to unmask]
Subject: [TN] z-axis voltage separation

maybe I'm having a "not enough coffee yet" morning, but I was asked how much separation I need between layers for high voltage.
I tried to search the TechNet Archives, but it doesn't seem to work as well as it used to!
anyway,

We have a design that may have 1700V in several places.
Since we are looking at a clearance into the board, layer-to-layer I'm pretty sure I can use the "internal" column B1 of Table 6-1 in IPC-2221 (using Table
6-1 for z-axis was discussed in an IPC committee meeting and no one disagreed)

but the number I get for 1700V is =
(.25 mm for the first 500V) plus (.0025 mm for each of the other 1200V, 3
mm)
equals 3.25 mm

For one thing, it already seems like I'm off-track because .25 for 500V doesn't correspond very well with 3 mm for 1200V, but if you can't trust IPC..... well, let's not go there.

My REAL question is that, although I'm safe using 3.25 mm, my board is not that thick!
Is there a smaller z-axis clearance that can be used for 1700V? across typical
FR4 material?
(we are using a RoHS compatible 170Tg /126)

What's the MINIMUM layer spacing I can use for 1700V?

thanks,
Jack


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