So your environment is pretty much the same as the engine compartment of a
motorboat powered with gasoline.
If you look at how they design the alternator for that environment, they just
assume it's going to spark, and put grounded screens/cages around any place
that could be susceptible to sparking. Strangely, things seem to be OK with
the exposed spark plug wires, although there are several boat explosions due
to this problem every year--it's not like the FAA accident investigation, even
though at least a few people die due to this every year.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Monday, June 23, 2014 7:22 PM
To: [log in to unmask]
Subject: Re: [TN] z-axis voltage separation
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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Please see this web page for our disclaimers and limitations:
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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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