Re: [SI-LIST] : Shielding Effectivness Question

[jrbarnes]

Neven,
To analyze the noise picked up by a wire that crosses a hole/slot in a shield,
look at:

[831] Lin, Guoxian, "Electromagnetic Excitation of a Wire Crossing a Long Slot
in an Infinite Plane," Electromagnetic Compatibility 1989, Zurich,
Switzerland, Mar. 7-9, 1989, pp. 89-92.

[938] Nakano, Hisamatsu, Yamauchi, Junji, Eda, Masahiro, and Iwasaki, Takeshi,
"Numerical Analysis of Electromagnetic Couplings Between Wires and Slots Using
Integral Equations," 4th International Conference on Antennas and Propagation
(ICAP 85), Coventry, UK, Apr. 16-19, 1985, pp. 438-442.

[966] Parmantier, J. P., and Aparicio, J. P., "Electromagnetic Topology:
Coupling of Two Wires Through an Aperture," Electromagnetic Compatibility
1991, Zurich, Switzerland, Mar. 12-14, 1991, pp. 595-600.

[1211] Taylor, Clayborne D., Marcum, Frank, Prather, William D., and Herrmann,
Carl C., "On Using a Sense Wire to Quantitate the Magnetic Flux Leakage Through
an Aperture in an Electromagnetic Shield," IEEE Transactions on
Electromagnetic Compatibility, vol. 31 no. 4, pp. 337-341, Nov. 1989.

My article on "Designing Electronic Equipment for ESD-Immunity" is being edited
by Printed Circuit Design magazine, and
should appear on their webpage http://www.pcdmag.com/ pretty soon now.
   Just last week I received several E-mails
from Andy Shaughnessy asking for clarification of points in my article.   My
list of references, which is supposed to be
posted with the article, covers 70-some books and booklets, and about 1300
pertinent standards/reports/papers/articles
gleaned from engineering and physics publications going back to the mid-1970's.


  John Barnes  Advisory Engineer
  Lexmark International





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RE: Shielding Effectivness Question

[George Tang]

RE: Product Safety: A Matter of Law or Litigation?Look for a book on RF or
microwave.  My favorite is Fields And Waves In Communication Electronics.
Look in the section on "waveguides."  Operate the waveguide below "cutoff."
A small aperture in a sheet metal with finite thickness is essentially a
very short waveguide.  Calculate the attenuation of a waveguide operating
below cutoff.  The radiated emission from one aperture may seem
insignificant, but when you have 100 apertures radiating in phase may cause
you to fail FCC A.

George Tang

  -Original Message-
  From: owner-emc-p...@majordomo.ieee.org
[mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Neven Pischl
  Sent: Tuesday, June 05, 2001 9:16 AM
  To: emc-p...@majordomo.ieee.org; si-l...@silab.eng.sun.com
  Subject: Shielding Effectivness Question


  I would appreciate if anyone could let me know if there are any references
(books, application notes, anythig ..) that deal with shielding efectivness
in cases when a source is close to an (electrically small) opening in a
shield (enclosure). In such a situation, the field will penetrate through
the hole and leak even if the size is much smaller than the wavelength. I am
particularly interested in situation when high-frequency source, such as a
PCB edge or a component operating at (say) 1 GHz and above is in proximity
of the venting holes, "small" gaps in the chassis etc.

  All references that I have deal with uniform plane wave propagating
incident to a metal plane with a slot or hole, in which case it is enought o
have electrically small size of the opening (e.g. lambda/10) to efficiently
block any field propagation through the barrier. I can't find any useful
reference that deals in any analytical way with the situation I am intersted
in.

  I believe I might get some answers using some of the simulation programs,
but at the moment I am more intersted in the analysis of the problem than in
simulating it.

  Thank you,

  Neven Pischl

Re: Product Safety: A Matter of Law or Litigation?

[Doug McKean]

I don't mean to speak for John, but I think he means 

The *scope of a directive* is to indicate WHAT to test. 

The *scope of a standard* is to indicate HOW to test. 

- Doug 



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Re: [SI-LIST] : Shielding Effectivness Question

[Doug McKean]

RE: Product Safety: A Matter of Law or Litigation?Neven, 

I did some rather intensive analysis of shielding effectiveness 
over the course of several weeks a while back.  Results were 
extremely informative. 

I've read a couple of fields and waves books for slot analysis. 
Rau is the one I prefer.  I'm at work, but I'll get the name of 
it and the ISBN as well. 

I've done some experiments with "small" stubs and loops with 
reference to wavelength.  And I was surprised to find a rather 
predictable approx -10dB rolloff per several decades from lo 
to high frequencies. Results were debatable. 

Converting the wire stubs over to slots by way of Babinet 
makes me think that the roll-off for slots is similar.  And with 
this information, I started my little analysis in the chamber 
on products. 

I haven't seen nor am I really worried about problems with 
holes.  Unless of course they're big.  Slots are your worst 
violator.  This is because of the mechanism by which apertures 
work.  It's, in my opinion, more difficult to impose a voltage 
differential across a round hole than it is across a slot. 

Slots are more directional than wires by a factor of roughly 10.  
That makes your product particularly susceptible during emissions. 

Anywho, I'll get you the name of the book and specifics for ya. 
And if you want, maybe you can pop over here to do some testing. 

- Doug McKean 



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Re: Shielding Effectivness Question

[Hans Mellberg]

Neven;

In the near field one must know whether the offending emmiter is mostly 
generated
from a magnetic loop or electric field antenna structure. Once that is known, 
the
eventual transmitted portion can be computed. If your shielding is non Fe, Ni 
or Co
and the source is of a magnetic loop nature then there will be minimal 
attenuation.
If the source resembles an electric field antenna (less likely at lower 
frequencies)
then the usual shielding formulae may apply since they are primarily based on
E-fields to some extent and you will have attenuation.  




=
Best Regards
Hans Mellberg
Regulatory Compliance & EMC Design Services Consultant
By the Pacific Coast next to Silicon Valley,
Santa Cruz, CA, USA
408-507-9694

__
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Re: [SI-LIST] : Shielding Effectivness Question

[Rob Hinz]

Neven,

The phenomenon you describe is pretty well understood and covered in the 
literature. What follows is from memory but is essentially the idea, so 
check the literature! Basically what you have, with a small opening, is a 
waveguide operating below its cutoff frequency. While it is true that power 
incident on the input port of such a waveguide does not propagate through 
it, in the sense of a propagating mode, electromagnetic fields will exist 
within the guide and will decay exponentially along the length of the 
guide. These are referred to as cutoff or evanescent modes. Unfortunately, 
when the evanescent mode reaches the other end of the waveguide, it will 
propagate in the space beyond, albeit attenuated significantly. Propagation 
through a waveguide is proportional to the complex exponential e^(-jBz). In 
a waveguide operating below it's cutoff frequency, beta (B), the 
propagation constant becomes negative-imaginary, B=-jA. The result is an 
exponential decay of the field strength as e^(-Az). The amount of 
attenuation is then dictated by the position within the guide, z, as 
measured from the input, and the value of A.


Beta, B is the propagation constant and is computed for rectangular wave 
guide as:


B = sqrt(k^2 -kc^2)
k = 2*pi*freq*sqrt(ue)
kc = sqrt( (m*pi/a)^2 + (n*pi/b)^2)
a = long dimension of waveguide cross section
b = short dimension of waveguide cross section
m and n are the mode indices

When kc > k the guide is in cutoff and B = -j(kc^2 - k^2) = -jA. In the 
case of a rectangular waveguide operating in cutoff we are only interested 
in the lowest frequency mode, TE10, The others operate at much higher 
attenuations and we want worst case. In this case m=1 and n=0. So my best 
guess is your attenuation should be something like 20log(e^(-Az)) dB, for a 
single rectangular waveguide, of length z, operating below cutoff. This 
could be easily extended to circular waveguide as well, if you have round 
holes.


Well, that's my WAG at your question. As I said, this is covered in the 
literature. Any EM text or microwave engineering text will have the 
governing equations. One of my favorites is Microwave Engineering by David 
Pozar. Perhaps others can suggest more.


Simulation could be done quite effectively using a field solver. We use 
Ansoft's HFSS. There are others as well. I would not dismiss simulation 
completely, but like you, I appreciate an analytical understanding. It 
keeps you out of trouble! Well most of the time anyway...


I hope this helps.

Cheers,

-Rob Hinz
Consulting Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR 97062-3090
(503) 885-1231
http://www.siqual.com/




At 09:16 AM 6/5/2001 -0700, Neven Pischl wrote:
I would appreciate if anyone could let me know if there are any references 
(books, application notes, anythig ..) that deal with shielding 
efectivness in cases when a source is close to an (electrically small) 
opening in a shield (enclosure). In such a situation, the field will 
penetrate through the hole and leak even if the size is much smaller than 
the wavelength. I am particularly interested in situation when 
high-frequency source, such as a PCB edge or a component operating at 
(say) 1 GHz and above is in proximity of the venting holes, "small" gaps 
in the chassis etc.


All references that I have deal with uniform plane wave propagating 
incident to a metal plane with a slot or hole, in which case it is enought 
o have electrically small size of the opening (e.g. lambda/10) to 
efficiently block any field propagation through the barrier. I can't find 
any useful reference that deals in any analytical way with the situation I 
am intersted in.


I believe I might get some answers using some of the simulation programs, 
but at the moment I am more intersted in the analysis of the problem than 
in simulating it.


Thank you,

Neven Pischl


Rob Hinz
SiQual Corporation
r...@siqual.com
phone (503)885-1231 x30
fax   (503)885-0550

RE: Product Safety: A Matter of Law or Litigation?

[Massey, Doug C.]

>>Mr. Woodgate says:
>>Because the scope of the standard is quite independent of the scope of the
LVD. 
 
Again, this begs the question - a matter of law or liability? The LVD is LAW
- if a product falls within the scope of the LVD, then the product must be
evaluated to the harmonized standard before placing said product in the
Community market. Using an independent third party to evaluate the product
(a wise choice) is optional. No arguement that the scope of a particular
standard is independent of a Directive. EN60950 would be the natural choice
of standard, but my question is, where is the law that supports this choice?
 
>>Mr. Woods says in an earlier reply:
>>...obtain approval marks for the US and Canada due to the potential of
litigation. You will have shown due diligence which may assist in limiting
any liability issues. In the EU, however, they have "no-fault" liability -
if someone is hurt by a defect in your equipment, you are at fault no matter
what approvals you have. A safety approval in Europe is for useful for
marketing purposes only.
 
Good point, and an interesting view. Let me clarify that "no-fault"
liability means that the injured party need only prove that the product was
defective, and that the defect caused injury or property damage; the injured
party does not have to show negligence on the part of the manufacturer, as
is the case here in the states. Also, a safety approval does not relieve the
manufacturer from liability in any court in the world, not just the EU.
Basically, evaluating a product to an accepted safety standard could be
viewed (cynically) as building your liability defense case before marketing
your product.
 
So far, I have found no iron-clad reason for evaluating a product outside
the scope of the LVD to the EN standard. Many compelling reasons, but it
seems to be a matter of litigation rather than law, and the choice is made
based on the level of liability risk the manufacturer is comfortable with.
I'm not debating the wisdom of said choices.
 
It could be argued that a wrist watch falls within the scope of the EN60950
standard. Modern watches have calculators in them, etc. However, to apply
that standard to a device with such low voltage, and low potential for harm
seems very extreme. So the risk is very low. However, a battery powered
device incorporating Li-Ion batteries, laser devices, LCD backlight
inverters, etc, even if the device does not fall within the scope of the
LVD, is on the high side of the risk factor scale. It may be a wise choice
to evaluate that product to EN60950 before marketing it in the EU.
 
Thank you all for sharing your valued opinions, insights, experience, and
knowledge.
 
Best Regards,
 
Doug

-Original Message-
From: Brian O'Connell [mailto:boconn...@t-yuden.com]
Sent: Tuesday, June 05, 2001 9:57 AM
To: 'emc-p...@majordomo.ieee.org'
Subject: RE: Product Safety: A Matter of Law or Litigation?



Once again, Mr. Woodgate has forced me to think... 

So, as a general policy, would it be valid to be driven by an indvidual
standard's scope rather than the Directives? 

Brian O'Connell 
Taiyo Yuden (USA), Inc. 


-Original Message- 
From: John Woodgate [ mailto:j...@jmwa.demon.co.uk
 ] 

, Massey, Doug C. 
 inimitably wrote: 
>If the LVD does 
>not apply to the product, then how can I argue that EN60950 applies to the 
>product? 

Because the scope of the standard is quite independent of the scope of 
the LVD. 
-- 

RE: Shielding Effectivness Question

[mkelson]

Here's an informal guideline that I use.  If memory serves, I got it off the
Futurebus specification years ago.  I imagine this guideline was based on
slower frequencies than we have nowdays. 
 
1. Enclosure should have 20 dB of RFI attentuation at 5 Ghz.
2. Maximum gap shall be less than 3 mm in any direction.
3. Holes for indicators, switches, or connectors that are not shielded shall
be limited to the following:
A. No source of high frequency within one diameter distance of the hole.
B. The thickness of the hole wall shall be more than:
0 for a 3 mm hole
2.5 mm for a 7.5 mm hole
10 mm for a 15 mm hole
4. Larger holes shall have back-shielding behind the light, switch, or
connector, and tied to chassis ground through low-impedance gasketing.
 
I would be interested also in any cook-book or analystical information
anyone else might have.
 
Max Kelson
Evans & Sutherland

-Original Message-
From: Neven Pischl [mailto:npis...@cisco.com]
Sent: Tuesday, June 05, 2001 10:16 AM
To: emc-p...@majordomo.ieee.org; si-l...@silab.eng.sun.com
Subject: Shielding Effectivness Question


I would appreciate if anyone could let me know if there are any references
(books, application notes, anythig ..) that deal with shielding efectivness
in cases when a source is close to an (electrically small) opening in a
shield (enclosure). In such a situation, the field will penetrate through
the hole and leak even if the size is much smaller than the wavelength. I am
particularly interested in situation when high-frequency source, such as a
PCB edge or a component operating at (say) 1 GHz and above is in proximity
of the venting holes, "small" gaps in the chassis etc.
 
All references that I have deal with uniform plane wave propagating incident
to a metal plane with a slot or hole, in which case it is enought o have
electrically small size of the opening (e.g. lambda/10) to efficiently block
any field propagation through the barrier. I can't find any useful reference
that deals in any analytical way with the situation I am intersted in.
 
I believe I might get some answers using some of the simulation programs,
but at the moment I am more intersted in the analysis of the problem than in
simulating it.
 
Thank you,
 
Neven Pischl


---
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RE: Product Safety: A Matter of Law or Litigation?

[Chris Maxwell]

Hi Doug, 

You've struck upon the same question that my company (and I assume, many
others) has wrestled with.  To answer your question directly (considering
only the European market).  Making your product safe is a matter of law.
Using a third party lab to evaluate the safety is a matter of litigation.
My reasons follow:

If your product runs from less than 75VDC or 50VAC, the law says that the
Low Voltage Directive does not apply.  

I assume that you have checked to see if your product  is covered by other,
more specific directives such as the Machinery Directive, Medical, R&TTE ...
I assume that none of these apply.  If you haven't checked this, I would
recommend it. 

Every product not covered by a is covered by the General Product Safety
Directive.  So here, the LAW says that you need to make a safe product.  The
LAW also says that the manufacturer is liable for the safety of the product.
However, the LAW does not absolutely require a 3rd party lab safety
evaluation to do so.  You can evaluate the safety of the product yourself.
You can even use EN 60950 if you technically feel that this is the most
applicable standard.

This is where litigation comes in.  If there ever was an accident with your
products, the third party safety evaluation MAY help keep the judgements
against your company down. However,  It is no guarantee.  

So, look at a bunch of factors:

1.  Are you absolutely sure that you have assessed your product and found
the correct directive that it is covered by?

2.  Does your company  feel comfortable that they can asses and mitigate all
of the liability risks (electrical shock, chemical, fire, battery
explosion...) of your products?  Would you or one of your engineering
colleagues be able to look a jury in the eye in the case of an accident?

3.  What are your customers expectation?  Do they expect a third party
safety eval?  Have they ever asked about it?

4.  What are your competitors doing?

5.  Do you have a cost estimate from a third party lab for the safety eval?
That way, you'll have a cost to compare against the benefits.

These are my opinions only and I admit that my opinions are the concoctions
of one mind trying to figure out a gillion confusing (and sometimes
contradictory) pages of regulatory documents.  

Stay compliant my man.

!PLEASE NOTE THE NEW EMAIL ADDRESS
Chris Maxwell
Design Engineer
NetTest
6 Rhoads Drive, Building 4
Utica,NY 13502
email: chris.maxw...@nettest.com
phone:  315-266-5128
fax: 315-797-8024


> -Original Message-
> From: Massey, Doug C. [SMTP:masse...@ems-t.com]
> Sent: Monday, June 04, 2001 8:47 AM
> To:   'IEEE Forum'
> Subject:  Product Safety: A Matter of Law or Litigation?
> 
> 
> Colleagues:
> 
> I have been tasked with justifying the need for independent, third party
> evaluations of the safety of our company's products to applicable
> standards.
> Our company manufactures various ITE equipment, either handheld, battery
> powered devices, or ITE devices powered by vehicle batteries. In
> particular,
> the scope of the LVD states that it is applicable to devices rated
> 50-1000Vac or 75-1500Vdc; most of our products are below 75Vdc. We market
> these products in 35 countries; North America, the EU/EFTA, and others -
> in
> fact, pretty much all of the countries participating in the CB Scheme.
> 
> In the US, OSHA regs justify this requirement, as our equipment is sold
> through direct channels solely for logistics applications - in other
> words,
> US workers will be using the equipment - it's not for general consumer
> use. 
> TITLE 29--LABOR PART 1910--OCCUPATIONAL SAFETY AND HEALTH STANDARDS--Table
> of Contents Subpart S--Electrical Sec. 1910.399 Definitions applicable to
> this subpart. Acceptable. An installation or equipment is acceptable to
> the
> Assistant Secretary of Labor, and approved within the meaning of this
> Subpart S: (i) If it is accepted, or certified, or listed, or labeled, or
> otherwise determined to be safe by a nationally recognized testing
> laboratory; or (ii) With respect to an installation or equipment of a kind
> which no nationally recognized testing laboratory accepts, certifies,
> lists,
> labels, or determines to be safe, if it is inspected or tested by another
> Federal agency, or by a State, municipal, or other local authority
> responsible for enforcing occupational safety provisions of the National
> Electrical Code and found in compliance with the provisions of the
> National
> Electrical Code as applied in this subpart; or ...
> 
> In the EU/EFTA, the justification is not so easy.  As I mentioned earlier,
> the products are exempt from the LVD. The General Product Safety
> Directive,
> and the Product Liability Directive, do not give me an easy justification,
> such as in the case of the OSHA regs stated in US Federal Code. My company
> has always had all products evaluated to the -950 standards, but has
> observed that other manufacturers of similar equipment do not have t

Shielding Effectivness Question

[Neven Pischl]

RE: Product Safety: A Matter of Law or Litigation?I would appreciate if anyone 
could let me know if there are any references (books, application notes, 
anythig ..) that deal with shielding efectivness in cases when a source is 
close to an (electrically small) opening in a shield (enclosure). In such a 
situation, the field will penetrate through the hole and leak even if the size 
is much smaller than the wavelength. I am particularly interested in situation 
when high-frequency source, such as a PCB edge or a component operating at 
(say) 1 GHz and above is in proximity of the venting holes, "small" gaps in the 
chassis etc.

All references that I have deal with uniform plane wave propagating incident to 
a metal plane with a slot or hole, in which case it is enought o have 
electrically small size of the opening (e.g. lambda/10) to efficiently block 
any field propagation through the barrier. I can't find any useful reference 
that deals in any analytical way with the situation I am intersted in.

I believe I might get some answers using some of the simulation programs, but 
at the moment I am more intersted in the analysis of the problem than in 
simulating it.

Thank you,

Neven Pischl

RE: Product Safety: A Matter of Law or Litigation?

[Brian O'Connell]

Once again, Mr. Woodgate has forced me to think...

So, as a general policy, would it be valid to be driven by an indvidual
standard's scope rather than the Directives?

Brian O'Connell
Taiyo Yuden (USA), Inc. 


-Original Message-
From: John Woodgate [mailto:j...@jmwa.demon.co.uk]

, Massey, Doug C.
 inimitably wrote:
>If the LVD does
>not apply to the product, then how can I argue that EN60950 applies to the
>product? 

Because the scope of the standard is quite independent of the scope of
the LVD.
-- 

Static to dynamic field converter

[Douglas C. Smith]

Hi All,

It is amazing what can be accomplished with simple, inexpensive test
fixtures when it comes to debugging designs. The Technical Tidbit this
month on www.dsmith.org describes a simple structure that generates
repeatable, intense EMI useful for finding weaknesses in system designs.
It also makes a good science fair
project for a student. This article is a continuation of last month's
article titled "Hidden Threats to Electronic Equipment" and is located
near the bottom of the index page. The material cost for this gaget is
about US$1 and it takes just a few minutes to build.

Doug
-- 
---
___  _   Doug Smith
 \  / )  P.O. Box 1457
  =  Los Gatos, CA 95031-1457
   _ / \ / \ _   TEL/FAX: 408-356-4186/358-3799
 /  /\  \ ] /  /\  \ Mobile:  408-858-4528
|  q-( )  |  o  |Email:   d...@dsmith.org
 \ _ /]\ _ / Website: http://www.dsmith.org
---

---
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