[SI-LIST] Re: ESD, shoes, and hospitals...
- From: Bill Wurst <billw@xxxxxxxxxxx>
- To: rchundru_umr@xxxxxxxxx
- Date: Wed, 16 May 2007 06:58:59 -0500
Patrick,
There are at least three variables here:
1) the material with which the shoe is constructed (keep in mind that
heel and sole may be different),
2) the flooring material, and
3) humidity level.
The charge generated should be proportional to the separation of those
materials in the triboelectric table. To test the charge build up on a
person you should get a static field meter which will give you a reading
in kV. I believe that the meter should be held by person grounded
through the conventional ESD wrist strap in close proximity to the
person wearing the shoes under test.
While I doubt that special ESD floors would be practical in your
situation, there is a wealth of information on such flooring material as
well as how floors contribute to ESD in general at:
http://staticworx.com/
Aside from banning shoes made of certain materials, one variable that
could be controlled in a hospital environment is humidity. Another
possibility is to use ion generators near sensitive equipment. Free
ions will help dissipate the charge on insulators. Many electronics
labs today are installing conductive flooring material (see ANSI/ESD
S7.1-2005 Resistive Characterization of Materials–Floor Materials for
more information) and requiring personnel to wear ESD shoes or foot
straps to prevent damage to very sensitive components. Equipment
manufacturers, as Tom pointed out, should have built in ESD protection
and done the testing to prove its worthiness. But protection can only
safeguard to a certain level. Dry environments and certain materials
may exceed those levels.
Hope this helps.
Regards,
-Bill
/************************************
/ billw@xxxxxxxxxxx /
/ /
/ Advanced Electronic Concepts, LLC /
/ www.aec-lab.com /
************************************
Ram Chundru wrote:
> Patrick,
> After sending my previous email at 2 am in the morning I realized that just
> measuring the induced voltages in a loop is not a good way to compare the
> static buildup of different footware. The main reason for this is that when a
> charged person is approaching an object at lower potential (the ground plane
> in this case), at one point the intervening air breaks down resulting in an
> arc (air discharge). The severity of air discharges (current, EM fields
> generated, etc) is strongly dependent on the arc length which in turn depends
> on several parameters such as speed of approach, air pressure, orientation of
> the electrodes, etc, etc. So, multiple air discharges at one voltage level
> from the same person wearing the same footware could produce different EM
> fileds and different induced voltages in the loop. So, measuring just the
> induced voltage without measuring the arc length will not be a good
> comparison between different footware.
>
> Thanks,
> Ram Chundru
> ESD/EMC Engineer,
> Texas Instruments
>
>
> Ram Chundru <rchundru_umr@xxxxxxxxx> wrote:
> Patrick,
> I read the original article and it looks like the hospital equipment had soft
> failures (malfunction but no damage). If you want to non-destructively test
> the equipment you should test it for indirect ESD where you take an ESD
> generator and zap large metallic structures near the equipment but not into
> the equipment. Indirect ESD typically causes soft failures since the EM
> fields generated by the ESD generator couple to the equipment and induce
> voltages and currents that are typically not sufficient to cause damage. On
> the other hand, if you test the equipment for direct ESD where you directly
> zap the equipment you can cause both soft and hard failures. Soft failures
> are harder to analyze as the coupling path is not always obvious.
> Regarding your other question about measuring the static buildup on different
> footwear, you can use an electrostatic voltmeter (also called a
> non-contacting voltmeter) to measure the voltages on charged objects. The
> probe of the fieldmeter typically has to be positioned relatively close to
> the test object which may not be practical in your case. The voltages on the
> charged persons could go up to 30 kV which may not be measurable by the
> electrostatic voltmeter.
>
> Another thing you can do is to use an ESD event detector which has a magnetic
> loop antenna that picks up the fields generated by ESD and reads out the
> number of events.
> Or you can build a simple small loop antenna and orient it such that it picks
> up the maximum magnetic field when the charged person discharges into a large
> grounded metal sheet. By doing repeated experiments with different footwear
> and measuring the induced voltages in the loop, you can compare different
> footwear.
>
> Thanks,
> Ram Chundru
> ESD/EMC Engineer,
> Texas Instruments
>
>
> Tom Dagostino wrote:
> Testing equipment will likely expose any issues with them and static. =
> When
> the issues are exposed there is a high probability that the instrument =
> will
> die. The response to static will be one of three, no effect, disrupts
> operation, destroys unit. It is likely that the manufacture of the
> equipment has already done the static discharge test as part of the
> certification process. There are likely standard to which these pieces =
> of
> equipment must pass.
>
> I think most semiconductor fabs have test equipment that everyone who =
> enters
> the fab must use to show the anti static protection measures the fab has
> employed is working. You might want to check out what they use.
>
> Tom Dagostino
> Teraspeed(R) Labs
> 13610 SW Harness Lane
> Beaverton, OR 97008
> 503-430-1065
> tom@xxxxxxxxxxxxx=20
> www.teraspeed.com=20
>
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> 401-284-1827
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
> On
> Behalf Of Zabinski, Patrick
> Sent: Tuesday, May 08, 2007 3:21 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] ESD, shoes, and hospitals...
>
>
> A recent article [1] states a Swedish hospital might ban
> Crocs shoes (all-plastic clogs) to avoid static interference. They do =
> not
> cite any specific failures or severe problems with static charge, but
> they're concerned enough to consider banning these particular shoes.
>
> This article caught the attention of a few folks here, and
> we thought about running some experiments to see how bad of
> a problem we have in our hospital environments, but we're
> not exactly sure of what experiments we should/could run
> to determine if we have a problem, how bad of a problem we have, and =
> what
> shoe types are of most concern.
>
> Got any ideas? =3D20
>
> Are there some simple tests we can run that measure static buildup (in
> Volts) on people wearing different footwear? Is there a simple way of
> characterizing the relative performance of different footwear (e.g., =
> ability
> to build charge more than other shoes)?
>
> We have thousands of pieces of medical equipment that might
> be susceptible to static electricity. Any ideas how we
> can non-destructively test the equipment?
>
> Thanks,
> Pat Zabinski
>
>
>
>
> [1] We believe the original article was published in
> The Local (http://www.thelocal.se/). A copy can be
> found at http://www.wcsh6.com/printfullstory.aspx?storyid=3D3D58243.
> ------------------------------------------------------------------
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