[SI-LIST] Re: ESD, shoes, and hospitals...
- From: Ram Chundru <rchundru_umr@xxxxxxxxx>
- To: rchundru_umr@xxxxxxxxx, tom@xxxxxxxxxxxxx, zabinski.patrick@xxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Wed, 9 May 2007 12:16:22 -0700 (PDT)
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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