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#1890 06/06/01 12:05 AM
Joined: Feb 2001
Posts: 308
S
Steve T Offline OP
Member
I was asked a question today which may not be exactly electrical in nature but here goes.

The parking meter guy has had problems with the electronics in the parking meters going bad. I believe they run on batteries (no power supply from an external source.) About a year ago an MRI office opened across the street from the meters and since then the problems have been occuring. (No problems before this to my understanding.) Is it possible that magnetic fields from the MRI machines are causing problems with the electronics of the meters?

The distance is a typical two-way street, a sidewalk and whatever the distance inside the building the machine is. I'd say roughly 50 to 75 feet.

I haven't done any real investigation, I was just posed the question today.

#1891 06/06/01 06:01 AM
Joined: Oct 2000
Posts: 5,392
S
Member
Steve;
i have heard similar concerns with pacemakers. a heavy magnetic field seems to directly impact electronics, this happens as a matter of proximity, strength of field.

this is a good one for the 'theory' section.

not to be a wise A**, but it also would be incentive for parking arrangements [Linked Image]

#1892 06/06/01 06:48 AM
Joined: Mar 2001
Posts: 2,056
R
Member
Wouldn't the metal meter enclosure shield the electronics from EMI? (isn't this what's known as a Faraday Cage)

#1893 06/06/01 07:49 AM
Joined: Oct 2000
Posts: 4,067
Likes: 3
Member
Check this link for some interesting info on this subject (EMI). Please let us know of any interesting things you find out.

http://www.fda.gov/cdrh/emc/emcref.html

As an interesting side note, This link came up in discussion that started off requesting information on known effects of electric wheelchair EMI on multiple sclerosis patients. Someone touched breifly on how the wheelchairs themselves have to be tested for effects from EMI
Quote
I also found it interesting that the manual specifically stated that the wheelchair also was compliant with a 20 V/M field from (IIRC) 30 MHz to 1000 MHz. That's nice, since personal electronic emitters (cell phones, PCS, handheld CB sets) typically generate about 5 to 10 V/M at about 1 to 2 meters distance. That increases the chances that the wheelchair user can safely use a cell phone without spinning in circles (or worse).

I hope that you don't mind this slight tangent. [Linked Image]

Bill


[This message has been edited by Bill Addiss (edited 06-06-2001).]


Bill
#1894 06/06/01 08:00 AM
Joined: Dec 2000
Posts: 4,289
Member
We had to have $3500 worth of shielding put around the service in a building we did for IEEE. The grounded service enclosure didn't stop the EMI, and it was causing monitors to act up. I'd really like to learn more about this, as I don't quite understand why all the high $ steel would make so much difference, and certainly don't want to go through the same drill again elsewhere.

#1895 06/06/01 08:21 AM
Joined: Dec 2000
Posts: 4,289
Member
Thank you, Bill! Looks like you were posting an answer as I was writing a ??. Your ESP must travel further than EMI.

#1896 06/06/01 09:01 AM
Joined: Oct 2000
Posts: 2,749
Member
Look here for some discussion on the parking meter issues:

http://www.jdray.com/Daviews/parkrant.html


Joe Tedesco, NEC Consultant
#1897 06/06/01 02:28 PM
A
Anonymous
Unregistered
>Wouldn't the metal meter enclosure shield the electronics from EMI?

If grounded, yes.

But whether something shields from a magnetic field depends on its magnetic permeability, e.g., aluminum, lead, and copper are ineffective.

#1898 06/06/01 02:37 PM
Joined: Oct 2000
Posts: 4,067
Likes: 3
Member
Joe,

Good one! [Linked Image] [Linked Image]

Redsy,

You bring up an interesting point. I don't know much about it but a place where I once worked had a room built of copper screening all around that I thought was sheilding something. ?? One of the products they made or tested was microwave antennas or something like that.

Bill


Bill
#1899 06/06/01 03:10 PM
Joined: Oct 2000
Posts: 4,067
Likes: 3
Member
Electure, Redsy,

I found an area that may have some information on this topic including sheilding

This one has links to archived articles.
http://www.rbitem.com/archivedarticles/archive.asp

This is specifically about building sheilded rooms and enclosures.
http://www.rbitem.com/archivedarticles/i97art19.htm


Bill


Bill
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