[Orcnet] [Orcnet-members] Strange bathroom fan behavior

[H. Joe Tabor]

Keith,

This could be a business opportunity, and a good
one involving public safety.  Not every home has
an EE or physicist.

Joe

-----Original Message-----
>From: Keith Lofstrom <keithl at kl-ic.com>
>Sent: Jul 28, 2010 8:39 PM
>To: orcnet <orcnet at eeconsult.org>, orcnet-members <orcnet-members at eeconsult.org>
>Subject: [Orcnet-members] Strange bathroom fan behavior
>
>Interesting electrical behavior!
>
>One of the idiosyncracies of the new house is a very strange
>bathroom fan and bathtub interaction.   When the bathroom
>fan is running, it induces a potential between the seemingly
>ungrounded bathtub and the faucets, two meters below. 
>
>The open circuit voltage is 90VAC, the short circuit current
>is 50uA.  Not dangerous, but startling.  The fan plugs into
>a socket in the fixture, and there is no potential when the
>fan is unplugged, or a resistive load is plugged in instead.
>
>I removed the fan, set it up on the bench, and ran it with
>a variac.  When running, the 6 inch metal strut across the
>back of the fan motor has a 10VAC potential across it!
>
>I'm guessing that the (induction?) motor is poorly designed,
>and the magnetic field circuit is not properly closed, with
>stray fields inducing quite a bit of dB/dt voltage in the
>surroundings.  Although the motor runs OK,  I will replace
>it anyway.
>
>Meanwhile, I grounded the drainpipe underneath the tub to
>the cold water line.  I don't know why the drain pipe is not
>conductive further down, that may indicate yet another problem.
>
>Keith
>
>
>
>
>-- 
>Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993
>KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
>Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs
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