[Orcnet] Strange bathroom fan behavior

[Keith Lofstrom]

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