Yesterday evening, I wandered around my house with my cheap non-contact voltage
sensor with the idea of locating the 12/2 romex within some walls without
actually opening said walls. The results have left me scratching my head and
perhaps the EEs and electricians who read this list can enlighten me.
Seems most of the outer walls and especially the ceilings directly below
the roof indicate the presence of an A/C voltage (sensitivity is supposedly
70V-440V). This happens even in places where I'm sure there are no wires
within several yards and where the only metal objects would be nails in the
aforementioned wall or roof. The ranges vary: some areas the sensor will begin
chirping about 12" from the surface; in other places it won't chirp until a few
inches from the surface. This little sensor even begins chirping when held near
my kitchen countertop.
As I understand, this class of sensor actually measures capacitive coupling
to ground through the operator (ie. me). But that in itself makes me scratch
my head: I'm wearing tennis shoes and standing on non-conductive carpet so I
should be floating (right?) so what, exactly, is coupling in this case?
Fluke's website suggests that in situations where you're floating (standing
on a wooden ladder, etc) that best practice is to ground oneself with one hand
while using the sensor in the other. When I tried this, my sensor detected
A/C *everywhere* -- including open air four feet from the nearest surface --
which
is somewhat less than useful.
So clearly I'm using this little sensor incorrectly or expecting it to work in
ways that it cannot. Anybody care to enlighten me as to the correct way to use
one? Is this guy really reporting a 70V+ A/C potential difference 12" from a
wall that contains no electrical wires? I could understand perhaps a volt or
two due to the aluminum gutters and drip edges acting as antennas but even then
I'd expect to detect it only near those structures.
JM
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