CWLee wrote:
My son recently replaced two wall on-off switches with
dimmer switches, new from Home Depot or Lowe's, for me.
They work fine and control incandescent lights. He
discovered, however, that even when completely off about
48-50 volts still flow through the new dimmer switches -
both of them.

We are both puzzled. We assumed that off would mean zero
volts. If this is normal isn't it a safety hazard?

Any insight appreciated.

I've read the various responses to date and they all have valid points. IFF
you're still confused, here's a definitive test you can use that will verify
whether the voltages are phantom or not.

With the same setup you've been using, measuring at the same place, find a
way to put a resistive load across the two points where you're measuring. A
standard light bulb will work fine.
Place the light bulb or resistor connections across the points you are
measuring. If the voltage drops to 0.0V, then it's phantom voltages and
nothing to worry about in any way. If it doesn't drop to 0.0, then there is
current available in the wires and something IS wrong, OR as another poster
noted, it doesn't switch all the way off by design.
If the voltage stays above, say, 30 V, then be VERY careful as there is a
possibility that it's high enough to hurt you if you get across those wires!
I'm going to suggest that won't happen; the voltage will go to 0.0 as you
place the bulb across the connected meter probes.

A handy little gizmo that makes it easier to measure like this is one of
those things you can screw a light bulb into and then plug it into a
standard receptable. Available at most hw stores. It's easier to hook
wires to the prongs of the device than to try to hold them against the bare
bulb base and center piece. Also safer for you and for your wiring should
you slip and it's not phantom voltagesg.
If you use a resistor instead of hte bulb, and have to buy one at RS or
wherever, get one about 2k ohms. That meanst about 2,000 ohms.
For safety sake, be CERTAIN to NEVER let more than one part of your body
(hand) contact anything metal at any time. Don't even try it if there is
any standing water around.

The other, probably simpler alternative, is to simply temporarily hook up
the lights, turn the power on, and measure the same points. See what the
voltages do then; I'm betting they'll go to 0.0 from around 120. It'd just
about have to if the dimmer is working.
This IS of course, a dimmer intended to dim flourescent lights, right?
If it's incandescent only, then you also have the wrong type of dimmer.

HTH,
Pop