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Is this stray voltage to a house light switch?

What would cause a reading of 49 volts when measured in series across the two hot terminals of a single pole light switch when the switch is turned off.  A single hot black wire is attached to each of the switch terminals.  Two white common wires to this circuit are tied together in the gang box.  When the switch is turned on the voltage drops to zero across the two switch terminals.  At the end of the circuit the meter reads 115 volts across the common and hot line in the circuit going to a light that is not yet installed.  Is it normal to have a 49 volt reading at the light switch?
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Posted 2016-04-29T00:59:49+0000  by busterbob busterbob
Are you using a digital multimeter to test this circuit.  Digital meters, under the right circumstances, will pick up stray or phantom voltage often in 40-70 volt range.  If you add some kind of resistance to the circuit, like a light bulb, the voltage will disappear.  Using a solenoid type tester (e.g., a "Wiggy") will eliminate it as well.

It's nothing to worry about.
Posted 2016-04-29T08:10:27+0000  by Adam444
Hello busterbob.  Welcome to the Community!

What you have measured is counter-intuitive, but perfectly normal.

Multimeters have an input resistance in the mega-ohm range.  As such, when you measure the hot leads across an open switch, the meter itself completes the 120 volt circuit, but at a close to zero current level.  Because there is now a flow, there will also be a measured voltage.  It is not stray voltage, as you have created the flow by hooking up the meter.  Meters like these are designed to measure voltage under conditions where their internal resistance is extremely high compared to the resistance in the circuit being measured.  The condition you have created is one where the resistance of the meter, although very high, is less than that of an open circuit, which has infinite resistance.  As Adam said, this is a phantom reading, and nothing to worry about.


Posted 2016-04-29T15:50:07+0000  by Chris_HD_CHI
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