51.4 volts on 3-way receptacle

[Dark Sparky]

Electrical engineer here - question on my own home for a change. Newer home, with a 3-way switch-controlled receptacle in one room. Over Christmas my wife unplugged the lamp that was being used on the recept, and plugged in some LED lights. We noticed that when the switch was off, they still lit (dimly). Finally got around to metering the receptacle today: 123 volts when the switch is on (OK). However, still 51.4 volts when the switch is off. Standard outlet tester indicates (red light off / 2 yellow lights on) that the outlet is ok from that standpoint. Thoughts on what to do / where to start 'troubleshooting'? I'm hoping this isn't a wiring issue. I'm a handyman also, but typically prefer projects by choice! Thanks in advance.

 

[gar]

160126-1848 EST

jonathankpa:

You are probably seeing what might be expected. Since you are an electrical engineer I suggest you devise some experiments to help you reason what is a possible cause of your observation.

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[GoldDigger]

Do you by any chance have a lighted switch in the mix? Or a three way dimmer set? Or are they just plain snap switches?

 

[Dark Sparky]

Do you by any chance have a lighted switch in the mix? Or a three way dimmer set? Or are they just plain snap switches?

Thanks, GoldDigger. No lighted switch. No dimmer. Indeed, just plain snap switches.

 

[Dark Sparky]

160126-1848 EST

jonathankpa:

You are probably seeing what might be expected. Since you are an electrical engineer I suggest you devise some experiments to help you reason what is a possible cause of your observation.

.

I assume you mean "expected in case of a problem with (whatever)", rather than expected/normal - ? I need to sit down and diagram possibilities, but hoping some of the expertise here will have ideas...

 

[GoldDigger]

Nope. Expected normal in some cases.
Ask yourself where limited current could be coming from.
Hint: do not limit yourself to resistive sources.

 

[ritelec]

I seem to recall some switches in the past that worked yet didn't work yet worked ??

Replacing switches solved the problem

For grins and five bucks. Replace both switches.

 

[ritelec]

Oh. And don't back stab and check the recept for back stab.

 

[gar]

160126-2220 EST

jonathankpa:

Think Thevenin's equivalent circuit and source impedance. What is the distribution of wires in a cable?

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[John120/240]

I don't know if it is Germane to the problem but, Is the whole receptacle switched or just one half of the receptacle ?

 

[gar]

160126-2227 EST

jonathankpa:

How can you approximately determine source impedance?

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[JFletcher]

I seem to recall some switches in the past that worked yet didn't work yet worked ??

Replacing switches solved the problem

For grins and five bucks. Replace both switches.

This. Chances are its voltage leakage thru the switches, enough to dimly light LEDs.

 

[kwired]

Neutral is the switched conductor instead of the hot conductor is my first guess, more details are needed for any further confirmation though.

 

[cpinetree]

I put led lights on the Christmas tree this year, they had an inline switch to turn them on/off.
First night I turn the lights off and I see a dim glow from the tree

The lights were plugged into an old extension cord that the neutral side of the light string would fit into the hot side.
Reversed the cord cap so the neutral was plugged into the cord correctly and the lights went fully off.

I did like the faint glow when it was the other way

 

[gar]

160127-0954 EST

Assuming the meter used that measured 51.4 V was a high impedance meter, then the 51.4 reading is of importance in the analysis of the problem.

It is easy to determined if it is a switched neutral.

.

 

[growler]

Standard outlet tester indicates (red light off / 2 yellow lights on) that the outlet is ok from that standpoint.

Neutral is the switched conductor instead of the hot conductor is my first guess, more details are needed for any further confirmation though.

It is easy to determined if it is a switched neutral.

.

His plug-in tester should tell him if there is an open neutral with the switch off (switched neutral).

I'm thinking there may not be anything wrong. with the hot and the travelers running in the same cable assembly between the three ways ( normal wiring ). What people often refer to as an induced voltage.

 

[Dark Sparky]

Recept half switched

Recept half switched

I don't know if it is Germane to the problem but, Is the whole receptacle switched or just one half of the receptacle ?

Just half the receptacle is switched...

 

[Dark Sparky]

His plug-in tester should tell him if there is an open neutral with the switch off (switched neutral).

With the switch off, no lights lit on the plug-in tester. (With the switch on, as noted, red light was off & both yellows were properly on.)

 

[mwm1752]

Is the nuetral tab broken on the receptacle? what is plugged in downstream of the sw recept? could be somewhere in the circuit a receot has hot & nuetral reversed.

 

[Dark Sparky]

Calcs

Calcs

I appreciate everyone's input.

160127-0954 EST
Assuming the meter used that measured 51.4 V was a high impedance meter, then the 51.4 reading is of importance in the analysis of the problem.
It is easy to determined if it is a switched neutral.
.

Using a Thevenin equivalent circuit as you suggested -
In essence a voltage source (120V) with 2 resistances in series (source resistance, Rth and multimeter's resistance Rm). Looked up meter's input resistance (Southwire 21010N meter) to be 10,000 ohms.

Thus the voltage across the source's internal resistance is Vs = Vth + Vm --> thus Vth = Vs - Vm = 120V - 51.4V = 68.6V.
Thevenin eq. resistance (source impedance) is Rth = (Vth x Rm) / Vm = (68.6V x 10,000 ohms) / 51.4V = 13,346 ohms.
Current, I, is: I = Vm / Rm = 51.4V / 10,000 ohms = 5.15 mA

Aside - I have noticed that the wall switches used by this builder aren't the greatest. They arc (you hear the crackle) when operated.

As some of you have said - leakage current through switches? Induced voltage in the wiring? Thoughts?