Weird voltage from switch single pole switch

[Jpflex]

has anyone ever encountered voltage although small 15 VAC showing up at switch leg from switch and with switch in off position?

Source is 120 VAC line power goes to lower switch screw as typical and switch leg is at second top screw (for typical two screw single pole switch)” “Tick tracer” beeped voltage at center pin of lamp holder inside of neutral threaded portion of lamp holder when switch was in off position.

Neutral from lamp holder has 0.5 ohms continuity to breaker panel bus and there is no partial closing of switch contacts in off position of switch, as switch contacts are OL when in closed position.

There was no bulb in lamp holder and no continuity between line and neutral pins of lamp holder

Not sure why 15VAC is showing up on switch leg between switch and lamp power or switch line leg terminal (center point of lamp holder?).

Circuit performs normal with heat lamp bulb in. Switch on turns in on and off turns it off?

 

[don_resqcapt19]

What did you use to read the 15 volts? Typical meters are very high impedance and can read voltage that has been capacitively coupled to a conductor that is not connected to a load or a voltage source.

 

[Fred B]

Had 30V being back fed from a neutral on a different circuit. It had all the neutrals from the 2 circuits tied together in an attic j-box. It didn't present until someone turned on the light in the other room, and the apprentice got a shock working on the "dead" circuit's switch.

 

[augie47]

Based on one of your earlier pictures it's likely due to the meter you are using (small leakage or capacitive voltage).. Us a wiggy !

or what don said

 

[Jpflex]

What did you use to read the 15 volts? Typical meters are very high impedance and can read voltage that has been capacitively coupled to a conductor that is not connected to a load or a voltage source.

Standard DVOM volt meter and I was away from other wires to avoid capacitive voltage. Volt meter red lead to black wire switch leg screw of lamp holder and black lead to white wire screw to lamp holder

 

[Jpflex]

Based on one of your earlier pictures it's likely due to the meter you are using (small leakage or capacitive voltage).. Us a wiggy !

or what don said

However I confirmed it with a DVOM and there is indeed voltage AC

 

[Jpflex]

Had 30V being back fed from a neutral on a different circuit. It had all the neutrals from the 2 circuits tied together in an attic j-box. It didn't present until someone turned on the light in the other room, and the apprentice got a shock working on the "dead" circuit's switch.

I was thinking it may of had to do with neutral and or back feeding but I can’t visualize how this would happen with an open line at switch and full continuity on neutral side

There is a difference of potential between switch leg and neutral but how?

 

[don_resqcapt19]

Had 30V being back fed from a neutral on a different circuit. It had all the neutrals from the 2 circuits tied together in an attic j-box. It didn't present until someone turned on the light in the other room, and the apprentice got a shock working on the "dead" circuit's switch.

If the neutral was is not open between the panel and the load, the only voltage to drive a shock from the neutral is the voltage drop on the neutral...30 volt drop on the neutral is not very likely.

 

[winnie]

I'm on the phantom voltage due to capacitive coupling bandwagon.

A DVOM is a very high impedance meter. The coupling between wires in a few feet of romex is sufficient for a reading.

Jon

 

[rambojoe]

I'm on the phantom voltage due to capacitive coupling bandwagon.

A DVOM is a very high impedance meter. The coupling between wires in a few feet of romex is sufficient for a reading.

Jon

Ok, but stop saying there _can_ be current on ground wires and such... Or ima tell the boss.

 

[Fred B]

I was thinking it may of had to do with neutral and or back feeding but I can’t visualize how this would happen with an open line at switch and full continuity on neutral side

There is a difference of potential between switch leg and neutral but how?

My scenario multiple circuit neutrals tied together at a random J-box, power returns on all possible paths, thru a resistive light bulb load, and splitting the load between all neutrals, then getting a feed back thru another bulb onto the switch leg then contact between the EGC and switch leg.
Same reason when having multiple fault return paths it requires a much higher fault current to trip a breaker when you have the multiple return paths than otherwise needed.

Capacitive coupling can throw phantom signal with the ticker, but haven't seen any real voltage metered, only fractional. Not saying it's not possible given enough contact length, but I haven't measured it.

 

[gar]

230612-2048 EDT

Jpflex:

You need a better understanding of electrical circuit theory.

I believe winnie has provided the reason for what you see.

To better understand what is happening there are several tests and analysis you need to perform.

I measured a 3 foot piece of #14 Romex between black and white with the ground conductor floating at about 10 pfd per foot. A typical DVM has about a 10 megohm DC resistance as its input impedance, and possibly 100 pfd of shunt capacitance.. With 120 V 60 Hz applied to one conductor of this 3 ft wire pair, and reading from the other wire with my Fluke 27 I read about 14 V AC.

Thus, it is quite easy to get the kind of voltage reading you have seen.

You need to study electrical circuit theory.

.

 

[Jpflex]

230612-2048 EDT

Jpflex:

You need a better understanding of electrical circuit theory.

I believe winnie has provided the reason for what you see.

To better understand what is happening there are several tests and analysis you need to perform.

I measured a 3 foot piece of #14 Romex between black and white with the ground conductor floating at about 10 pfd per foot. A typical DVM has about a 10 megohm DC resistance as its input impedance, and possibly 100 pfd of shunt capacitance.. With 120 V 60 Hz applied to one conductor of this 3 ft wire pair, and reading from the other wire with my Fluke 27 I read about 14 V AC.

Thus, it is quite easy to get the kind of voltage reading you have seen.

You need to study electrical circuit theory.

.

I have studied and completed books 100s of pages each on electrical theory. This is a question on one situation but on a question on basic theory as you assumed.

Just because in theory a voltage can be induced in another wire by induction (or capacitance) does not mean this is the cause.

I do have FCC Central electronics tech license since back in my 20s, ASE advanced electrical L1, ASE electrical certification, and personal study of AC systems

 

[winnie]

Ok.

What is the impedance spec for the meter you used to measure the 15V?

What voltage do you measure when you shunt this with say a 1K resistor?

Voltage can come from:
Capacitive coupling (say from hot to switched wire in a switch loop)
leakage across the switch
external magnetic fields inducing voltage in your measurement circuit
something else???

-Jon

 

[Jpflex]

Ok.

What is the impedance spec for the meter you used to measure the 15V?

What voltage do you measure when you shunt this with say a 1K resistor?

Voltage can come from:
Capacitive coupling (say from hot to switched wire in a switch loop)
leakage across the switch
external magnetic fields inducing voltage in your measurement circuit
something else???

-Jon

Most DVOM meters I worked with are within 10 mega ohms of impedance.

I did not get a chance to go deeper into finding the root cause of this issue because owner just wanted this circuit to run when heater lamp was not working. A bad connection was the fault, but like I said I got circuit working so I was asked to start another project

I’m confident the line power wire adjacent to switch leg was just inducing a voltage, although I never had such a high voltage induced in another wire that could be picked up by “tick tracer pen”.

 

[Joethemechanic]

Modern high impedance meters are good for a lot of things, but they can fool people when working on basic electrical power circuits. It happens all the time. When I suspect capacitive coupling voltage I break out one of these.


Or These

 

[OldBroadcastTech]

Ok.

What is the impedance spec for the meter you used to measure the 15V?

What voltage do you measure when you shunt this with say a 1K resistor?

Voltage can come from:
Capacitive coupling (say from hot to switched wire in a switch loop)
leakage across the switch
external magnetic fields inducing voltage in your measurement circuit
something else???

-Jon

" shunt this with say a 1K resistor"

Or obtain a Simpson 260 and use it

A 260 ( Series 6 and previous) has an ohms-per-volt spec of 1000 ohms per volt on the AC scales

 

[OldBroadcastTech]

Modern high impedance meters are good for a lot of things, but they can fool people when working on basic electrical power circuits. It happens all the time. When I suspect capacitive coupling voltage I break out one of these.

View attachment 2565757
Or These
View attachment 2565758

You beat me to it !

 

[Joethemechanic]

Or obtain a Simpson 260 and use it

You can get them for next to nothing at flea markets. I bought a really nice one for 10 bucks complete with the leather case. I sent it to Simpson for reconditioning and even that was cheap. I forget how much but it was well under 100 bucks.

 

[masterinbama]

Based on one of your earlier pictures it's likely due to the meter you are using (small leakage or capacitive voltage).. Us a wiggy !

or what don said

Augie? who besides the 3 or 4 here even own a wiggy, let alone know what it is?