Weird voltage from switch single pole switch

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Jpflex

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
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

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
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

Senior Member
Location
Upstate, NY
Occupation
Electrician
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

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
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

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
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

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
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

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
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

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
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

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
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

Senior Member
Location
phoenix az
Occupation
Wireman
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

Senior Member
Location
Upstate, NY
Occupation
Electrician
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

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
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

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
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

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
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

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
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

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
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.

s-l1600.jpg
Or These
1A587_AS01.jpg
 

OldBroadcastTech

Senior Member
Location
Western IL
Occupation
Retired Broadcast Technician
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
 

Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
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.
 
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