troubleshooting question...

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journeyman0217

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hey guys, i know this is going to be a very vague question but any advice would help...was troubleshooting a ceiling light, started by testing voltage at the fixture. i got 120v between H and N with switch on which was a good thing, but then I was getting about 50v between H and N with switch off?? i know it will be hard to say exactly what could be causing odd readings like this, number one reason being, hiring a handyman man to do electric work which i believe is the culprit, but can anyone lead me in the right direction on what to look for...? anyone run into anything like this before?
 
journeyman0217 said:
hey guys, i know this is going to be a very vague question but any advice would help...was troubleshooting a ceiling light, started by testing voltage at the fixture. i got 120v between H and N with switch on which was a good thing, but then I was getting about 50v between H and N with switch off?? i know it will be hard to say exactly what could be causing odd readings like this, number one reason being, hiring a handyman man to do electric work which i believe is the culprit, but can anyone lead me in the right direction on what to look for...? anyone run into anything like this before?
Click to expand...

Had the same problem myself years ago. The wiring had no ground. The ceiling light was on the second floor (unswitched) and panel was in the basement. I was working alone and didn't know which fuse fed that circuit; what to do? I took my clock radio and screwed one of those "converters" into the socket and cranked up the radio. Went back down to the basement leaving all the doors open so I could hear and loosened the fuses until the radio died. Went back up and started taking everything apart and got bit! Turns out there was still 77 volts on the hot; just not enough to let the radio run but it surprised me. IIRC, there was backfeed somehow from one of the other circuits. I just opened fuses until that 77 volts (checked with the meter now, and for always) disappeared.
 
journeyman0217 said:
sounds like you may have been dealing with a MWBC? no the light is not on a lighted switch, and to make things worse i got the same reading on another light...
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Are you using a high input impedance meter? The most likely explanation is a capacitively coupled phantom voltage. If it were are "real" 70 volts the light would stay on dim.
 
journeyman0217 said:
sounds like you may have been dealing with a MWBC? no the light is not on a lighted switch, and to make things worse i got the same reading on another light...
Click to expand...
May be phantom voltage. Are you using a digital meter? Do the circuits have an equipment grounds?

If you have ruled out phantom voltage I would get a an extension cord wired up to a known good outlet and start checking hot to hot, hot to neutral, hot to EGC, neutral to EGC, neutral to neutral and see what you have.
 
What were you troubleshooting in the first place? Did you read 50V H-N with the switch removed completely or across the contacts?

ftr, it's been my experience any time there is something hosed up, replace the switch first. I havent done a ton of troublehooting but switches can fail in the most bizarre ways, and are usually very cheap to replace. Seen a pair of Decora 3 ways both fail so that after numerous switch cycles, the hallway light always stayed on. Moving the switch a 1/4 mm sideways caused it to link common to both travellers. It didnt appear damaged/worn....
 
lady originally said hall light didn't work, there was no fixture installed when i got there, i was metering the wires in the fixture box. i had 120 v when switch was in the on position but then this 50 some volts between h and n when the switch was in the off position. so what exactly is phantom voltage and how do you rule it out?
 
journeyman0217 said:
lady originally said hall light didn't work, there was no fixture installed when i got there, i was metering the wires in the fixture box. i had 120 v when switch was in the on position but then this 50 some volts between h and n when the switch was in the off position. so what exactly is phantom voltage and how do you rule it out?
Click to expand...
Is this light on a three way switch?
 
GoldDigger said:
Are you using a high input impedance meter? The most likely explanation is a capacitively coupled phantom voltage. If it were are "real" 70 volts the light would stay on dim.
Click to expand...
I agree and that's the direction I would go. Simply placing a load across the points in question and the voltage most likely drop the zero.
 
journeyman0217 said:
the hall light is on a three way...
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Depending on how it is wired can increase capacitive coupling potential as there is often going to be an energized conductor in same raceway/cable with your leads to the light fixture in question.

One other possibility is the grounded conductor is the switched conductor, will still be capacitively coupled voltage that is measured at the light in question but if you measured to some other grounded object you would read full voltage to ground.
 
I believe you have what I call a "shared neutral" meaning that teo different circuits are using the same neutral. I have seen this countless times in the homes I work on. It can usually be found where there are two switches usually at least one being a 3 way switch, next to each other. They typically used 14/2 wire rather than 14/3 and they used the neutral from circuit #1 for circuit #2. As far as i know, that is allowed as long as the breaker for the two circuits is a two pole breaker so that both circuits are on different phases and also because you cant turn off only one of thr circuits. The handle forces both to be on or both to be off. If you research "multiwire branch circuit" you will get an idea of the logic, but a mwbc is usually run with one cable rather than two.

Sent from my LG-D415 using Tapatalk
 
Take another voltage reading with the light bulb screwed in, the light switch switch ON and the breaker OFF and see of the stray voltage goes away with a load back in the circuit.
 
jmattero said:
I believe you have what I call a "shared neutral" meaning that teo different circuits are using the same neutral. I have seen this countless times in the homes I work on. It can usually be found where there are two switches usually at least one being a 3 way switch, next to each other. They typically used 14/2 wire rather than 14/3 and they used the neutral from circuit #1 for circuit #2. As far as i know, that is allowed as long as the breaker for the two circuits is a two pole breaker so that both circuits are on different phases and also because you cant turn off only one of thr circuits. The handle forces both to be on or both to be off. If you research "multiwire branch circuit" you will get an idea of the logic, but a mwbc is usually run with one cable rather than two.

Sent from my LG-D415 using Tapatalk
Click to expand...
That alone doesn't cause OP's troubles, something has to be malfunctioning.
 
jmattero said:
I believe you have what I call a "shared neutral" meaning that teo different circuits are using the same neutral. I have seen this countless times in the homes I work on. It can usually be found where there are two switches usually at least one being a 3 way switch, next to each other. They typically used 14/2 wire rather than 14/3 and they used the neutral from circuit #1 for circuit #2. As far as i know, that is allowed as long as the breaker for the two circuits is a two pole breaker so that both circuits are on different phases and also because you cant turn off only one of thr circuits. The handle forces both to be on or both to be off. If you research "multiwire branch circuit" you will get an idea of the logic, but a mwbc is usually run with one cable rather than two.

Sent from my LG-D415 using Tapatalk
Click to expand...
An open shared neutral that has a load will read the full voltage to ground and zero voltage to a hot on the same leg when measured with a meter.
 
journeyman0217 said:
lady originally said hall light didn't work, there was no fixture installed when i got there, i was metering the wires in the fixture box. i had 120 v when switch was in the on position but then this 50 some volts between h and n when the switch was in the off position. so what exactly is phantom voltage and how do you rule it out?
Click to expand...

Newer high impedance (ohm/volt) digital meters need very little current (load) to measure voltage, thus they have a very high "impedance" or ohms/volt, meaning they put very little load on the circuit being measured. This can cause the meter to read voltage that is really only a "charge" and is not really a voltage source. If you were to use an older analog meter or a Wiggy" you would see no voltage because the "source" would not be able to supply enough current to power the measuring circuit, resulting in a reading of near zero volts. Probably much clearer ways to explain it, but bottom line is, voltage readings not under load can be misleading and may not be actual usable circuit voltages at all. Thus the name "phantom". They ain't really there. You rule it out by placing a load on the circuit. If the voltage drops to near zero, it's phantom.
 
A more technical way to describe the "charge" on the open conductor is that you are seeing capacitive coupling.
If you look at the two wires there is a capacitance between them, just as there is a capacitance between any conductive objects that are locally insulated from each other.
You can draw an equivalent circuit for the open wire by drawing one (small) capacitor between the wire under test and any wire near it (like running in the same raceway or cable) that has line voltage on it. And another (also small, but maybe a bit bigger) capacitor between the wire under test and any grounded metal near it. (such as raceway or EGC and neutral in the same NM.)

With no meter attached at all the voltage is determined by a voltage divider effect of the two capacitances, and using a non-contact tester you will see that full voltage.

If you now attach a voltmeter you are adding a resistance in parallel with the capacitor that is going to ground/neutral.
If the voltmeter looks like a very high resistance it will not affect the voltage divider much. If is it a lower resistance, like an old analog meter, a wiggy, or a newer meter that deliberately shows a low input impedance, the voltage read will be lower or even appear to be zero.
As you turn on and off other circuits that run in the same raceway, you may change to value of the phantom voltage.
 
The "charge" is real, it is just that the "capacitor" isn't very high value of capacitance and can not deliver even a few milliamps of current which ends up dropping the voltage to near nothing with even the somewhat high impedance level of what is called a low impedance meter.
 
journeyman0217 said:
hey guys, i know this is going to be a very vague question but any advice would help...was troubleshooting a ceiling light, started by testing voltage at the fixture. i got 120v between H and N with switch on which was a good thing, but then I was getting about 50v between H and N with switch off?? i know it will be hard to say exactly what could be causing odd readings like this, number one reason being, hiring a handyman man to do electric work which i believe is the culprit, but can anyone lead me in the right direction on what to look for...? anyone run into anything like this before?
Click to expand...

infinity said:
Is the switched a lighted switch?
Click to expand...


I have run into this but the light I dealt with had a dimmer. As Infinity stated a lighted switch or dimmer will also give these readings .
 
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