Low Voltage Reading on Circuit

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Little Bill

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I was checking a nonworking light in a business this week. I checked the voltage and got a low reading so my first thought was either open neutral or phantom voltage. I read 72V but thought I would rule out the reading by checking with a load. I took my pigtail socket with a 65W incandescent bulb and connected it to the leads for the light. The bulb lit up, so I was sure the voltage was real.

Now my question is what would cause such a low voltage?
I went to a receptacle and checked my meter to see what voltage I got there. It read 120V so pretty certain my meter was reading right.
Also, the bulb in the nonworking light was a MR16 with a GU10 base. I tried several bulbs but none would light except the regular incandescent in my pigtail socket.
I went into the attic to trace down the wiring but ran out of time as the business was about to open and I would be in the way. I plan on going back next week earlier in the morning so I have time to check. Just thought I would get some opinions before going back.

Is this real or Halloween VOO-DOO?:)
 

GoldDigger

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My guess is that the ballast is bad and is reducing the output voltage via a transformer or reactive voltage divider effect. Maybe a multivoltage ballast that thinks it should be getting a higher input voltage?
An arc lamp (which is what fluorescent and gas discharge lighting really is) will generally not start the arc at that low a voltage, while an incandescent lamp will just run at a lower current, temperature and light output.
 

GoldDigger

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How was an open neutral ruled out?
I do not think that it has been conclusively ruled out, but if it is an open neutral there must necessarily be some devices on the MWBC which are seeing 165V or so all the time. (Neutral offset of about 45V to produce 74 at the device in question.)
 

iwire

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I do not think that it has been conclusively ruled out, but if it is an open neutral there must necessarily be some devices on the MWBC which are seeing 165V or so all the time. (Neutral offset of about 45V to produce 74 at the device in question.)

Well yeah. :)
 

growler

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Atlanta,GA
I checked the voltage and got a low reading so my first thought was either open neutral or phantom voltage.

Now my question is what would cause such a low voltage?

Is this real or Halloween VOO-DOO?:)


It can always be the neutral but can be a poor connection of the hot conductor in one of the earlier junction boxes. Once a connection goes bad it starts to arc and this causes a resistance to current flow.

If you only have one light that's not working I would look at the first juntion box that's feeding this fixture. From there you start toward the source.
 

meternerd

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Athol, ID
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retired water & electric utility electrician, meter/relay tech
what was hot to ground voltage reading...neutral to ground. Open neutral should still show 120V hot to ground.
 

gar

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Ann Arbor, Michigan
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EE
151030-1609 EDT

Little Bill:

I believe you have a series impedance in either the hot or neutral wiring that is much greater than the wire alone.

Measure the voltage at the socket with no bulb installed. This you have done and it is about 72 V.

Next connect your 65 W test bulb and read the voltage across the bulb. You have done half of this. The bulb glowed, but you did not measure the voltage.

Next connect a known good 250 W bulb and read its voltage. From these measurements we can estimate the magnitude of the source resistince.

Also with the 250 W load connected measure voltage at the bulb from hot to a known good ground-netrual reference point, and from neutral at the bulb to the same reference point. This will give you an idea of whether the problem is in the hot or neutral or both.

With the 250 W bulb as the load, then try to find another voltage measurement point on the same circuit between the 250 W bulb and the source breaker. Measure the same voltages here. This will tell you whether the problem is up stream or down stream of the measurement point.

.
 

GoldDigger

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151030-1609 EDT

Little Bill:

I believe you have a series impedance in either the hot or neutral wiring that is much greater than the wire alone.

Measure the voltage at the socket with no bulb installed. This you have done and it is about 72 V.
.
If the problem is indeed a high resistance, then the lower voltage with no bulb installed would have to be the result of another load on the same circuit beyond the same fault that is dropping the voltage. If all other equipment is working properly that is not particularly likely. Or else the idling current of the ballast for that lamp is dropping the voltage.
The OP can amp clamp the breaker feeding the circuit to see whether there is current flowing with that lamp disconnected. Then look for reduced voltage at other outlets on the same circuit, as you described for trying to locate the fault.

My first troubleshooting step would be to measure the line voltage at the input to the ballast and then if that is also low measure the line voltage with the ballast disconnected.
 

Little Bill

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My guess is that the ballast is bad and is reducing the output voltage via a transformer or reactive voltage divider effect. Maybe a multivoltage ballast that thinks it should be getting a higher input voltage?
An arc lamp (which is what fluorescent and gas discharge lighting really is) will generally not start the arc at that low a voltage, while an incandescent lamp will just run at a lower current, temperature and light output.

The lamp/bulb is a MR16 halogen, don't think it has a ballast unless it's internal to the lamp.

A0BD_1_20111110_12251051.jpg


How was an open neutral ruled out?

To be honest, after I tested the conductors with my pigtail socket and the bulb lit, I didn't check much further for the neutral. Also, I ran out of time.

It can always be the neutral but can be a poor connection of the hot conductor in one of the earlier junction boxes. Once a connection goes bad it starts to arc and this causes a resistance to current flow.

If you only have one light that's not working I would look at the first juntion box that's feeding this fixture. From there you start toward the source.

I checked one JB but turns out the MC cable just ran by it, not from it. I will check further when I go back. Everything is fed from the attic and I couldn't leave my ladder there while they were open

If the problem is indeed a high resistance, then the lower voltage with no bulb installed would have to be the result of another load on the same circuit beyond the same fault that is dropping the voltage. If all other equipment is working properly that is not particularly likely. Or else the idling current of the ballast for that lamp is dropping the voltage.
The OP can amp clamp the breaker feeding the circuit to see whether there is current flowing with that lamp disconnected. Then look for reduced voltage at other outlets on the same circuit, as you described for trying to locate the fault.

My first troubleshooting step would be to measure the line voltage at the input to the ballast and then if that is also low measure the line voltage with the ballast disconnected.

As I mentioned, I don't think there is a ballast. I am going back to check this out more thoroughly next week as I didn't have time for much troubleshooting the first time.
 

gar

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Location
Ann Arbor, Michigan
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EE
151031-1353 EDT

An MR16 GU10 seems to be identified as a 50 W halogen incandescent bulb, except there is confusion because there are now LED versions, and also one source has two different base versions, one for incandescents and one for LEDs (120 V vs 12 V), with apparently a similiar identification.

A 50 W halogen load should not be much different than your 65 W standard incandescent. Since you mention 65 W I assume it is a reflector bulb. An incandescent would not normally use a ballast (whatever a ballast might mean --- for a gas discharge bulb it means an approximate constant current source).

A standard 25 W 120 V incandescent
20 V --- 0.09 A --- 1.8 W --- 222 ohms --- couldn't see any glow with some ambient light
40 V --- 0.12 A --- 4.8 W --- 333 ohms --- not much glow
80 V --- 0.17 A --- 13 W --- 470 ohms
120 V --- 0.21 A --- 25 W --- 571 ohms

A standard 60 W 120 V incandescent
20 V --- 0.19 A --- 3.8 W --- 105 ohms --- barely see glow
40 V --- 0.27 A --- 11 W --- 148 ohms
80 V --- 0.39 A --- 31W --- 205 ohms
120 V --- 0.48 A --- 58 W --- 250 ohms

Glow magnitude can be some indicator of voltage.

.
 

Little Bill

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UPDATE

UPDATE

Ok, I was able to get back to the nonworking light this morning well before the business opened. So I was able to spend more time checking things.

I'm going to put up a small defense for myself and just turn :ashamed1: for the most part though!:weeping:

It turns out the light was on a dimmer, hence the low voltage reading. I didn't know about the dimmer as it was not in the area where the light was. (part of my defense)

I do not carry these types of bulbs (MR16 with GU10 base). I was given 7 or eight bulbs that I assumed were new. (more of my defense) None of them worked but my pigtail socket w/65W incandescent did. They were able to get me a known working bulb from another fixture. I put it in and it worked. However, it didn't last long before it burned out. The light is a 4" recessed can and I can only guess that between the bulb being used already and the heat it gives off, that is the reason it failed. The can housing holds too much heat and the bulbs won't last.

I replaced the halogen with an LED of the same bulb type. Just have to wait and see if it will hold up in the recessed housing. Most LED bulbs say to not use them in a totally enclosed housing. Since the can doesn't have a closed front/lens/trim I'm hoping there will be enough air flow that it won't harm the LED. The face of the bulb is the lens, similar to any of the larger recessed can lights, except the bulbs in these are held at the opening and not up in the housing.

Now given my (small) defense, let the "flogging" begin!:D
 

gar

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Ann Arbor, Michigan
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EE
151102-2333 EST

Little Bill:

No flogging. Rather I thank you for coming back with some usefull informtion that makes sense. There are many that present a question and never provice any feedback.

An incandescent, regular or halogen, can tolderate a lot of heat without damage. Short life might imply over-voltage, and/or mechanical shock or vibration.

An incandescent is not very efficient in conversion of electrical energy into visible energy. But a lot of the wasted energy is radiated as infrared away from the fixture just like the visible light. So the percentage of the input energy to the bulb of the total input energy that remains in the fixture will be smaller than you might expect.

An LED is much more efficient in converting the total input energy to the bulb system ( the LED driver and the LED emitter ) than is an incandescent. But there is not much high temperature infrared energy radiated from an LED system compared to an incandescent. There is low temperature heat, below 200 F, and this mostly stays in the fixture, In the LED system the driver (electronics) can not tolerate anything near what and incandescent bulb can. Thus, much less heat buildup in an enclosure with an LED can produce a greater problem for the LED than a higher temperature for an incandescent.

I ran some temperature rise tests on an LED in an insulated can fixture a year or two ago. I believe I provided some of that data on this forum. See if you can find it. I am real concerned about putting LED bulbs in a regular fixture in place of an incandescent where there is no good ventilation, and the affect on LED bulb life.

.

.
 

growler

Senior Member
Location
Atlanta,GA
It turns out the light was on a dimmer, hence the low voltage reading. I didn't know about the dimmer as it was not in the area where the light was. (part of my defense).

Now given my (small) defense, let the "flogging" begin!:D


No "flogging" needed because things like this happen all the time. People think you can walk in the door and know exactly how a building is wired and yet there is never a set of prints showing the building wiring.

I went on a service call a few years ago and spent the first 15 minutes looking for a problem and then looked across the room and saw the switch. The problem was that this switch had been in the on position around the clock for long that the people had forgetten that the light even had a switch. To keep everyone (myself included ) from looking so dumb I wrote it up as a bad connection.
 

kentirwin

Senior Member
Location
Norfolk, VA
No "flogging" needed because things like this happen all the time. People think you can walk in the door and know exactly how a building is wired and yet there is never a set of prints showing the building wiring.

I went on a service call a few years ago and spent the first 15 minutes looking for a problem and then looked across the room and saw the switch. The problem was that this switch had been in the on position around the clock for long that the people had forgetten that the light even had a switch. To keep everyone (myself included ) from looking so dumb I wrote it up as a bad connection.

I had the same thing happen on a service call to a shoe store. Lights were out in the stock room. Checked the breaker I was told controlled the lights. Opened a ceiling tile and looked down the row of fluorescents and saw a conduit leaving a junction box and turning down into a wall. Walked to where it turned down and moved a bunch of shoe boxes on the shelf and voila, a forgotten light switch.
 
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