Go Back  DoItYourself.com Community Forums > Electrical, AC & DC. Electronic Equipment and Computers > Electrical - AC & DC
Reload this Page >

Getting Neutral-Ground voltage, and low Hot-Ground voltage

Getting Neutral-Ground voltage, and low Hot-Ground voltage

Reply

  #1  
Old 10-26-13, 04:36 PM
Member
Thread Starter
Join Date: Oct 2013
Posts: 5
Getting Neutral-Ground voltage, and low Hot-Ground voltage

Hi, I've just bought an older house (built 1960's) and have come across a wiring problem. On one of the circuits in the house, I get the following readings on a multimeter set to measure AC voltage:

Hot-Neutral: 120V
Hot-Ground: 78V
Neutral-Ground: 42V

As far as I know that's wrong and very bad and I'm going to die. Devices plugged into this circuit work, as do switches and lamps, but I'm guessing none of them are grounded properly.

There's only 1 circuit in the house with this problem. All the rest measure:
Hot-Neutral: 120V
Hot-Ground: 120V
Neutral-Ground: 0V

...which is I think correct.

Is there a DIY fix to this, or should I be calling an electrician? And is this something even an electrician can fix without tearing out half the wiring?

Thanks!
 
Sponsored Links
  #2  
Old 10-26-13, 05:14 PM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,374
I'll bet you are using a digital multimeter, right? If so the odd voltages are most likely what we call "phantom voltages" and are caused by the high input impedance of the voltmeter. Or in other words, you do not have a decent equipment ground connection. To test further connect a standard incandescent light bulb in parallel with your meter's test leads and re-do the tests. The load of the light bulb overcomes the high impedance of the meter.
 
  #3  
Old 10-26-13, 06:02 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
Welcome to the forums!

I've... come across a wiring problem. On one of the circuits in the house, I get the following readings on a multimeter set to measure AC voltage:

Hot-Neutral: 120V
Hot-Ground: 78V
Neutral-Ground: 42V

As far as I know that's wrong and very bad and I'm going to die. Devices plugged into this circuit work, as do switches and lamps, but I'm guessing none of them are grounded properly.
First of all, thanks for the clear information with meter readings.

Yes, if those readings are real, that is puzzling. You shouldn't be seeing those readings. I'm not convinced, though, that what you're seeing presents a clear and present danger.

For one thing, there's no reason to suppose that whatever is causing this hasn't been present for years, and hasn't caused a catastrophe in all that time. For another, it's interesting that the sum of the "errant" readings is 120V.

Some questions, to help us understand this better:
  1. Are you using an analog or a digital multimeter?
  2. How did you determine that this is only happening on one circuit?
  3. Have you checked every receptacle on that circuit?
  4. Are you testing on disconnected wires or in the receptacle slots?
  5. Where are the receptacles on this circuit located?
  6. And, what did you observe that prompted you to test for voltage at your receptacles?

There's only 1 circuit in the house with this problem. All the rest measure:
Hot-Neutral: 120V
Hot-Ground: 120V
Neutral-Ground: 0V

...which is I think correct.
Yes, that's correct.

Is there a DIY fix to this, or should I be calling an electrician?
If there's an actual problem - which your answers to our questions will help us determine - it should be something you can fix yourself.

And is this something even an electrician can fix without tearing out half the wiring?
We won't know the answer to that until we determine the nature and source of the problem.
 
  #4  
Old 10-26-13, 06:50 PM
Member
Thread Starter
Join Date: Oct 2013
Posts: 5
Thanks for the help guys.

How I found this:
I was attempting to replace a simple 2 position light switch with a motion activated switch. The motion activated switch has to be grounded. It had 4 wires - 2 wires replacing the ones off the existing switch (hot and the line to the light I guess) and 2 wires that had to be connected to ground. There was a bare copper wire in the box, connected to the frame itself, which I'm assuming is ground and what I connected it to.

But the motion switch won't work. Won't work as a simple switch, and no indicator lights on indicating it's even getting power. Its manual seems to indicate that it's not working because it's not connected to ground.

Which is how I found myself plugging multimeter leads into my walls seeing what was grounded or not. (I really should get a 3 prong tester, I know).

On this circuit, I've observed the same 120V/78V/42V issue on three outlets in the hall and neighbouring bedrooms, as well as the bare wires from within the switch I was trying to replace. I've determined they're on the same circuit, because everything goes off in those rooms when I trip that particular breaker.

I've tried every other accessible outlet in the house (tried about 15 in total), and have found them all to be 120V/120V/0V as discussed. Which is how I figure this problem is only on this circuit.

I was using a digital multimeter. A previous poster suggested wiring it in parallel with an incandescent light and testing again. That results in:

Hot-Neutral: 120V (light comes on)
Hot-Ground: 0V (no light)
Neutral-Ground: 0V (no light)

That doesn't look right to me. I should be seeing the light come on when I connect Hot-Ground right?
 
  #5  
Old 10-26-13, 06:55 PM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 32,667
2 wires that had to be connected to ground.
Very unlikely it has two ground wires. If one of those four wires on the switch is a white wire it does not go to ground. It goes to neutral which many older switch boxes do not have.
 

Last edited by ray2047; 10-26-13 at 08:16 PM.
  #6  
Old 10-26-13, 07:04 PM
Member
Thread Starter
Join Date: Oct 2013
Posts: 5
Sounds unlikely, but i guarantee they're both supposed to go to ground:

http://www.1000bulbs.com/pdf/lutron-...itch-specs.pdf
(It's the MS-OPS2. Wiring diagram on page 8).

Green wire comes from within the switch. The bare wire visibly terminates in the plastic frame of the switch without contacting anything electrical.
 
  #7  
Old 10-26-13, 08:24 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
i guarantee they're both supposed to go to ground:
That's what the instructions you linked to say. The troubleshooting guide from the manufacturer, Lutron says white to neutral and bare to ground.
 
  #8  
Old 10-26-13, 08:32 PM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 32,667
but i guarantee they're both supposed to go to groundl
Extremely weird. It may use the ground as current return path. That is permitted if the draw is 5 Ma or less.

I really should get a 3 prong tester, I know).
Not as good as a multimeter and an analog multimeter is best. Are you using a digital or analog?

Does the box have a ground wire? If not is it wired with metallic cable? If non metallic cable (AKA Romex) and no ground wire then you may need to replace the cable. or add a ground wire. As a test only I would run a insulated wire from a known good ground in a receptacle box to the green wire on the switch and see if the switch works. If it does you will have to add a ground wire in an approved way either by replacing the cable or running a separate wire for ground from the breaker box.
 
  #9  
Old 10-26-13, 08:45 PM
Member
Thread Starter
Join Date: Oct 2013
Posts: 5
The troubleshooting guide from the manufacturer, Lutron says white to neutral and bare to ground.
That's for a different model number. The MS-OPS6M2N-DV and MS-VPS6M2N-DV. I have the MS-OPS2, which doesn't have a white wire coming off of it.
 
  #10  
Old 10-26-13, 09:49 PM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 32,667
But is that wire green as shown in the diagram or is it white?

Name:  insert21.jpg
Views: 17526
Size:  25.0 KB

I'm guessing it is the exact same switch as the ones with white wires and they just changed to green for switch boxes with no neutral. It is the same "return path". See my previous post.
 
  #11  
Old 10-26-13, 09:51 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
hat's for a different model number. The MS-OPS6M2N-DV and MS-VPS6M2N-DV.
So it is.

Time to grab an analog meter and start troubleshooting. See Troubleshooting a dead receptacle or light, Basic Terminology & Other info. Yes, I know your switch isn't dead, but troubleshooting is troubleshooting.
 
  #12  
Old 10-27-13, 12:20 AM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,374
Going back to post #4 the testing I suggested in post #2 proves there is NO EQUIPMENT GROUND on this circuit. Somewhere the bare copper wire in this circuit is broken or at the very least does not make a complete circuit back to the circuit breaker panel. It could be in any of the receptacles, light switches or light fixtures on this particular circuit or in the panel itself.
 
  #13  
Old 10-27-13, 03:08 PM
Member
Thread Starter
Join Date: Oct 2013
Posts: 5
Thanks guys. Would an appropriate troubleshooting routine from this point on look a bit like this?

1) Shut off all Circuit Breakers
2) Shut off Main Breaker
3) Remove cover revealing the circuit breaker wiring
4) Verify the trouble circuit is grounding. The white and copper wires from this circuit will be grounding to a single bar, or to separate bars, depending on how this home was wired. I'll have to figure out which it is, and then figure out whether this circuit's ground connection isn't there, or somehow unlike the others.

If that all looks fine, I would:
5) Put the breaker panel back together. Turn on the main breaker and all the other circuit breakers.
6) Make a list of all the fixtures on the trouble circuit. Basically turn the breaker for the trouble circuit off and plug in an incandescent light in all the fixtures, making a list of which ones aren't powered.
7) One at a time, starting closest to the breaker panel, take each fixture apart and see if its grounding is disconnected, not present, or somehow messed up.

Do I have that right?
 
  #14  
Old 10-27-13, 03:21 PM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,374
I would go the other direction. Starting at the switch, receptacle or junction box closest to the problem inspect all the wiring on this problem circuit back to the circuit breaker panel.
 
  #15  
Old 10-27-13, 09:16 PM
Nashkat1's Avatar
Member
Join Date: Mar 2012
Location: USA
Posts: 8,470
Remember, you only need to find and repair the splice where the grounding conductor for this circuit wasn't carried forward.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
'