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Help needed: bizarre voltage readings

Feeling that my system can sometimes sound a lot worse than usual, I got a voltage meter to check if something was wrong with my AC power. Imagine my surprise when I found that one of the receptacles measured 122V between hot and neutral, 4V between ground and neutral, and 4V between ground and hot. HOW IS THIS EVEN POSSIBLE? If hot and neutral are 122V apart from each other, they simply cannot both be 4V away from ground, right? I tried taking the readings several time and they didn't change.

I tried the meter on several other outlets, including a couple that are on the same circuit as the problem outlet, and they all measured okay: 120-122V between hot and neutral, 120-122V between hot and ground and 0V between neutral and ground. So at least we know the meter is okay.

Please help. I'm really confused.
cpclee
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jea48
4,412 posts
 
Imagine my surprise when I found that one of the receptacles measured 122V between hot and neutral, 4V between ground and neutral, and 4V between ground and hot. HOW IS THIS EVEN POSSIBLE? If hot and neutral are 122V apart from each other, they simply cannot both be 4V away from ground, right?
Cpclee
You have an open equipment ground.
It could be caused by several things.
* Not made up on the receptacle. Wire may be broken at recept.
*No equipment grounding conductor at the recept rough-in box.
* Open joint at a feed through of another recept rough-in box, or a junction box.

4 Vac reading you are getting? Phantom voltage..... Typical with digital meters.

Connect a load like a 25 watt light bulb in parallel with the meter leads and take a reading....
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shadorne
7,477 posts
 
BTW - since a path to ground via a grounding rod may not be very good - this also bypasses the safety protection on your equipment with three prongs - the idea is that the chassis is both grounded and connected to neutral at the panel which means that in the event of an equipment fault you will not encounter high voltages when you touch a chassis and something else that happens to be grounded.
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shadorne
7,477 posts
 
Appreciate the article. But if the ground outlet's ground is not connected to anything, should the reading give 0V instead of 4V between hot and ground and between neutral and ground?

Voltages are everywhere...if the ground is connected to a grounding rod (earth) and not to the neutral at the panel then you are simply measuring the voltage between the floating transformer from your utility and "earth" wherever your "earth ground rod" happens to be buried.

This is not a good thing - if you have a lightning storm and it hits the powerlines then these may carry very high voltages into your components within your home as lightning seeks a path to "earth"...in finding a path of least resistance it may burn something - sometimes even several items can get fried!
cpclee OP
3 posts
 
Hi Dpac996: The answers to your questions are (1) TRUE and (2) FALSE. That's why I've been so confused.
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dpac996
692 posts
 
Cpclee,
True or False:
1)You measured this with ALL your components unplugged
2)The receptacle (where you measured 122-0-0) was on a different circuit (off a diff breaker) from the outlet that measured 122-4-4.

If 1 and 2 are both true, I have no idea why the 4 volt offset is there.

If 1 is false: In some power supplies there are diodes that are connect the chassis to ground in both directions. The chassis could be completing a strange trip back to earth ground and is causing the offset.

If 1 is true and 2 is false seek immediate shelter (just kidding)

Please let me know as much about what was going on that day
gs5556
1,233 posts
 
Get a 3-prong circuit tester and see if the receptacle is wired correctly. If it is, get an electrician to check the wiring at the panel. It sounds to me like you have a reversed hot neutral with an open ground somewhere.
cpclee OP
3 posts
 
Appreciate the article. But if the ground outlet's ground is not connected to anything, should the reading give 0V instead of 4V between hot and ground and between neutral and ground?
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shadorne
7,477 posts
 
Check this Whitlock document out about ground and ground loops.
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shadorne
7,477 posts
 
Neutral is normally grounded at the panel - it seems your ground is not connected to anything.
cpclee OP
3 posts
 
On a related note, does anyone know what effect a non-zero ground voltage can have on the sound quality? Thanks.
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