Odd voltage on electrical outlet

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  #1  
Old 02-17-07, 05:10 PM
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Odd voltage on electrical outlet

Yesterday I lost power on two outlets, so I did soem testing.

There are at least 10 outlets on a Ground Fault breaker. I used a multimeter to test across the non working outlets and found the following:
P to N = 86 volts
P to G = 118 volts
N to G = 16 Volts

I tested other outlets in, both in the same room in in different rooms, but on the same breaker, there results were
P to N = 118 volts
P to G = 118 volts
N to G = 0 Volts

Any idea what would cause the odd behavior on the two outlets?
 
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  #2  
Old 02-17-07, 05:15 PM
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Put away your digital multimeter. Do not use it for electrical testing until you learn how to interpret the results. Using an analog meter or a two wire tester would have provided you with results you could understand.

You have an open neutral. Somewhere a neutral connection has come open. It is at the first non-working location on the circuit or at the last working location. Check every junction box on the circuit. Remake an wire nut connections with new wire nuts and move any back stabbed connections to screw terminals.
 
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Old 02-17-07, 06:13 PM
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For those of us that have digital meters, and know how to read them, just how do you interpret the readings?
 
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Old 02-17-07, 06:15 PM
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OPEN neutral. You can read transiant voltages that can mess up a novice.

Always use an analog or coil type tester, and use bothh referance points, Ground AND neutral. One is not enough.
 
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Old 02-17-07, 07:56 PM
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Digital meters will read transient voltage, often referred to as ghost voltage. While the voltage is real, there is no current behind it, so anything that puts a load on the circuit, like an analog meter, will show nothing (no light, no voltage, etc.). The digital meter reads the voltage because it does not load the circuit.

Using an analog meter, or using a digital meter and understanding the results, the proper statement would have been,
P to N = 0 volts
P to G = 118 volts
N to G = 0 Volts,
instead of what was actually stated.
 
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Old 02-17-07, 08:37 PM
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The only thing a digital voltmeter is good for is telling the difference between 120 and 240 volts. For all other purposes, a simple $2 neon circuit tester is far more useful. The main thing I use my digital voltmeter for is testing batteries using the battery tester.

If you do use a digital voltmeter, interpret anything between 110 and 130 as 120, anything between 220 and 260 as 240, and every other possible reading as zero. And google "phantom voltage".

Keep in mind that a voltmeter only tells you the relative voltage between two points. You can test between two wires and get a reading of 0, and then still get killed touching the wires. You always need to test between one wire and a known good ground. You can even use your body for a ground if you have to.
 
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Old 02-17-07, 08:52 PM
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I use a digital meter but I also use a tester made of two 120 volt bulbs in series. dim =120 bright = 240. Make sure both bulbs are the same wattage.
 
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Old 02-18-07, 12:22 AM
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Originally Posted by John Nelson View Post
The only thing a digital voltmeter is good for is telling the difference between 120 and 240 volts. For all other purposes, a simple $2 neon circuit tester is far more useful. The main thing I use my digital voltmeter for is testing batteries using the battery tester.

If you do use a digital voltmeter, interpret anything between 110 and 130 as 120, anything between 220 and 260 as 240, and every other possible reading as zero. And google "phantom voltage".

Keep in mind that a voltmeter only tells you the relative voltage between two points. You can test between two wires and get a reading of 0, and then still get killed touching the wires. You always need to test between one wire and a known good ground. You can even use your body for a ground if you have to.
YEah it really unnerves some people at work when I hold one meter wire in one hand and touch the other to a live circuit.
 
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Old 02-19-07, 11:30 AM
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mcandka - Don't toss your DMM just yet. It is fine for measuring voltage as long as you recognize the possibility of an erroneous indication due to "phantom" voltage. Your DMM can be used to measure voltage accurately in a properly wired circuit and it can also be used to check continuity which can be handy when you're trying to figure out which buried wire goes where.

IMO when it comes to testing receptacles, nothing beats a 3 prong plug-in tester with 3 indicator lights. They are cheap, quick and simple to use. The ones I have even have a label that tells you how to interpret various light indications. It should be in every home owners electrical repair box.

Phantom voltage is nothing more than a charge on an open wire (such as your suspected open neutral) coupled from a nearby current carrying conductor. It is not a transient. A transient is more accurately used to describe short term voltage abberations such as the voltage spikes sometimes attributed to circuit interruptors (a switch). The voltage measured by a DMM can be whatever, depending on the coupling factors involved. An analog meter with a low input impedance will discharge the phantom voltage. A DMM will not.

There is a safety issue involved and that involves misinterpreting an actual voltage as "phantom". Say if you measured 85v on a circuit with a DMM , and wrongly assumed it was a harmless phantom voltage and grabbed on you could be injured.
 

Last edited by Wayne Mitchell; 02-19-07 at 11:46 AM.
  #10  
Old 02-19-07, 12:18 PM
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I highly endorse those three-prong three-lamp receptacle testers (usually sold under the less-precise name "outlet tester"). However, they have their own problems. The most notable problem is that (1) they often report open neutrals as hot/ground reverse. Other problems are that (2) they often quit working accurately after they've been dropped a few times, and (3) sometimes bleed-through from one lamp to the next makes you think a lamp is on that is not.
 
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Old 02-19-07, 02:50 PM
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John - The first "outlet tester" that I bought had the bleed over problem you mentioned. It was a hard semi translucent plastic. A couple of years ago I came across some cheapo's made in China from hard opaque rubber. I bought 3 or 4. I think you could play hockey with them and they would still work.
 
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Old 02-19-07, 03:51 PM
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Testing procedures aside, your problem is an open neutral. That means that the white wire has either come loose or burned off at one of the connection points along the circuit. The most likely place is at the last working receptacle or at the first non-working receptacle. To correct the problem, you must open up receptacles, switches and light fixtures on the circuit until you find the bad connection.

Chances are the problem is at a receptacle with a backstabbed connection. The wires are simply pushed into small spring-loaded holes in the back of the receptacle rather than wrapped around screws and tightened. The backstab connection weakens and fails over time. The solution is to move the failed backstabbed wires to the adjacent screw terminals on the receptacle. The open neutral could also be at a wirenut connection, so don't overlook those when you are looking for the break.
 
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Old 02-19-07, 06:09 PM
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Wayne: your explanation is great. ...
came across some cheapo's made in China .... (A whole 'nother web site..)

John: I like the 3-prong aswell And now you can (simply test GFCIs')


TESTERS: DMM's are great, Coil types are great, analog's are great, proximity's are great.
The big thing is to know how to use and read each one, they all have limitations. Any and all of these testers,used with out the knowledge of knowing how to interpret their answers, Is useless.

In any event... You still have an open neutral..... And all a tester can tell you is THAT. Now you must find it.
 
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