Strange AC Power Issue

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francesco
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Strange AC Power Issue

Post by francesco » Tue Aug 07, 2007 5:01 pm

I have 3 UPSs. All of them say there is a building wiring fault, but my outlet tester says everything is good. What could possibly be wrong?

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jwax
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Post by jwax » Tue Aug 07, 2007 5:27 pm

Need some detail- what exactly is the indicated "fault"?

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jollyrgr
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Post by jollyrgr » Tue Aug 07, 2007 9:33 pm

Get a digital multimeter that can measure AC line voltages.

Place one probe in the narrow outlet slot (HOT), the other in the wide outlet slot (NEUTRAL). You should see 110V to 120V AC.

With the first probe still in the narrow slot, move the other to the ground hole. Again you should see 110V to 120V AC.

Next move the probe out of the ground hole and to the screw holding the wall plate in place. (If painted you may need to unscrew it a bit and touch the threads of the plate screw.) Yes, you are touching the faceplate screw and expecting to see the 110V to 120V AC between the narrow slot and the ground screw.

Move the probe you have been moving around back to the wide slot and confirm the above voltages. This time move the probe from the narrow slot to all the other locations. If you get more than a volt or two you have a serious problem. Three to four volts (according to professional electricians) can sometimes be seen from the NEUTRAL to the GROUND in an isolated ground situation.
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francesco
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Post by francesco » Tue Aug 07, 2007 10:03 pm

jwax wrote:Need some detail- what exactly is the indicated "fault"?
Hi John, the UPSs don't give any details. The red LED indicator is just on. Didn't have that at the other apartment.

francesco
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Post by francesco » Tue Aug 07, 2007 10:07 pm

Ok, these are the results:

hot+neutral=118V
hot+ground=75V
neutral+ground=25V

What could the problem be?
jollyrgr wrote:Get a digital multimeter that can measure AC line voltages.

Place one probe in the narrow outlet slot (HOT), the other in the wide outlet slot (NEUTRAL). You should see 110V to 120V AC.

With the first probe still in the narrow slot, move the other to the ground hole. Again you should see 110V to 120V AC.

Next move the probe out of the ground hole and to the screw holding the wall plate in place. (If painted you may need to unscrew it a bit and touch the threads of the plate screw.) Yes, you are touching the faceplate screw and expecting to see the 110V to 120V AC between the narrow slot and the ground screw.

Move the probe you have been moving around back to the wide slot and confirm the above voltages. This time move the probe from the narrow slot to all the other locations. If you get more than a volt or two you have a serious problem. Three to four volts (according to professional electricians) can sometimes be seen from the NEUTRAL to the GROUND in an isolated ground situation.

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jollyrgr
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Post by jollyrgr » Tue Aug 07, 2007 10:32 pm

francesco wrote:Ok, these are the results:

hot+neutral=118V
hot+ground=75V
neutral+ground=25V

What could the problem be?
This is a good one. Find and unplug everything on that circuit. I'm expecting that there is a "minor" short between HOT and ground somewhere. This could be windings in a motor or transformer partially shorted to ground. I also suspect that the ground is not truly grounded. This is because you are seeing 25V between neutral and ground. This should be ZERO volts in an ideal situation. If after unplugging everything on that circuit you still get the strange results, there is likely a short in the building wiring.

Another test is to find a SOLID ground you know is good (like a copper cold water pipe at the water meter, a ground rod at the service entrance, a ground rod for an antenna, satellite, cable TV, or phone box, etc.). Measure from this ground to your outlet ground. If you see 25V there you need to fix the grounding circuit in your electrical service.

I suspect SOMETHING that has a ground lug on that circuit has a short or other problem. Hot and ground voltage measurements should be the same as hot and neutral. To have 25V between neutral and ground is a serious issue. One other test you could perform is to hook a light bulb between neutral and ground. If the bulb glows dimly you need to correct a serious problem.
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dyarker
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Post by dyarker » Wed Aug 08, 2007 7:53 am

Or a high resistance (relatively for AC wiring) in a neutral connection. If neutral to ground voltage decreases as load is disconnected this might be the problem. If neutral voltage stays the same, or drops suddenly as loads are disconnected, its probably a short as mentioned above.

Moved appartment. So you don't own it. Either way problem is, it is serious. Landlord needs to call electrician NOW!
Dale Y

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Post by Robert Reed » Wed Aug 08, 2007 9:58 am

Hello Franscesco
First of all - give your "AC tester" the deep six, as it doesn''t appear to be working. If there was any appreciable resistance in the hot and neutral leads back to the service entry, you would have noticed this immediately upon loading it. I see this strictly as a poor ground connection somewhere along the line. However, before condemning the whole system, try this - take similar voltmeter readings at a different outlet in your apartment and maybe even in an adjacent apartment. If they look good, the problem may be as simple as a bad ground connection on the duplex receptacle that you are operating off of. If not, then its definately time to call the electrician.

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dacflyer
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Post by dacflyer » Wed Aug 08, 2007 1:45 pm

i have tested outlets also with a heavy wattage lamp,
hot to neutral and hot to GND.
once had a problem in a old house that still used cartridge fuzes (shot gun shell looking fuzes ) and screw in types.. all was normal one day then the lights went very dim,,the owner was using the dryer, and a A/C at the same time, and it was enough to blow one of the main fuzes, well when it did this the power inthe house was upset,, when there was a heavy load the lights were brighter,, and as you turned things off the lights would get dimmer..lol the proper fuzes were replaced and then all was back to normal... 4 yrs later. the fuze box went neuclear! the material the box was made of,melmac?? somehow started conducting..it became a fireworks show between the mains before the main fuzes. :O
needless to say its a good thing he was home back then..saved his house from burning down,, a new electrical box was in order..

so beware of bad connections where ever they may be.. bad connections become arcs when under load. and then can become fires..

i am so glad solid aluminum wire has been banned,,but theres a lot in houses here still.. to me that sort of wire is like fuze wire..i hate it.

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Chris Smith
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Post by Chris Smith » Wed Aug 08, 2007 1:56 pm

Sounds like poor grounding.

My work shop had that problem, 17 volt loss due to old wiring.

Also check if your pole, main box and / or circuit breakers has a ground, and or a ground to natural.

For a quick check rig up a extension cord with the natural and ground wires available.

Wear gloves or touch nothing!

Then find or make a great ground out side and hook up a decent wire to one and then the other, and then both.

Then see if there is a fault using the DVM and then the UPS and then both.

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jollyrgr
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Post by jollyrgr » Wed Aug 08, 2007 7:49 pm

I agree with there being a possible ground circuit issue (hence my suggestion to measure between a cold water pipe and electrical ground). But SOMETHING has to be putting voltage on the ground leg. Stray inductance and an open ground is one thing. But if a usable current exist there is a REAL DANGEROUS situation that needs to be corrected.

I'll expand on my lamp suggestion. Take a lamp socket and connect two wires to it. You can separate a line cord for this. Using insulated probes or being careful with twisted ends of wire, connect the wires as you did in the voltage measuring task I posted about. You should get the same brightness from the light when going from the HOT and NEUTRAL as between HOT and GROUND. In no other combination should you see any light. If the bulb glows even a dim amount, there is a serious problem.

I'm still going to guess there is a problem with the ground circuit and there is something plugged in, with a ground plug, that is shorting to ground. And the ground circuit is NOT doing its job.
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dacflyer
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Post by dacflyer » Wed Aug 08, 2007 8:29 pm

ya,, thats what i was trying to say Jolly, about my lamp test.. :)

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Chris Smith
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Post by Chris Smith » Wed Aug 08, 2007 8:34 pm

Caution is the story here.

Touch nothing!

Also is your wall sockets two plug, or three?

Older houses in the 50s or less had two prongs, the neutral was always grounded at the box, as well as to neutral on the line.

Mine is newer, with the pole line neutral and the local ground being the same.

Then there is the house.

The house panel is also to a ground pole, which happens to be the neutral wire and negative.

The neutral leads in the house can float to read a ground interrupt, but everything is dumped way before it goes down the mains. [A difference]

Three prong is similar but different, but grounds are needed regardless as the main safety, even if it’s the water pipes, a 6 foot copper rod, or what ever they have chosen or you then chose to install.

Extra grounds don’t usually hurt, they merely insure a proper “dumpingâ€

Robert Reed
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Post by Robert Reed » Wed Aug 08, 2007 9:01 pm

QUOTE:
Extra grounds don’t usually hurt, they merely insure a proper “dumpingâ€

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Chris Smith
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Post by Chris Smith » Wed Aug 08, 2007 10:08 pm

Is that two wires or three?

In the Two wire system it is called a ground, A.K.A. neutral.

The sole purpose of the modern "Ground" is to sense a fault in the three wire system, which by the way, is also attached to "GROUND" at the mains.

Just because it supposed to be passive and not carrying the load, doesn’t for one second mean it doesn’t represent a “Groundâ€

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