Line fault at the outlet -- do I need an electrician?

almarg9,459 posts

03-23-2020
2:59pm


Hilde45, when you measured the 4 volts while the system was plugged in, were the components in the system turned on, or were they turned off or at least in standby? And as far as you know was anything else that is on the same branch turned on at the time?

If a lot of stuff was turned on at the time it increases the likelihood that the explanations cited by Erik and Jim apply. If not, it increases the likelihood that my hypothesis applies, namely that applying 120 volts to the AC neutral input of the component(s) resulted in excessive AC leakage to ground, mainly via their power transformers. Or, as heaudio123 alluded to, if significant current was being drawn by the components or other things the cause could have been a combination of both factors.

If my hypothesis was the main contributor, though, that issue should go away once the hot and neutral connections are reversed to what they should be.

Best regards,
-- Al

Al, (almarg),

First off we don’t know if the OP’s test is valid. We are only assuming that there is/was an actual difference of potential of 4 volts between the neutral conductor and the equipment grounding conductor.

What I do know unless a load is connected to a branch circuit there will not be an actual difference of potential between the neutral conductor and the equipment grounding conductor. (To quantify that statement we assume the equipment grounding conductor is connected to the neutral conductor at the main electrical service panel)

On the same dedicated circuit that I ran tests on yesterday, using a Fluke 87 DMM, with nothing plugged into the duplex outlet I just measured a mains voltage of 121.8Vac. From the neutral contact to the equipment ground contact I measured 1.0mV - 1.1mV. Jmho, that is a phantom voltage and not a real, actual voltage reading.

I could intentionally add series resistance to the neutral conductor, or to the equipment grounding conductor and that would not cause, on its’ own, a difference of potential, voltage, between the two conductors. (No Connected load)


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For all:

What can, does, cause an actual voltage to exist on the equipment grounding conductor is a connected load on the branch circuit. Can’t have a difference of potential between the neutral conductor and equipment grounding without a connected load on the hot and neutral conductor. The voltage can be an induced voltage caused by the current carrying Hot and neutral conductors. Though in that case the voltage will be normally in the mV range.

The other thing that causes a voltage to exist between the neutral conductor and the equipment grounding conductor is VD (Voltage Drop) on the circuit caused by a connected load. No load, no VD....
Causes of voltage drop:
http://www.adamselectric.coop/wp-content/uploads/2015/02/Voltage-Drop.pdf

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Al, (almarg),

If a lot of stuff was turned on at the time it increases the likelihood that the explanations cited by Erik and Jim apply. If not, it increases the likelihood that my hypothesis applies, namely that applying 120 volts to the AC neutral input of the component(s) resulted in excessive AC leakage to ground, mainly via their power transformers.

I gave your theory some thought and I can see where there would be an increase of current on the equipment grounding conductor as more leakage was present but I cannot see where the voltage would increase. If anything the voltage would decrease. Wouldn’t more leakage translate into a lower neutral to equipment ground resistance? The lower the resistance, the higher the current, the lower the voltage.
Extreme example. 0 ohm resistance between the neutral and equipment grounding conductor.

Jim
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I gave your theory some thought and I can see where there would be an increase of current on the equipment grounding conductor as more leakage was present but I cannot see where the voltage would increase. If anything the voltage would decrease. Wouldn’t more leakage translate into a lower neutral to equipment ground resistance? The lower the resistance, the higher the current, the lower the voltage. Extreme example. 0 ohm resistance between the neutral and equipment grounding conductor.

Hi Jim,

With hot and neutral reversed and a component turned off 120 volts will be applied to the end of the primary winding of the power transformer in the component which normally receives AC neutral. AC neutral presumably goes nowhere in the component in that situation (other than perhaps to a line filter capacitor), assuming the component doesn’t have a standby mode or is not in standby mode, because its power switch is turned off. The 120 volts will leak to some degree to the chassis/AC safety ground of the component via stray capacitance in the transformer. That degree being different and I would imagine probably greater (perhaps much greater) than under normal circumstances when 120 volts is applied to the other end of the transformer via the power switch. So what I’m envisioning is that the reversal of hot and neutral will result in a significant voltage being applied to the safety ground, resulting in significant current through the safety ground wiring while no current exists in the neutral wiring, resulting in a significant voltage developing between safety ground and neutral as a result of that leakage.

I could be wrong about that, and admittedly creation of a 4 volt difference would seem to require a great deal of current. But nevertheless it seems to me to be a possible explanation. And as I said, if that is the main contributor to the 4 volts the issue will go away when hot and neutral are connected properly, without the need for further corrective action involving the wiring and connections.

Best regards,
-- Al

Al, (almarg),

I drew 3 vertical parallel lines on a piece of paper. One representing the Hot, one for the neutral and the other the equipment ground. At the top of the drawing I drew a horizontal line connecting both the neutral line and equipment line together. (Representing how the equipment grounding conductor is connected to the neutral at the electrical panel.)

At the bottom of the neutral and equipment ground lines I connected a digital meter.

Above the meter I drew the symbol of a resistor. I connected one lead to the hot and the other lead to the equipment ground. The resistor represents the leakage you spoke of in your post.

With the power switch of the equipment in the off position there will not be any hot and neutral current carrying load. Therefore there can not be VD. No VD, no difference of potential between the neutral conductor and the equipment grounding conductor. (At least originating within the branch circuit, and the piece of equipment connected to the branch circuit.)

Using your leakage theory I see what you mean when the mains Hot conductor is directly feeding the primary winding of the power transformer. I can see if there is a leakage to the chassis there would be current on the equipment grounding conductor. I am not sure you would measure a difference of potential from the equipment ground to the open switch open neutral though. Especially if the neutral conductor and equipment ground conductor are at the same ground potential.

Now if the leakage turns into a ground fault event and there is then considerable current causing a VD in the Hot / equipment ground (ground fault) circuit then indeed there would be a difference of potential from the equipment ground to the open switch open neutral.

I could be wrong though..... It wouldn’t be the first time.

Jim
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@ cissado

cissado19 posts

03-23-2020
11:26am



2 tips. Be conscious of any switches outlets when testing, so there are no surprises.
When trying to find the first receptacle box, you can disconnect one pair of wires from the receptacle. THEN reapply power to find out if you’re in the beginning or middle of the circuit.

@ cissado
Reading your entire post from 03-23-2020 you have a background in electrical wiring/methods.


This was my response to your post.

jea483,307 posts  

03-23-2020  
 12:03pm  

@ cissado

Good post except in the case of where the duplex receptacle device may have been used as a junction for the make up of neutral and Hot conductors coming in and going out of the outlet box.

We don’t know what type of wiring materials/methods were used for the branch circuit wiring.

What year NEC was in effect at the time? What were the AHJ electrical code standards/requirements back then for where he lives? Was conduit required in basements back then? Is there a chance the branch circuit is part of a multi wire branch circuit? What happens if he breaks the feed neutral at an outlet and the other circuit of the multi wire branch circuit has a connected load on it?

The OP is not an electrician. An electrician would know what to look for. Like another Hot circuit conductor passing through the box he is about to open a neutral. An open neutral on a multi wire branch circuit has killed many a electricians.

This was your response to my above post.

cissado19 posts  

03-23-2020   
4:17pm

I’m not sure what this means exactly. Maybe because I wasn’t clear in my post. I usually am not clear... it’s my bad writing skills.
I just meant the first receptacle may have been fed from a light fixture. Also, that the wiring inside the light fixture box could be wrong.

It was this part of your post from 03-23-2020 that I was addressing.

When trying to find the first receptacle box, you can disconnect one pair of wires from the receptacle. THEN reapply power to find out if you’re in the beginning or middle of the circuit.

Maybe I was not clear. The point I was trying to make, because the OP admittedly called himself a novice and knows very little about electrical power systems and or electrical wiring and wiring methods, it may not be a good idea to open a branch circuit neutral even though the OP turned off the breaker that feeds the circuit he is working on.
We don’t know, and the OP definitely doesn’t know, if the branch circuit he is dealing with is a 2 wire with ground branch circuit ( Hot, Neutral, and ground), or possibly part of a 3 wire multiwire with ground branch circuit. Multiwire, therein 120/240V 3 wire multiwire branch circuit consisting of 2 hot conductors with a shared neutral conductor.
(Two 120V circuits that share a common neutral conductor.)


Best regards,
Jim

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