Strange AC Potential

[AxisCat]

I am trying to understand the underlying mechanics of why I am getting a voltage potential from a metal cabinet to my body.

The electrical cabinet has an LED light fixture in it. I live in a very old house so there is no ground wiring. I was noticing a very slight shock when I brushed up against the cabinet but after the initial shock holding my hand on it, I felt nothing.

I took my multimeter on the AC setting and read 68v by touching one lead to the metal cabinet and the other being held in my hand.

I then ran a wire from a know ground in the house and got the same voltage reading between the metal and ground.

I tried putting a 120v 75 watt incandescent bulb in series with the metal and ground to see if there was any noticeable current flow, I didn't see any. Checking voltage across the bulb showed the same 68v.

My first thoughts is a short or something in the LED fixture to the metal cabinet. But if it was a short I would expect to current to flow when I gave it a path to ground.

I want to recreate this condition for another reason on a different application, anyone got any deep electrical theory they want to share?

Thanks,
Axis

 

[pc1966]

I am trying to understand the underlying mechanics of why I am getting a voltage potential from a metal cabinet to my body.

The electrical cabinet has an LED light fixture in it. I live in a very old house so there is no ground wiring. I was noticing a very slight shock when I brushed up against the cabinet but after the initial shock holding my hand on it, I felt nothing.

I took my multimeter on the AC setting and read 68v by touching one lead to the metal cabinet and the other being held in my hand.

I then ran a wire from a know ground in the house and got the same voltage reading between the metal and ground.

Generally modest voltages with low current are the result of capacitive coupling, often between the line (live/hot) and any metal or CPC conductor (earth/ground) that is not actually grounded.

I tried putting a 120v 75 watt incandescent bulb in series with the metal and ground to see if there was any noticeable current flow, I didn't see any. Checking voltage across the bulb showed the same 68v.

If you had 68V over a 120V lamp it would be glowing for sure! Are you sure it was a working lamp?

It is most likely a result of not having grounding in your house. It is a real risk and here in the UK that would be a failure on electrical inspection from before WW2 (except for light circuits, they were only mandatory with ground from 1966). However, the USA has other regulations and you might be OK with that if adequately protected by GFCI.

Someone like @Megawatt is better qualified than me to comment here.

 

[AxisCat]

If you had 68V over a 120V lamp it would be glowing for sure! Are you sure it was a working lamp?

Good point, I better check the lamp to make sure it is good...but yes that is the reading I had across the lamp.

It is a real risk and here in the UK

Right. It is a rental so I probably need to bring it up to the landlord....

 

[Lucien Nunes]

A multimeter will sense voltage even when the available current is in the order of microamps. Capacitive coupling can easily cause this level of current to be available from metal bodies nearby to energised NM and other unshielded cables. I am skeptical about whether your lamp test was valid, I agree with @pc1966 that the lamp might was probably open-circuit when the 68V was still being registered across it. My hunch is that a working lamp will cause the voltage to collapse to near zero.

 

[Megawatt]

I am trying to understand the underlying mechanics of why I am getting a voltage potential from a metal cabinet to my body.

The electrical cabinet has an LED light fixture in it. I live in a very old house so there is no ground wiring. I was noticing a very slight shock when I brushed up against the cabinet but after the initial shock holding my hand on it, I felt nothing.

I took my multimeter on the AC setting and read 68v by touching one lead to the metal cabinet and the other being held in my hand.

I then ran a wire from a know ground in the house and got the same voltage reading between the metal and ground.

I tried putting a 120v 75 watt incandescent bulb in series with the metal and ground to see if there was any noticeable current flow, I didn't see any. Checking voltage across the bulb showed the same 68v.

My first thoughts is a short or something in the LED fixture to the metal cabinet. But if it was a short I would expect to current to flow when I gave it a path to ground.

I want to recreate this condition for another reason on a different application, anyone got any deep electrical theory they want to share?

Thanks,
Axis

If you had on shoes then you did not give it a path to ground and that’s why you didn’t get shocked because there is no difference of potential between you and your cabinet. Drive a price of pipe or anything metal into the ground and measure the voltage between the grounded electrocuted and your cabinet. Could you please send us a picture of your main panel and your ground rod for your panel.

 

[AxisCat]

Thanks for the input friends. My initial thought was it acted like a capacitor, when I touch the metal housing with one lead and held the other in my hand I thought I saw the voltage drop to zero after a few short seconds. More investigation proved this to be false, the voltage remained steady.

My experiments have been a lil on the hurried side with just jabbing wires here and there into the ends of extension cords, all the things you shouldn't do; Tomorrow I will do some more regimented testing and will report back the results.

 

[AxisCat]

Thanks Megawatt, actually I was barefoot on a wood floor. And I did get a little shock on the initial touch but nothing further after holding my hand on the metal cabinet.

I did run a wire from a known ground in the house wiring and got the same voltage reading to that conductor. I was afraid to just completely short the cabinet to ground as I don't know how much current might flow, I don't want to burn something up.

This is a very old house I am renting, and the main panel is a nightmare.... I am used to working around high voltage stuff 460 3-phase so it doesn't bother me too much. If I sent you a picture it would be a great example of all the things that one shouldn't do.

 

[Megawatt]

Thanks for the input friends. My initial thought was it acted like a capacitor, when I touch the metal housing with one lead and held the other in my hand I thought I saw the voltage drop to zero after a few short seconds. More investigation proved this to be false, the voltage remained steady.

My experiments have been a lil on the hurried side with just jabbing wires here and there into the ends of extension cords, all the things you shouldn't do; Tomorrow I will do some more regimented testing and will report back the results.

Keep in touch my friend

 

[Megawatt]

Thanks Megawatt, actually I was barefoot on a wood floor. And I did get a little shock on the initial touch but nothing further after holding my hand on the metal cabinet.

I did run a wire from a known ground in the house wiring and got the same voltage reading to that conductor. I was afraid to just completely short the cabinet to ground as I don't know how much current might flow, I don't want to burn something up.

This is a very old house I am renting, and the main panel is a nightmare.... I am used to working around high voltage stuff 460 3-phase so it doesn't bother me too much. If I sent you a picture it would be a great example of all the things that one shouldn't do.

Short it to the cabinet and see what happens

 

[AxisCat]

Short it to the cabinet and see what happens

So a better test determined with the lamp in series with the cabinet and ground the voltage did collapse to 0.

What is the best way to characterize what I am seeing here?
Voltage leaking from the main somewhere to cabinet that is creating a capacitance effect?

Do you think it is possible that I can measure the capacitance in say uF or pF?