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?

 

[pc1966]

If you have a multimeter that can measure down to a few mA then measure the current 'I' to a rod in the ground. You can measure your AC voltage 'V' as well, or just assume 120V, then the impedance is

Z = V / I

If you can assume the AC is basically a sine-wave of 60Hz then your capacitance C is given by:

C = 1 / (2 * 3.141592 * 60 * Z)

Typically power cable is in the 150-300pF/m range so from C and typical cable values you can estimate the length of cable causing it. If it is a sane answer, like 5-50m, then you know. If massively wrong then check your measurements and units again!

 

[AxisCat]

If you have a multimeter that can measure down to a few mA then measure the current 'I' to a rod in the ground. You can measure your AC voltage 'V' as well, or just assume 120V, then the impedance is

Z = V / I

If you can assume the AC is basically a sine-wave of 60Hz then your capacitance C is given by:

C = 1 / (2 * 3.141592 * 60 * Z)

Typically power cable is in the 150-300pF/m range so from C and typical cable values you can estimate the length of cable causing it. If it is a sane answer, like 5-50m, then you know. If massively wrong then check your measurements and units again!

Great info!!! My meter does have a setting for mA. It has both AC and DC mA but I have only used the DC setting in the past where the meter is in series with the load. To be honest I don't even know how to measure AC mA with a meter... can you give me a quick tutorial? Is the meter in series like when measure DC current?

 

[AxisCat]

I figured out how to measure AC capacitance with my meter

So here is my math:

I don't have a feel for these values, do they look 'sane' to you?

 

[Lucien Nunes]

Yes and no.

First, when you are calculating the capacitive reactance, consider what capacitance you are interested in: the capacitance to the hot wire only, or the capacitance to everything including neutral and ambient ground in parallel with the capacitance to the hot wire?

The voltage on the cabinet when floating is below the line voltage as a result of a voltage divider comprising the capacitance to hot and the capacitance to neutral/ground. When you connect the ammeter to ground, you are effectively grounding the cabinet through its very low resistance, shorting out the stray capacitance to ground and measuring the current when the full line voltage is across the capacitance to hot. Therefore if you are specifically trying to find the capacitance to hot, the relevant voltage is the 120V supply voltage, not the voltage measured on the cabinet when floating. Using the latter voltage will give you the source impedance of the touch leakage but this includes the reactance of the stray capacitance to the world outside, which although real is not part of the electrical equipment.

As it happens, the measured O/C voltage is about half the line voltage, so the two capacitances, to hot and to neutral/ground, are similar, and the result differs between the two scenarios by a factor less than two.

1.2mA is a 'sane' value for the touch leakage which could give you a noticeable but not painful sensation.
Using 120V we get Xc=104kΩ and C=25nF or using the measured voltage we get 46nF as per your calcs.
This is a high value for cable leakage, corresponding to hundreds of feet of NM cable with a ground conductor present but not grounded. Yet you describe the circuit as not having a ground. This kind of value is high even for suppression capacitors from hot and neutral to ground, although such a scenario would also account for the near symmetry that is causing the voltage on the cabinet to lie close to the midpoint of hot and neutral.

Is it possible that the cabinet is connected, via say the ECG in a 3-conductor cord to a 3-prong duplex outlet shared with an appliance that contains a suppression capacitors? If the outlet itself does not have a connection to ground, leakage produced by the appliance could be present at the cabinet.

Another consideration is that if the LED driver is leaking to the cabinet at frequencies other than 60Hz, both the measurement and the calculation could be invalid. However this would not typically result in over a millamp of leakage.

 

[pc1966]

As @Lucien Nunes says it is likely around 25nF from hot to ground, which is a lot of cable if it were a single circuit, but easily explained by a whole installation and/or a good few devices with filter capacitors in them (most things with electronic supplies will have that, and that is most appliances these days).

But it also suggests that all of the grounds are connected together in your installation, just not connected to the actual ground (i.e. the Earth)!

 

[Lucien Nunes]

However the OP specifically calls attention to the lack of ground conductors in post 1, which I took to mean including the circuit powering this light.

 

[AxisCat]

However the OP specifically calls attention to the lack of ground conductors in post 1, which I took to mean including the circuit powering this light.

That is correct, there is not any type of ground on the LED fixture in the metal cabinet. I had to run a wire from circuit that did have an actual ground to make these measurements.

I really appreciate all the help from you guys... great stuff!!!

However I am going to have to chew on this a while as my knowledge of electricity at this level is very limited. I have only used capacitors to start single phase AC motors or the occasional crossover on one my loud speaker projects.

 

[AxisCat]

Like I mentioned, I am struggling to understand but; and this is probably over simplifying things.... would I get the same result as connecting a 104kΩ resistor from the hot to the cabinet with the LED light removed?

 

[pc1966]

Like I mentioned, I am struggling to understand but; and this is probably over simplifying things.... would I get the same result as connecting a 104kΩ resistor from the hot to the cabinet with the LED light removed?

You would get a similar result, if the AC supply was a pure sine wave. But it is not a good idea to try that!

For safety you should use components rated not just for the nominal voltage (so USA 120V = sqrt(2)*120 = 170V peak) but also for the kV level of voltage spikes that appear from large motors switching and any nearby lightning. If you look up class X and class Y capacitors you will see what I mean.

That is one reason why you need to be careful with using a multimeter on AC power circuits - you really should be using a meter that is at least CAT-III rated and preferably CAT-IV rated for the nominal supply:

Measurement category - Wikipedia

en.wikipedia.org

https://www.digikey.co.uk/en/blog/what-are-multimeter-cat-safety-ratings

The other reason that using a multimeter for safety checks is frowned upon (certainly here in the UK) is the fact they can do so many different measurements (AC volts, AC current, DC volts, DC current, resistance, etc...) and if you had it set wrong then you get the wrong readings which could be "no danger" as DC volts sees only a couple of V on a 230V AC supply.

And if not of adequate safety category that can result in the meter exploding in your face! And I mean literally doing so, quite a few "electrical" deaths every year are actually due to burns and not shock.

 

[AxisCat]

You are exactly right, thanks for hand slap. I promise to be careful with anything I do that probably isn't a good idea.

I looked up the class X-Y capacitors and understand the point you make about dangers when something fails. A good reminder to myself that when dealing with anything potentially lethal always try to design for failsafe. I read through the other links you provided too. Good stuff.

I have 35 years in the HVAC field so I have become somewhat relaxed working with electricity, combustible fuels, heavy rigging... just all manner of things that can could really hurt a person. We kid around sometimes that safety is for sissies when we are doing something right on the edge but always approach the task with a good dose of seriousness.

Appreciate the safety reminder.