Understanding AC wall outlet multimeter readings

Thread Starter

gp21

Joined Apr 1, 2020
5
Hi all

I am a software engineer, new to electronics, trying to understand how things work and I am a bit confused.

I live in an apartment in europe.I have single phase AC in my home.

I am using a multimeter, set to voltage mode, to measure the wall outlets in my apartment.
I have done the following tests:

Plugging the red lead to the hot and the black to the neutral and multimeter reads around 230V.
Plugging the red lead to the hot and the black to the ground and multimeter reads around 230V.
Plugging the red lead to neutral and the black to the ground and multimeter reads around 0.5V.
Pressing the Hz button multimeter reads around 50 Hz.

Also tried with a tester screwdriver and when I plug it to the hot it lights up bright while in neutral and ground nothing happens.

So far so good that was what I expected to see.

I noticed that when I plug the red lead to the hot and the black lead is in the air far apart touching nowhere the multimeter reads a voltage value around 20-25 V.That confused me a lot in the start because I expected to see 0V in an open circuit.But from what I have read that is because the capacitance of my multimeter to Earth completes the circuit.

What I still cannot understand and cannot explain is:

When I plug the red lead to the hot and I touch the black lead to the floor or to a wall nothing changes ( still reading that 20-25V like when the black lead is in the air).

From my understanding the reason someone would get electrocuted if touches the hot wire of an AC wall outlet and their feet is in the floor (without rubber shoes or insulated floor) or touches a wall, is because the floor or the wall have a potential difference with the hot wire and not (not much at least) potential difference with the ground.So as a result electrical energy flows inside the body through the floor or the wall back to its source.So what I would expect to see is a reading around 230 V or at least over 50V.

Thanks
 

LesJones

Joined Jan 8, 2017
2,638
The input impedance of your meter is probably around 10 Meg ohms so it needs very little current to register. The black lead wil have capacitance to ground via you and any other objects close to it. A capacitor has a reactance with AC that is providing a path to ground. Reactance is similar to resistance but also creates phase shift. The reactance of a capacitor is 1/(2 x Pi x F x C) The test screwdriver only works because of this. The neutral is at ground potential as it is connected to ground at some point in the distribution system. This could be at the sub station or where the power cable enters your house.

Les.
 

Thread Starter

gp21

Joined Apr 1, 2020
5
Thank you for your answer

What I do not understand is:

If I put one hand at hot wire and the other at neutral wire (theoretically speaking of course...) I would get electrocuted and probably die because the potential difference between hot wire and neutral is 230V. So logically the multimeter reads 230V when I plug it into hot and neutral.

If I put just one of my hands in hot while at the same time my bare feet touch the floor of my apartment I would again get electrocuted because the potential difference between hot wire and the floor of my apartment is again 230V.Right?

If that is right then why when I put one lead to the hot and the other to the floor the multimeter does not read 230V?

Μaybe the answer is hidden in your reply but unfortunately I cannot see it.

Thanks
 
Your "floor" does not have a good path to ground. If it's carpet, it's effectively an insulator.

A washer with a metal cabinet might be. The washer might be powder coated, so that's an insulator. But, if a hot wire went astray inside the washer, that's a direct path to ground which would blow the fuse.

The electrocuted part needs some clarification. Somewhere around 10 mA across the heart muscle will disrupt the heart beating.
getting 10 mA across the heart might be hard to do. That's like 23,000 ohms at 230V. measuring resistance between your two hands might get you 10K if your hands are wet.

There is a bulk property of a material called resistivity. For nylon (a rug) might be p=1E13 ohm-cm. It would depend on temperature and being wet.

To convert that into a resistance R=pL/A, L is length in cm and A is cross-sectional area in cm^2

Your meter's internal resistance of 10 M ohms plays a part too.

So, the resistance of the rug to ground is too large.
 

LesJones

Joined Jan 8, 2017
2,638
The capacitance between the wire and ground is VERY small so the reactance is very high. This forms a potential divider with the meters input resistance. As the reading on your meter was about 25 volts that is about one tenth of the mains voltage so the voltage across the capacitive reactance will be 9 times that. We will assume your meter input resistance is 10 Meg ohms so the capacitive reactance will be 90 Meg ohms. So the total resistance in series with the 250 volt mains is 100 Meg ohms. so the current will be 250/100000000 (Amps) = 2.5 uA. You would net even feel this small current. (The simple addition I used of reactance and resistance is not quite correct. It should be vector addition but it will not make much difference and simplifies the explanation.)

Les.
 

Thread Starter

gp21

Joined Apr 1, 2020
5
Thank you for your answers

Your "floor" does not have a good path to ground. If it's carpet, it's effectively an insulator.
It is not a carpet or wooden floor.I have also tried touching the black lead to the wall instead of the floor and had the same result.

The capacitance between the wire and ground is VERY small so the reactance is very high. This forms a potential divider with the meters input resistance. As the reading on your meter was about 25 volts that is about one tenth of the mains voltage so the voltage across the capacitive reactance will be 9 times that. We will assume your meter input resistance is 10 Meg ohms so the capacitive reactance will be 90 Meg ohms. So the total resistance in series with the 250 volt mains is 100 Meg ohms. so the current will be 250/100000000 (Amps) = 2.5 uA. You would net even feel this small current. (The simple addition I used of reactance and resistance is not quite correct. It should be vector addition but it will not make much difference and simplifies the explanation.)
I have definitely a lot of reading to do to catch up.Reactive capacitance I new term for me so I got confused a bit more.
But from what I understand the multimeter has a very large resistance so everything connected in series with it will get very little current.

So the total resistance in series with the 250 volt mains is 100 Meg ohms. so the current will be 250/100000000 (Amps) = 2.5 uA.
So that small current goes throught the multimeter and the leads when I plug the red in the hot and I touch the floor (not my hand or anything else just the floor...) of my apartment or a wall with the black lead?

So to clarify some things to see if I understood what you said or got ti completely wrong...

1)The floor of my apartment (if we assume its not from an insulating material) has a potential difference with the hot wire
probably big enough to cause harm or at least be felt.If that wasn't the case putting my finger in the hot of the wall outlet would be safe (again theoretically speaking. I will not try it that's why I am asking...)
2)The multimeter cannot read this potential difference because of capacitive resistance between the wire and the floor.

The above statements are true?

Thanks
 

ericgibbs

Joined Jan 29, 2010
10,030
hi gp21,
It is obvious from your post that you have little or no knowledge with regard to working with mains supply voltages and the trial and error method you are using could lead to you or some other person receiving a lethal electrical shock.

My best advice is to immediately stop working on mains voltage testing until you have the knowledge and experience required.

E
 
2)The multimeter cannot read this potential difference because of capacitive resistance between the wire and the floor.

Not exactly. reactance is frequency dependent. The equlivelent circuit is a series circuit of 240 VAC voltage source + 90 M-ohms + 10 M-ohms where as the equivalent circuit of Hot to GND is 240 VAC + 0.5 ohms + 10 M-Ohms. An ideal voltage source has a resistance of 0. In the hot-GND scenereo, the meter (10 M-ohms) does not affect the measurement.

Anytime, you do a measurement you disturb the circuit. The ammeter function of MOST DVM's insert a resistor which could be significant.

At work, I routinely could measure currents as small as +-4 pA at -100 to +100 volts DC. 4 pA is 4e-12 Amps. A voltage source and an ammeter is used, because a voltmeter would affect the measurement. The cables used were triax which had a center conductor and two shields. The inner shield was kept at the potential of the center conductor and the outer shield was ground.
At these levels, you have to worry about the triboelectric effect and the piezoelectric effect or motion and flexing generating current. The cables had graphite between the shields.

When measuring low resistances, you typically use the Kelvin probe technique where you measure the current through the DUT (Device Under Test) and the voltage across the DUT. 10 M-ohms in parallel with 0.010 ohms is insignificant.
 

Thread Starter

gp21

Joined Apr 1, 2020
5
hi gp21,
It is obvious from your post that you have little or no knowledge with regard to working with mains supply voltages and the trial and error method you are using could lead to you or some other person receiving a lethal electrical shock.

My best advice is to immediately stop working on mains voltage testing until you have the knowledge and experience required.

E
Thank you for your advice.It's a good advice and I will take it.I was not planning to do more tests or work with the main voltage anyway and I hope all the tests I did I think were pretty safe as I kept my multimeter always on voltage mode where it has a big internal resistance and never touched the leads.

I usually do tests with small batteries or in a breadboard and that's what I am planning to do and in the near future.

The reason I posted this is because I am trying to understand how electricity works and for someone new to electrical/electronics (I had some classes in the university but I didnt get much out of it since I was more of a software guy) sometimes the texts are a bit confusing.Especially about "ground" and what is considered "ground".

My questions are theoretical questions to understand "ground" especially because testing some things is dangerous and cannot be done practically so any help would be appreciated.

Is my apartment's floor and walls considered "ground"?

From my understanding since the building is connected directly to earth and if it is not completely insulated it should be considered "ground" and have the same potential as "ground" so it should have a 230V difference with the "hot" wire and touching them and the hot wire in the same time should cause a considerable current flow.

If that is the case then why the multimeter cannot meter that.That's why I am trying to understand.

If it is something wrong with my thinking or it has to do with the way multimeters work and their internals characteristics.

Thanks
 
The main purpose of ground or that earth-neutral connection is to give the building a reference AND to protect against lightning strikes.

During a thunderstorm, the earth can have a potential gradient across it.
 

MrChips

Joined Oct 2, 2009
21,119
Here is a simple way of looking at it.
Floor Ground.jpg

Yes, the floor is an insulator, a leaky insulator perhaps. In that case, put some resistors across each capacitor and capacitors across each resistor. Just remember that the resistance values are very high while the capacitance values are very low.

Along with that, we are dealing with AC currents and voltages. With AC we introduce reactance and impedance along with resistance. The resistance and reactance is very high. Hence the currents involved are very small.

Now, your foot is in contact with the floor at some point along the chain. The chain is infinitely long (or almost infinitely long). Even though you may not be in electrical contact with the leads of the test meter, your body influences the reading on the meter. Note that the resistance of the meter is typically 10MΩ. It only takes 1μA for the meter to register 10 on the scale.

As already stated, don't mess around with AC mains if you don't understand what is going on.
 

Thread Starter

gp21

Joined Apr 1, 2020
5
Not exactly. reactance is frequency dependent. The equlivelent circuit is a series circuit of 240 VAC voltage source + 90 M-ohms + 10 M-ohms where as the equivalent circuit of Hot to GND is 240 VAC + 0.5 ohms + 10 M-Ohms. An ideal voltage source has a resistance of 0. In the hot-GND scenereo, the meter (10 M-ohms) does not affect the measurement.
Now, your foot is in contact with the floor at some point along the chain. The chain is infinitely long (or almost infinitely long). Even though you may not be in electrical contact with the leads of the test meter, your body influences the reading on the meter.
Now that starts making some sense.

Thank you all for you help and pointing me in the right direction for further reading.
 
Let's take a tree. https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch03.pdf The tree is grounded and gets hit by lightning.
Electrical conductivity is 1/(Resistivity). The unit is Siemens. Your multimeter will have trouble measuring resistance. there is the bark which is dry and the first ring is where the transport occurs, so it's somewhat conductive. At the large voltages/currents of lightning, it will conduct electricity. Resistivity is a material property. Resistance is the V/I relationship of a particular current path,

Be careful reading the paper. Differentiate between thermal and electrical.
 

crutschow

Joined Mar 14, 2008
24,957
Most floor and wall housing materials are basically insulators unless they are damp, so touching them with a probe will result in very little current from the main's voltage.
Your body capacitance is sufficient though, that you will likely experience a very noticeable (as in I won't to that again) but not likely lethal shock if standing on that floor when you touch the hot main's lead.
 

dl324

Joined Mar 30, 2015
10,737
I am a software engineer, new to electronics, trying to understand how things work and I am a bit confused.

I live in an apartment in europe.I have single phase AC in my home.

I am using a multimeter, set to voltage mode, to measure the wall outlets in my apartment.
I have done the following tests:
Don't do this. Novices shouldn't be probing line voltage; you can get hurt.

When I'm working with 220VAC, I'm always reminded of someone who worked as a lineman for a power utility. He thought he had turned off the breaker for a dryer outlet. Before touching anything with his hand, he stuck a screwdriver across the line. It was still live and gave him a nasty surprise. After that, he pulled the power meter from the house.

When I was in college, I was taught to keep one hand in your pocket and to touch things with the back of your hand to see if they were live. That way, a shock would cause you to move your hand away. If you're gripping something conductive that contacts a hot wire, the shock will make you grip harder.

If you want to do AC measurements, get a step down transformer and do your experiments at lower voltages. Less than 50V is generally considered non-lethal.
 

DickCappels

Joined Aug 21, 2008
6,384
The floors in my house are glzaed tiles and I am almost always barefoot in the house. I have had quite a few painful zings from appliances that leak current from the power line.
 

MrChips

Joined Oct 2, 2009
21,119
Not everything electrical can be explained with what you believe.
The knob on your bedroom door is insulated.
Your body is insulted. Yet you get zapped just by touching the doorknob. Go figure.
 

MisterBill2

Joined Jan 23, 2018
6,055
hi gp21,
It is obvious from your post that you have little or no knowledge with regard to working with mains supply voltages and the trial and error method you are using could lead to you or some other person receiving a lethal electrical shock.

My best advice is to immediately stop working on mains voltage testing until you have the knowledge and experience required.

E
A bit of caution is what is needed in working with mains voltages and so I do not suggest a panic mode of operation. Certainly mains voltage can be lethal, and so it is important to avoid getting shocks.
For most of what we do it is simple enough to unplug a thing before we work on it. While it is unplugged the shock hazard is also unplugged. Then it is also important to avoid contacting conductive thing while they are connected to the mains voltage.
 
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