Confused about "earth ground"

[Special K]

So if I understand this correctly, every DC circuit will have a ground which is just an arbitrary 0 voltage reference point for the other voltages in that particular circuit. If I were to measure the potential between 2 DC grounds in 2 completely separate, isolated circuits, I could very well measure a potential difference between them, right?

What confuses me is the earth ground. I know in residential 120VAC power systems there is line, netural, and earth ground, with neutral being tied to earth ground. Now I have heard earth ground described as being the "absolute zero" of voltage reference points, is that the case? I thought voltage was all relative to whatever point you define to be 0, just like potential energy. Since earth ground is literally a connection to the building's structure, which goes into the ground, would all earth grounds be at the same potential? What if I were to measure the potential difference between the earth ground in one house, and another many miles away? Would the difference be 0?

Sorry if that is a bit rambling

 

[luigi1]

The whole concept of ground, common, earth ground, netural is an often misunderstood area. An example, netural is tied to earth ground at some point (usually the main panel) but we intend to run current on netural wires and ohm's law tells us that we will allways develop a voltage in responce to a current through a resistance. All wires have some resistance so netural is only at ground potential at the point that it is grounded (I'm assuming that some electricity is being used here, if current is zero then netural is still at ground potential). Lets talk about utility ground a bit. Thats the ground that the electrical company provides as part of the service. Every power pole, every service entrance is grounded and tyed together by utility ground. This massively parallel network provides a low impeadence high current capacity ground. So yes the potential of 2 houses miles appart is the same as long as they are on the network. Now if one of those houses were isolated earth ground could be different. But thats the difference between earth ground and utility ground.

 

[f95toli]

The point with earth ground is that Earth (our home planet) is pretty big, hence it does not matter much if you add/remove some electrons to/from it; the the total charge won't change much.

In order to "use" earth ground the only thing you need a good connection planet Earth, i.e. push a long copper rod into the ground.
In some applications (e.g. some radar installations) they bury a big metal mesh into the ground to make sure they have a good connection. sometimes they even add some salt and water to increase the conductitivty of the soil.

 

[TuxDave]

You are correct that there is no 'absolute zero' of voltage since it is all relative. However, you don't want to have an electrical system that has a floating ground (tied to nothing) because your system's ground may be 100's of volts above 'earth ground' and so if some poor sod touches anything in your system he'll most likely get zapped. Since his body's potential is most likely hanging around 'earth ground'.

 

[Special K]

Originally posted by: TuxDave
You are correct that there is no 'absolute zero' of voltage since it is all relative. However, you don't want to have an electrical system that has a floating ground (tied to nothing) because your system's ground may be 100's of volts above 'earth ground' and so if some poor sod touches anything in your system he'll most likely get zapped. Since his body's potential is most likely hanging around 'earth ground'.

But are all circuit's earth grounds at the same potential, considering they are all connected to the same earth, even though it may be separated by thousands of miles?

 

[Mday]

when in a circuit, there are typically two different potentials that may be considered "ground" or 0 V.

The first is a virtual ground, where the circuit is isolated and not actually grounded to "earth" or another object. This is the case with portable electronics. This also happens to other electronics, but nowadays, for electronics that requires grounding, virtual ground is somewhat dangerous in terms of either producing noise on a signal, or causing power problems. Also, within a portable device, it is possible to have multiple virtual grounds if the circuitry is designed poorly or falling apart.

What is normally ground is grounding yourself to the planet (earth). This can be done by touching a water pipe if you're indoors. If your house wiring was done correctly, your ground prong should be connected to the "earth" ground.

Note on computers: when you are installing a computer, grounding yourself typically means grounding yourself to the case by touching some metal part of it. This is usually virtual ground, unless your PS is plugged into the wall and installed onto your case. When you use a wrist strap to install components, the clip is recommended to be clipped onto your computer case (metal part).

FYI: there are some schools of physics\EE that call ground, earth. That is, EARTHed is a synonym used for being grounded.

 

[bobsmith1492]

Also, it's not a good idea to tie neutral to earth ground; maybe it works in houses, but anywhere else, i.e. control boxes with different AC lines in, it can (and will) cause trouble.

On a side note, I'm sort of confused as to when to tie signal grounds to earth ground. In one application, it was necessary to eliminate noise; in others, it causes ground loops and whatnot. Any ideas? It probably has something to do with whether or not the power sources are isolated or not?

 

[Geniere]

Originally posted by: TuxDave?

However, you don't want to have an electrical system that has a floating ground (tied to nothing) because your system's ground may be 100's of volts above 'earth ground' and so if some poor sod touches anything in your system he'll most likely get zapped. Since his body's potential is most likely hanging around 'earth ground'.

Not so. Consider a 120vdc battery atop a wooden ladder and with one hand you are grasping a pipe that is ?Earthed?. If with the other hand you touch either of the battery terminals you will not be shocked because there is no return pathway to the other terminal. Current cannot flow unless there is a complete circuit i.e., source>load>source.

Ignoring distributed capacitance the same would hold true for a transformer when neither of the secondary windings is connected to ground. Label the secondary windings L1 and L2 with 120vac measured between them. As with the battery scenario no current will flow if the person touches L1 or L2 but not both. (Because of distributed capacitance, some small micro/nano-ampere current will occur). If you measure either leg to ground, you will measure some voltage if the voltmeter has high input impedance. In the scenario I described, you?ll measure about 50-90vdc. With a low input impedance meter (analog type) you?ll measure near 0vac. If you bond either L1 or L2 to Earth you convert the secondary to a ground referenced system. Then you will get a shock when touching the non-grounded terminal. Hospitals use non-ground reference systems in their Operating Rooms if they use explosive anesthetics, as well as (I think) in grain silos, gas stations? any where one would want to try to reduce the likelihood of a spark to ground which is known to be more likely to occur than a terminal to terminal spark.

Many countries (France for one) do not reference their power transmission systems to ground, as does the US. There?s pros and cons for both methods, some countries ground via high impedance. The system they chose is one they favor to protect the system components, not an individuals safety. Safety concerns begin at point of use.

 

[TuxDave]

Originally posted by: Geniere

Originally posted by: TuxDave?

However, you don't want to have an electrical system that has a floating ground (tied to nothing) because your system's ground may be 100's of volts above 'earth ground' and so if some poor sod touches anything in your system he'll most likely get zapped. Since his body's potential is most likely hanging around 'earth ground'.

Not so. Consider a 120vdc battery atop a wooden ladder and with one hand you are grasping a pipe that is ?Earthed?. If with the other hand you touch either of the battery terminals you will not be shocked because there is no return pathway to the other terminal. Current cannot flow unless there is a complete circuit i.e., source>load>source.

My electrical engineering side agrees with you but I need a mental exercise to really make this make sense to me. Can you explain the 'rubbing your feet against the carpet and then touching a metal doorknob to get a shock' in terms of current loops? I'm envisioning a floating system to be equivalent to your body after rubbing against the carpet and the doorknob to be the 'grounded person'. Maybe it's late but it's not making sense...

 

[Geniere]

Originally posted by: TuxDave?My electrical engineering side agrees with you but I need a mental exercise to really make this make sense to me. Can you explain the 'rubbing your feet against the carpet and then touching a metal doorknob to get a shock' in terms of current loops? I'm envisioning a floating system to be equivalent to your body after rubbing against the carpet and the doorknob to be the 'grounded person'. Maybe it's late but it's not making sense...

The key words are ?metal doorknob?, as you cannot discharge via a non-conductor. As you approach the doorknob, the electrical field strength increases. The mobile electrons of the doorknob are repelled thus creating a potential difference across the air gap. When the voltage increases to a value sufficient to ionize the air, a spark results.

I'm retired so early or late seems the same to me.

 

[Special K]

Alright, here's a related scenario:

Let's say you have an isolation transformer with 120 VAC on the primary windings. The lower end of the winding is tied to earth ground. The secondary windings are allowed to float. Now let's say you take a voltmeter and measure between the lower end of the primary (which is grounded) and the lower end of the secondary, which is floating. Will you measure a potential difference in this case? The isolation transformer is 1:1 so you should see 120VAC across the secondary but if it's floating, how far will it float above/below earth ground?

And another scenario:

Same situation as above, except let's assume you measure 10VAC between earth ground and the lower end of the secondary. Now lets say you touch both of these windings. Do you get shocked?

 

[Special K]

Oh and no one has yet answered my question:

Since all earth grounds are connected to the same "earth", are they all at the same potential?

I suspect they wouldn't, because wouldn't some materials cause the earth to have different potentials at different points?

How well does the earth conduct, anyway?

 

[f95toli]

Originally posted by: Special K
Alright, here's a related scenario:

Let's say you have an isolation transformer with 120 VAC on the primary windings. The lower end of the winding is tied to earth ground. The secondary windings are allowed to float. Now let's say you take a voltmeter and measure between the lower end of the primary (which is grounded) and the lower end of the secondary, which is floating. Will you measure a potential difference in this case? The isolation transformer is 1:1 so you should see 120VAC across the secondary but if it's floating, how far will it float above/below earth ground?

It depends, usually not much but it can be tens of volts (at least in my lab).

And another scenario:

Same situation as above, except let's assume you measure 10VAC between earth ground and the lower end of the secondary. Now lets say you touch both of these windings. Do you get shocked?

Yes, but 10VAC does not hurt much.

 

[f95toli]

Originally posted by: Special K
Oh and no one has yet answered my question:

Since all earth grounds are connected to the same "earth", are they all at the same potential?

They should be, yes.

I suspect they wouldn't, because wouldn't some materials cause the earth to have different potentials at different points?
How well does the earth conduct, anyway?

I don't think it has anything to do with materials, however it takes time for electrical signals to travel through the ground so I guess the potential might vart a bí because of that.

The conductivity of the earth varies a great deal (many orders of magnitude), as I wrote above you need to add salt and water to the soil if you want to be sure that you have good connection. Dry soil, sand etc is more or less insulating.

 

[ScottMac]

If you drive two grounding rods into the ground some distance apart, you'll get measurable current, sometimes quite a bit.

Putting grounding rods into a (usually wetter) clay soil will be a much better gournd than if you plant it in sandy / loam soil. In some instances, you need to put in things like a saline drip to get a decent ground in less-than-wonderfully conducting soil.

Other things that will affect your "earth ground" with respect to the electrical system are things like the quality of the grounding connection to your service panel. GO out and look at some of the ground stakes. Most of the time it's ~3/8 to 1/2" copper clad, with a clamp attaching the wire to the panel.

That connection corrodes or oxidizes after a while ... go look. That's not a great ground (from your panel's point of view). If you connected two locations, one has a decent ground, one has a poor ground AND maybe comes off a different leg of the transformer (or a different transformer) ... there's going to be potential there.

If you connect devices in each of those places together without some form of entrance protection, there's a risk of (at the least) damaging the equipment or (worse case) death.

Ground can be ground, ground might be ground, ground might NOT be ground ... as mentioned before, it's a situational thing.

FWIW

Scott

 

[Geniere]

Originally posted by: Special K
Alright, here's a related scenario:

Let's say you have an isolation transformer with 120 VAC on the primary windings. The lower end of the winding is tied to earth ground. The secondary windings are allowed to float. Now let's say you take a voltmeter and measure between the lower end of the primary (which is grounded) and the lower end of the secondary, which is floating. Will you measure a potential difference in this case? The isolation transformer is 1:1 so you should see 120VAC across the secondary but if it's floating, how far will it float above/below earth ground?

There is no lower end of the secondary winding; both terminals will float above ground equally. There is no way (even in a theoretical sense) to construct a transformer without some coupling between the primary and secondary winding. There will always be a current flow to ground, it might be 1 electron per hour or several microamperes. The transformer can float ?to the peak voltage?, but you will not be able to measure it since the voltmeter will cause the voltage to drop. When isolation is the primary design factor in a power transformer the core would be torroidal and the secondary winding would be physically separated from the primary. That type of transformer is less efficient as desirable inductance is lessened as well as undesired capacitance.

Originally posted by: Special K And another scenario:

Same situation as above, except let's assume you measure 10VAC between earth ground and the lower end of the secondary. Now lets say you touch both of these windings. Do you get shocked?

I don?t really follow the question, but there is no lower end. The voltage reading will be heavily influenced by the measuring device, design of the transformer, humidity?

Originally posted by: Special K
Oh and no one has yet answered my question:

Since all earth grounds are connected to the same "earth", are they all at the same potential?

The short answer is no. For the purpose of electrical safety, it doesn?t matter. A ham radio operator may wish to establish a ground plane to better transmit his signal. The Earth is capable of generating electrical potentials at different points on its surface via many mechanisms; seismologists use some to try to predict earthquakes.

Originally posted by: Special KHow well does the earth conduct, anyway?

Are you standing on a sand dune in the Sahara Dessert or wading in the Great Salt Lake? If I recall correctly the NEC wants 25ohms between electrodes spaced 6 feet apart. There are specific instruments and measuring techniques required, an ohmmeter doesn?t cut it.

The purpose of the grounding rod (referencing the residential electrical system to ground) has nothing to do with preventing someone from getting a shock. It is there to protect the electrical system ( and connected devices) from high voltage surges, usually from lightning.

 

[Intelia]

How would that apply if you if the test were performed in load stone?

 

[Special K]

Originally posted by: Geniere

Originally posted by: Special K
Alright, here's a related scenario:

Let's say you have an isolation transformer with 120 VAC on the primary windings. The lower end of the winding is tied to earth ground. The secondary windings are allowed to float. Now let's say you take a voltmeter and measure between the lower end of the primary (which is grounded) and the lower end of the secondary, which is floating. Will you measure a potential difference in this case? The isolation transformer is 1:1 so you should see 120VAC across the secondary but if it's floating, how far will it float above/below earth ground?

There is no lower end of the secondary winding; both terminals will float above ground equally. There is no way (even in a theoretical sense) to construct a transformer without some coupling between the primary and secondary winding. There will always be a current flow to ground, it might be 1 electron per hour or several microamperes. The transformer can float ?to the peak voltage?, but you will not be able to measure it since the voltmeter will cause the voltage to drop. When isolation is the primary design factor in a power transformer the core would be torroidal and the secondary winding would be physically separated from the primary. That type of transformer is less efficient as desirable inductance is lessened as well as undesired capacitance.

Originally posted by: Special K And another scenario:

Same situation as above, except let's assume you measure 10VAC between earth ground and the lower end of the secondary. Now lets say you touch both of these windings. Do you get shocked?

I don?t really follow the question, but there is no lower end. The voltage reading will be heavily influenced by the measuring device, design of the transformer, humidity?

Originally posted by: Special K
Oh and no one has yet answered my question:

Since all earth grounds are connected to the same "earth", are they all at the same potential?

The short answer is no. For the purpose of electrical safety, it doesn?t matter. A ham radio operator may wish to establish a ground plane to better transmit his signal. The Earth is capable of generating electrical potentials at different points on its surface via many mechanisms; seismologists use some to try to predict earthquakes.

Originally posted by: Special KHow well does the earth conduct, anyway?

Are you standing on a sand dune in the Sahara Dessert or wading in the Great Salt Lake? If I recall correctly the NEC wants 25ohms between electrodes spaced 6 feet apart. There are specific instruments and measuring techniques required, an ohmmeter doesn?t cut it.

The purpose of the grounding rod (referencing the residential electrical system to ground) has nothing to do with preventing someone from getting a shock. It is there to protect the electrical system ( and connected devices) from high voltage surges, usually from lightning.

Here, this crude pic should explain what I am asking:

pic

The transformer is 1:1, and the secondary is floating. What does the DMM measure, and would I get shocked if the DMM measured a voltage and I touched the 2 points the DMM is connected to?

 

[f95toli]

There is no way to tell what the DMM will measure, it depends on leakage currents etc; som value between 0-120VAC.
Most likely a few volts, there is always a finite impedance between the two sides which tends to equalize any difference in potential, in some cases people even use large resistors, 1 MOhm or so, to lower this impedance even further.

And yes, as I wrote above, you can get a shock.
I use several isolation transformers in my lab and most of the equipment, including a shielded room, is floating.
We actually had to get a permit before we were allowed to build it like that; precisely because in principle you can get a shock if you e.g. touch the shielded room (which is connected to the measurement ground of the equipment) and some piece of metal
connected to earth (building ground).
A few volts won't hurt but if some piece of equipment is malfunctioning it the voltage difference can actually become deadly, therefore we have safety circuits in place that monitors the voltage difference (I guess it is basically a standard circuit breaker) between earth and the isolated measurement ground.

 

[Geniere]

Originally posted by: Special K
Here, this crude pic should explain what I am asking:..

The transformer is 1:1, and the secondary is floating. What does the DMM measure, and would I get shocked if the DMM measured a voltage and I touched the 2 points the DMM is connected to?

As f95toli stated there is no way to know. Your DMM will in all probability read a much higher voltage than would a non-electronic analog meter because the DMM has a higher input impedance. Whatever the reading is, it has no significance. If you attach a 100-ohm resistor across the meter, both types of meter will read 0-volts. You do not want to touch the secondary windings because only the wire insulation isolates you from the mains. To determine if the insulation is not damaged, disconnect the primary from the mains and measure the DC resistance using the same setup as in your drawing. A good transformer will measure infinite resistance between primary and secondary.

When working with electricity, you are usually only two failure modes away from getting a shock if the EGC (Equipment Grounding Conductor or Safety Ground) is intact and properly terminated. Without that wire (older homes) you are only one failure mode away from a shock? that is unless you use a GFCI (ground fault circuit interrupter). These devices DO NOT require a grounding wire to function and are a great way to improve electrical safety in the older homes. A GFCI monitors the hot leg current and compares it to the neutral current. If there is a difference, the GFCI assumes the difference current is going to ground and opens the circuit.

Originally posted by: Intelia
How would that apply if you if the test were performed in load stone?

The NEC requires a specific range of readings regardless of the local soil composition. I would think that lodestone would improve the readings because of the metallic content.