relative voltage

[timelessbeing]

I need help understanding voltage sources.

If you connect a voltmeter across mains and earth ground, it will show your mains voltage. However, if you connect it across a DC source, such as a battery, and earth ground, it will show zero. Why?

 

[colin55]

The mains coming into your house is really only a single wire. The wire is called active.
The other wire is a metal rod pushed into the ground. This is called the neutral.
That's why touching the active and ground will kill you.
A battery is not connected to the gound. That's why you have to connect to both terminals to get a voltage reading.

 

[KeepItSimpleStupid]

I'll also add that Neutral and Earth are bonded at one point usually near the service entrance to your house. This is also where the metallic water pipes get bonded too as well. Ideally, the telephone protector and cable should be bonded to that same point too.

What this does, is it makes everything in the house share the same reference. It also means if say lightning strikes a single circuit in the house, the reference still stays the same. The circuit that got the hit would be susceptable to the most damage.

Now, it turns out that the ground can sometimes not be the same potential everywhere especially during periods of lightning and there are special grounding rules for pools.

 

[timelessbeing]

OK, I already knew those things, but I still don't understand. I get that there is a large electrical potential difference between mains and ground. (I've even experienced it first hand). So why is there no potential difference between a charged battery terminal and ground. Or we could take a capacitor as an example because I know it has a plate that is crowded with free electrons. Do they not want to flow to ground?

You guys say that mains neutral is bonded to ground, and a battery is not. You are implying that the earth ... the soil, rocks, rivers and all that... forms part of a circuit. Lets take a long power transmission line as an example. I don't know exactly how closely spaced the physical bonding points are, but I'm quite certain that in some cases it could hundres of meters to such a point. There could be billions of ohms of resistance in between. And yet, we know that touching a fallen transmission line can kill you. You also imply that if I drive a conductive rod into the ground, connect it to the negative terminal on a battery, and measure the voltage between the positive terminal and any earth ground point, that you will see a potential difference. I have tried this and there is not.

 

[ericgibbs]

hi,
Get a 12v car battery, connect the -V terminal to Earth, say a metal cold water pipe, you will then be able to measure from the +V terminal to Earth ground.

Without that connection to the cold water pipe there is no electrical path to Earth ground, the case of the battery is an insulator.

 

[KeepItSimpleStupid]

Think of the power company as an AC battery (there is no such thing). They connect one end of their generator to the earth. Your DC battery is not connected to the earth. The power companies currents are low because the voltages are high. For residential US it's probably around 10,000 Volts. This energizes the primary of the transformer. One side of the secondary is grounded.

In order to get a low resistance to ground for your battery, you need to drive the copper rod nearly 5 to 6 feet into the ground usually near the water table. Dry ground is a poor conductor of electricity as you have found out.

 

[crutschow]

"Ground" is just a reference point and is earth is used as that reference point. The ground resistance varies greatly with the type of soil and its moisture content. Thus to get a good ground you drive a rod deep into the ground where there normally is moisture. Where I worked, when a reliable ground connection is required for testing, such as EMI shielded rooms, they would place the ground rod in the soil with added salt and a continuous drip of water to minimize the resistance of the rod to the earth.

If you drive two conduction rods into the ground some distance apart into normally moist soil and you connect one terminal of your battery to one rod, you should be able to measure some voltage between the other battery terminal and the other ground rod.

It's all a matter of the resistance of the soil between the two ground points. Completely dry soil, rocks, etc. obviously would generally have a very high resistance. Damp soil has a much lower resistance. Downed power lines often occur after a storm when everything is wet and obviously you then would have a very low resistance to earth ground.

 

[timelessbeing]

I think part of the problem was my faucet has a plastic pipe section. Thanks for the clarification.

 

[olly_k]

Think of the power company as an AC battery (there is no such thing). They connect one end of their generator to the earth. Your DC battery is not connected to the earth. The power companies currents are low because the voltages are high. For residential US it's probably around 10,000 Volts. This energizes the primary of the transformer. One side of the secondary is grounded.

In order to get a low resistance to ground for your battery, you need to drive the copper rod nearly 5 to 6 feet into the ground usually near the water table. Dry ground is a poor conductor of electricity as you have found out.

It doesn't really work like that does it? I mean, Earth is quite specifically a fault return path. The power company surely distributes the electricity as 3 phases which are then reduced by the local sub-station. The Substation is also Earthed where the transformer coils meet (star) and this is also where the Neutral is formed for single phase use. Only parasitic or leaky current will ever flow through the earth connection back to the sub station or Power Station..

@op if you get an isolating transformer then you will get 240VAC (sorry, 120VAC?) with no reference to earth, so you can touch either side without risk of electric shock. (You could get a capacitive tingle but no significant current will ever flow) However, stick one end into the ground and all of a sudden that side becomes 'neutral', basically the same potential as you and earth. However the other side now becomes HOT and is considered dangerous to touch! Incidentally, if you were to earth one side of the isolation transformer, you would use an neutral wire and consider the earth a separate entity, even though it *should* be at the same potential as the earth.