What is the purpose of grounding in AC systems?

[Haecifer]

Hello everyone!

There is something about grounding in AC systems that has been bothering me for a while.
I have read almost all of the threads about grounding here, but I haven't found my specific question on any of them.

In my house, there are 3 wires that come from my outlet, the hot wire, the neutral wire and the ground wire.

So let's say that I plug something on the wall and let's represent it by a dark box.
What we would basically have is Figure A (Where H stands for hot wire, N for neutral and GND for ground).
Now, the reason why we grounded our dark box is because we want to keep it at 0V to ground, so that people who touch it don't possibly get shocked.
This all I understand, but I am not catching the actual circuit that exists behind it.

What I thought the actual circuit could like is Figure B, where our dark box is represented by a resistor R, a capacitor C and an inductor L.
If my representation of the circuit behind all this is incorrect, please tell me how it actually is, otherwise, I have a question.
Here is the deal, I was told that there is generally no current flowing through the ground wire. However, if you look at figure 2, you will notice that, considering that the neutral wire and the ground wire should be so-so at the same potential, current would flow through both of them.
After all, in order to current to flow, you just need a potential difference.

Thanks in advance!

 

[NascentOxygen]

Hi, Haecifer. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

Neutral and ground are not connected together at your house; they come in as separate circuits from the street transformer (GND often coming via the earth). H and N carry the normal operating currents, leaving GND to carry no operating current. Neutral may be some volts above ground due to Ohms Law of current in the conductors coming from the street transformer. So there could exist some volts between N and your kitchen sink and plumbing pipes, but no volts between GND and your plumbing pipes (except under fault conditions).
http://img803.imageshack.us/img803/4666/holly1756.gif

 

[berkeman]

Hello everyone!

There is something about grounding in AC systems that has been bothering me for a while.
I have read almost all of the threads about grounding here, but I haven't found my specific question on any of them.

In my house, there are 3 wires that come from my outlet, the hot wire, the neutral wire and the ground wire.

So let's say that I plug something on the wall and let's represent it by a dark box.
What we would basically have is Figure A (Where H stands for hot wire, N for neutral and GND for ground).
Now, the reason why we grounded our dark box is because we want to keep it at 0V to ground, so that people who touch it don't possibly get shocked.
This all I understand, but I am not catching the actual circuit that exists behind it.

What I thought the actual circuit could like is Figure B, where our dark box is represented by a resistor R, a capacitor C and an inductor L.
If my representation of the circuit behind all this is incorrect, please tell me how it actually is, otherwise, I have a question.
Here is the deal, I was told that there is generally no current flowing through the ground wire. However, if you look at figure 2, you will notice that, considering that the neutral wire and the ground wire should be so-so at the same potential, current would flow through both of them.
After all, in order to current to flow, you just need a potential difference.

Thanks in advance!

Welcome to the PF.

In order to prevent such ground currents from flowing, Neutral and Earth Ground are usually only tied together at the breaker panel. It would be against safety rules to tie Neutral and Earth Ground at each appliance.

The Earth Ground connection in devices is used to ground all exposed metal pieces (like the metal enclosure and any exposed metal switches). This is done so that if there is a single fault in the wiring coming into the device, like if the Hot wire comes loose and touches the metal enclosure on the inside), there is a low-impedance path to ground for the high voltage, which will either blow the device's fuse, or will trip the breaker at the breaker panel.


EDIT -- N.O. beat me to it!

 

[russ_watters]

Hi, Haecifer. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

Neutral and ground are not connected together at your house; they come in as separate circuits from the street transformer (GND often coming via the earth). H and N carry the normal operating currents, leaving GND to carry no operating current. Neutral may be some volts above ground due to Ohms Law of current in the conductors coming from the street transformer. So there could exist some volts between N and your kitchen sink and plumbing pipes, but no volts between GND and your plumbing pipes (except under fault conditions).
http://img803.imageshack.us/img803/4666/holly1756.gif

Sorry, but pretty much all of that is wrong:

1. The ground comes from the ground itself (an actual stake, planted in the ground, for residential).
2. Ground and neutral are connected, at the electrical panel. So:
3. There should never, ever be a voltage difference between your neutral and ground.

Since the neutral point of an electrical supply system is often connected to Earth ground, ground and neutral are closely related...

All neutral wires of the same electrical system should have the same electrical potential, because they are all connected through the system ground...

Neutral wires are usually connected at a neutral bus within panelboards or switchboards, and are "bonded" to Earth ground at either the electrical service entrance, or at transformers within the system...

http://en.wikipedia.org/wiki/Ground_and_neutral

 

[Haecifer]

Hmmm I understand. Thanks all of you!

One more question though, If ground and neutral are connected in the electrical panel why doesn't current normally flow from the hot wire to ground through this connection?
(Take a look at Figure C to see what I am thinking)

 

[dlgoff]

Hmmm I understand. Thanks all of you!

One more question though, If ground and neutral are connected in the electrical panel why doesn't current normally flow from the hot wire to ground through this connection?
(Take a look at Figure C to see what I am thinking)

It does. Here's what keeps you safe when a fault occur.

 

[russ_watters]

No, Haecifer meant why doesn't it normally flow to ground.

Ground and neutral are both zero potential, so there is no flow between them. One of its mainfunctions is to act like a universal neutral, ensuring everyone has the same reference.

And don't confuse neutral with negative: the Earth isn't sucking-up electrons from the wire.

 

[Haecifer]

I think I got it all now!

Thanks a lot everyone!

 

[Studiot]

Haecifer, this is question important.

Are you in Europe or USA?

This is also why you have been offered different answers that seem to conflict.

In the UK it is illegal to connect the neutral to ground.

In the USA it is illegal not to.

Both systems are correct in their own way but they are different, so it is important to establish which we are talking about.

 

[Haecifer]

I live in Brazil, where the system is similar to the USA system.

 

[Studiot]

Then follow the advice of those who say that your 'neutral' should be grounded at your control panel. The US system has an isolating transformer between the supply and the consumer. The centre point of the secondary of this transformer forms the neutral and should be grounded.

This system is completely different from the European supply configuration.

 

[Haecifer]

Ok, understood!

 

[jim hardy]

Thought Experiment:

In your fig B
1. Disconnect your G wire from N at the appliance (R, L, C). Remove that dot from the vertical N wire.
2. Draw a dotted line encircling the appliance (R, L, C) but not touching anything else.
3. Connect your G wire to that dotted boundary encircling the appliance.

Now - the dotted boundary represents the skin of your appliance. It forms almost a Faraday cage surrounding the active components, certainly a cage that keeps tiny fingers away from hazardous parts..that's up to the mechanical designer to provide.
That cage is connected to the ground prong on your appliance cord.

Now - take a close look at your plug on end of appliance cord.
The wide blade is Neutral, earthed near the panel. But it's a large conductor all the way back to your transformer so that it won't develop noticeable voltage unless something is significantly wrong.

The skinny blade is Line, to hot side of panel

The round prong is Earth ground and tied to the Faraday cage. . Notice it's longer than the other two - first to connect on plugging in, last to disconnect on unplugging.
That's what keeps applianve safe. Any current in ground wire comes from a fault in appliance. Ground wire is sized so as to carry enough current to trip the breaker without developing lethal voltage drop.

That's as succinct as i can put it, hope it helps.

old jim

EDIT oops i see it got answered whilst i was typing. I do "hunt & peck"...

 

[Haecifer]

Thank you as well jim!
It is always good to confirm if you have learned something correctly.

 

[sophiecentaur]

Surely the whole point of having an Earth conductor is that it should ensure that any exposed metal is at the same potential as any other metal, floors, paths,walls etc so tht you can never get a shock, whatever anyone does with the mains supply. That's a general principle and the Earth is the 'crowbar' protection for us all. Any other protection relies on fuses and sensitive enough breakers and is up to the particular method that's used for supplying the electrical power. A simple Earth connection will do nothing to reveal a small, but potential lethal leakage onto some exposed metal work. Disconnect a metal basin from the cold tap and the faulty local heater could kill you if the system didn't 'tell you about' a 100mA leak to ground.

UK and US have different arrangements and most of the arguments we have on this topic are because that isn't acknowledged. I just read, above, somewhere that the Neutral is "always at zero potential" (to Ground) Well, I could say that, in the UK it is Never at Zero potential. You can always measure a volt or two (or more) and that varies throughout the day. Your neighbours on different phases of the Mains Voltage Three Phase supply from the transformer that supplies the street are connected, alternately, to Phase 1,2,3,1,2,3,1,2,3 as you go up the street. As they alter their loads, the neutral volts shift a bit wrt ground.
The Earth in most houses is supplied on the outer casing of the supply cable and this is also connected to all services. As most services are now on plastic pipe, there's not much of a connection to the actual Earth unless someone goes to the trouble of putting in an Earth spike and I don't think it's a requirement in the UK these days (I think it is, in fact, not permitted). So you can't say "always" or "never" when making general statements about mains supplies on international Fora.

 

[dlgoff]

No, Haecifer meant why doesn't it normally flow to ground.

Ground and neutral are both zero potential, so there is no flow between them. One of its mainfunctions is to act like a universal neutral, ensuring everyone has the same reference.

And don't confuse neutral with negative: the Earth isn't sucking-up electrons from the wire.

Well. They are very close to zero potential, but being the knit-picker I am, ... Since there's a LITTLE resistance in the neutral wire leading from the distribution/pole transformer and the panels ground wire leading from the homes Earth ground point, there's a small potential. So there is a little current in both. Just sayin'

 

[sophiecentaur]

Well. They are very close to zero potential, but being the knit-picker I am, ... Since there's a LITTLE resistance in the neutral wire leading from the distribution/pole transformer and the panels ground wire leading from the homes Earth ground point, there's a small potential. So there is a little current in both. Just sayin'

It's not just the resistance in the connecting cable, the resistive components within the transformer will also contribute to unbalanced voltages when the load on each side is unbalanced. If this effect can't be detected then the transformer was badly designed (i.e. too good for the job)

 

[jim hardy]

It might (or might not) help here to note something that's in Digoff's annotated version of the sketch, in posts 6 & 16.

Observe that:

IF you imagine yourself very small and standing just inside left edge) of breaker panel,(not right edge)
AND you draw a dashed circle around just the H and N wires;

When NO fault is present (post 16), ALL the current traveling out on the H wire comes back on the N wire, so your dashed circle encloses equal and opposite currents whose sum is zero.
When a fault IS present (post 6) SOME of the current going out the H wire comes back via the G wire instead of the N wire. So our dashed circle now encloses two unequal currents whose sum is no longer zero.


Aha ! A faulted appliance can be detected before the fault current rises to point of tripping the breaker. Just measure sum of currents on H and N wires, if they're unequal then SOME current is sneaking back through a fault.

That's what GFCI breakers do - both H and N wires(but not G wire) pass through a tiny current transformer and a non-zero result trips the breaker. It's worth taking one apart to see its constuction for it'll cement the grounding concept in your mind.
http://www.fairchildsemi.com/ds/RV/RV4141A.pdf


If your fridge trips the GFCI breaker only once every day and runs fine otherwise,,, look for a fault in the defrost heater circuit... mine was arced through about halfway.

old jim

 

[sophiecentaur]

The 'residual current' circuit breaker is a very clever piece of kit - being able to detect a few mA of imbalance in a 100A circuit. Must have saved many lives since the idea was introduced.
There is another form of Earth Leakage Trip which was used in a house I lived in as a boy. There was a very long two wire feed (1km+) to the house with a very skimpy third (earth) wire. Also, an Earth spike. Between the 'earth' terminal on the consumer unit and these Earth connections there was a current operated relay that tripped at a few hundred mA, disconnecting the supply. It's a dim and distant memory now but I guess it was because an ordinary fuse wouldn't necessarily blow whilst there was an unsafe voltage on the exposed case of the fridge etc. We were only allowed a small supply fuse and no electrical cooking or water heating with that supply, iirc.

 

[dlgoff]

The 'residual current' circuit breaker is a very clever piece of kit - being able to detect a few mA of imbalance in a 100A circuit. Must have saved many lives since the idea was introduced.
There is another form of Earth Leakage Trip which was used in a house I lived in as a boy. There was a very long two wire feed (1km+) to the house with a very skimpy third (earth) wire. Also, an Earth spike. Between the 'earth' terminal on the consumer unit and these Earth connections there was a current operated relay that tripped at a few hundred mA, disconnecting the supply. It's a dim and distant memory now but I guess it was because an ordinary fuse wouldn't necessarily blow whilst there was an unsafe voltage on the exposed case of the fridge etc. We were only allowed a small supply fuse and no electrical cooking or water heating with that supply, iirc.

Probably a little off topic but when you mentioned "(1Km+)" I thought of the Single wire Earth return transmission line which avoids the need for a neutral (transmission line neutral).