Question regarding ground wire

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Old 06-02-03, 07:08 PM
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Question regarding ground wire

If neutral wire and ground wire are connected to the same bar then why current is not passed to ground wire? They are all metal connection so shouldn't current passed from neutral wire to bus bar to ground wire?
 
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Old 06-02-03, 07:20 PM
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If there is a problem with your neutral wire then sometimes there is current in the ground wire in the panel. This would only happen on the power company side of the neutral/ground connections. That is why you never connect the neutral and ground together after the main sirvice disconnect.
 
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Old 06-02-03, 09:21 PM
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Current flows from a point of higher potential to a point of lower potential. Some current does flow to ground. But because the earth has such a high resistance relative to metal, it is a very miniscule fraction of the current flowing back to the transformer through the neutral. I wish I could quantify it for you, but I'm sure the number has a bunch of zeroes after the decimal point before you get to a non-zero.
 
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Old 06-10-03, 03:59 PM
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This is what my understanding of electricity.
The black (hot) wire comes from electrical company to the main circuit breaker to the individual circuit breaker. From there to my light bulb's black wire (Assuming this light bulb does not have any switch). Light Bulb's white wire is connected to the neutral wire that came from neutral/ground bus and from neutral bus current goes back to the electric company.
In the event when I accidentally touch the black wire of the light bulb, most of the current instead of passing through me will pass through bare ground wire to the bus bar and back to the electric company. Also, circuit breaker will trip.

Questions:
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1. In the normal condition since all the neutral wires of my house are connected to the same metal bus bar shouldn't the current that came from my light bulb go all around to other ground and neutral wires? I understand it will not go to ground since it has the most resistance.
2. Why neutral and ground wires are needs to be grounded in the first place?
3. I know in the event of over current, circuit breaker will trip because it has magnetic strip, which will pull the switch and disconnect the black wire of light bulb from main black wire that came from the electric company. Since ground wire is never connected to the circuit breaker, in my above example how does circuit breaker know it needs to be tripped?
 
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Old 06-10-03, 05:19 PM
DaveB.inVa
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==>In the event when I accidentally touch the black wire of the light bulb, most of the current instead of passing through me will pass through bare ground wire to the bus bar and back to the electric company. Also, circuit breaker will trip. <==

Not so, if you touch the black wire while the light bulb is in operation you will be shocked. It could be fatal. The current will continue to flow its normal course through the bulb but an amount of current will also go through you to the earth. The circuit breaker most likely will not trip unless it is of the GFCI variety.

I think you have a couple misconceptions about electricity. One is that it takes the path of least resistance. This is untrue. It takes ALL paths, its just that the path with the least impedance will get the most current.

Second electricity does not try to go to ground! It merely tries to get to its source, the transformer. The centertap of the transformer (the neutral) is grounded to limit voltage to earth and to dissipate lightning strikes and contact from higher voltage lines. If you did not ground the transformer everything would work just the same, but by capacitance voltages to the earth could rise to a level higher than the insulation on equipment and wires could take, this would basically cause a blowout ;-).

To your questions.

1. No current will not flow out to all other wires whether they are the neutral or ground. Because the current is trying to get back to its source, the transformer. Its way back to the source is through the neutral (in this case)***. Also remember from before where it was stated that the neutral is grounded to limit voltage to ground. Well this makes the neutral and the earth the same voltage (for a distance). You must have voltage for current flow. With the earth the same voltage as the ground wire, no current flow. The neutral however can have voltage in relation to the actual earth, this is due to current flow and voltage drop along the circuit. This is also why you only ground the neutral once at the main service.

2. The neutral is grounded for the reasons stated above. The ground is there to provide a low impedance path for current flow. If the hot were to short to the frame of equipment, then the ground wire would carry a lot of current, enough to trip the breaker. This keeps you from getting shocked. This is also why you must never ever ground equipment to a merely driven rod. The impedance of the earth is simply too high to allow enough fault current to flow to trip a breaker!!!!

3. The breaker knows to trip because the low impedance path to the transformer, by the ground wire, causes enough current to flow to trip the breaker. For instance if there is a fault from the hot to the equipment case then it is what you could call a short.


*** The neutral is only the "return" for 120v loads. For 240v loads no neutral is needed. The current flows along the two hots. If the equipment needs both 120v and 240v youll need a neutral plus 2 hots and an equipment ground. Like a dryer.

If the equipment needs just 240v then all you need is 2 hots and an equipment ground. Like a water heater.

If the equipment needs just 120v then all you need is 1 hot the neutral (actually called the groundED conductor) and the equipment ground (actually called the groundING conductor. This could be like a vacuum.

You must always switch the hot wire!! If you switch the neutral, sure the equipment wont run, but if someone were to touch the equipment then they can be electrocuted.
 
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