White and ground wires

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  #1  
Old 10-05-15, 06:59 AM
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White and ground wires

In wiring my latest project, I was afforded the opportunity to actually work in the breaker box where I learned that white and ground wires are joined together at the busbar.

Why are the whites and ground connected there, but are not allowed to be connected anywhere else? I understand not to, but I do not know the "why".

Thank you,
 
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Old 10-05-15, 08:07 AM
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The "ground" wire, more properly called the "equipment grounding conductor" is NOT a current carrying conductor except in a fault condition. Literally volumes have been written on the subject and still the majority of people are just like you, they know the proper way to treat the two conductors but they don't understand the why.

The equipment grounding conductor has ONE purpose and that is to provide a low impedance (low AC resistance) path back to the source of the power to facilitate the actuation of the circuit's overcurrent device under fault conditions. Or, to put it in Americanized English, the purpose of the "ground" wire is to cause the fuse to blow or the circuit breaker to open when the so-called "hot" wire comes into contact with any electrically conductive substance in contact with the earth.

For example, consider the grounding conductor from a receptacle all the way back to the circuit breaker panel is solidly connected and at the CB panel (we are assuming the "service" or "main" panel here) is connected to the so-called "neutral" bus which is also solidly connected to the "grounding electrode" (ground rod and/or metallic water service piping) via the "grounding electrode conductor" or GEC. This connection to the earth holds the neutral conductor at earth potential and gives a reference point for the voltage measurement of the hot conductor. Now consider a metal-cased drill motor with a standard three-wire cord and plug. The green insulated conductor in the cord is attached to the metal case on one end and the round "ground" connection at the plug. When plugged into the receptacle the metal case is solidly connected to the earth via the equipment grounding conductor and the grounding electrode and its conductor.

Under normal circumstances the the equipment grounding conductor plays no part in the operation of the drill, it will still work just as well if the equipment grounding conductor is disconnected, either intentionally by breaking off the round prong of the plug or inadvertently by the equipment grounding conductor between the receptacle and the CB panel breaking.

However, when a fault occurs, such as the hot conductor's insulation fraying a bit inside the drill case to the point where the hot conductor is now touching the metal case there will be a proportional current flow through BOTH the white neutral conductor AND the green equipment grounding conductor back to the source of the power. If the equipment grounding conductor is somehow severed, that proportional current flow will pass through the hand holding the case, through the body and into the earth, the current flow being a function of the impedance offered by the body, shoes and earth itself.

This is where so many people get confused. They think, incorrectly, that electricity takes the path of least resistance back to the source. It doesn't! Electricity takes ALL paths back to the source and the amount of current on any particular path is dependent upon the impedance of that path. So while 90% of the current will return on the proper neutral conductor there will also be 10% of the total current returning through the body and the earth. (Percentages are stated only as an example.) In this example assume that the drill uses 5 amperes then 4.5 amperes returns along the normal path and 0.5 amperes returns via the body and earth. Considering that 0.5 amperes is the same as 500 milliamperes and that it takes less than 75 milliamperes to stop the heart you can see that you want to keep that fault current as low as possible. That is the purpose of the equipment grounding conductor, to provide a much lower impedance path back to the source than the human body path.

Further, the total current in a circuit is dependent upon two factors, the applied voltage and the total impedance. With a fixed voltage the current flow varies inversely with the impedance, a low impedance means a high current flow. It is the low impedance path that causes a high current to flow and this high current causes the circuit breaker to trip or the fuse to blow.

Does this help you to understand?
 
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Old 10-05-15, 08:13 AM
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This is actually a pretty big topic, but I'll start with the simplest reason: The ground wires are there as a safety net to protect people and property should certain kinds of faults occur in wiring or appliances/devices connected to the power lines. For them to provide this safety net properly, it is important that NO electrical current flow through the wires under normal conditions, but only when there is a fault. If the white wires were connected to the ground wires anywhere else but at the main service, there would be some current flowing in the ground wires at all times and this would prevent them from properly serving their safety function.
 
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Old 10-05-15, 08:16 AM
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The neutral wire carries the current equal to the hot wire (current flows out, current flows back). Ground wires are not supposed to carry current except during emergency or fault situations, and then only long enough for the breaker to trip and current to be cut off.

If the neutral and ground were joined out in the circuit, half of the "return" current would flow on the neutral and half would flow on the ground. Since ground wires are connected to all exposed metal on appliances, in that situation you would have current flowing on all the exposed metal surfaces. This is a potential shock hazard. It becomes a much greater shock hazard in kitchens, bathrooms and laundry where a person can touch a metal appliance with one hand and a copper pipe or metal faucet with the other hand and thus the person becomes yet another path for the neutral current to flow. This situation is made much more dangerous if the copper plumbing is broken at some point and a piece of plastic pipe used to repair. You now have two different "ground" systems, each capable of shocking someone. This sort of thing happens all the time in old houses where people will feel a tingle in the shower when they're standing in a cast iron tub or while standing on a concrete floor and touching the washing machine.
 
  #5  
Old 10-05-15, 08:22 AM
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Thank you all for the long and short answers.

I guess what I have trouble wrapping my head around is that; if electricity takes all paths, how being connected at one point is any safer than being connected at other points. Barring a diode, electricity can still flow back into my body.

But, I now have another starting point for my pondering.
 
  #6  
Old 10-05-15, 08:39 AM
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Because the path from the main panel back to the power company transformer is much, much lower impedance than any other paths due to the large diameter service entrance wires. Some power will flow back out the ground wire(s), through the shared plumbing, through the earth itself, etc. However, the amount of current that flows on each path depends on how good of a conductor that path is compared to the others. That means that something like 99% will flow through the service entrance wires which are excellent conductors and 1% through the other paths which are not nearly as good conductors (approximation).

Compare that to a branch circuit where the ground and neutral are both #12 copper of the same length -- they have nearly equal impedance yielding something more like a 50%-50% split of current flow.
 
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