Ground / Neutral connection at the Main
I was reading the “Wiring a Subpanel†chat and have a follow up question regarding the ground / neutral connection at the Main panel. PM22 mentioned in his response that “there is as much electricity flowing through the neutral as flows through the hot wire. You get a spark when you cut a neutral which is carrying a load. Learned this at Glen Ellen high school. Also turn your wire cutters into wire strippers [the new notch].â€
My question is that since the neutral carries a load (I understand this concept completely) and the neutral and bare ground wires (from the branch circuits) are attached to the same bus bar at the Main panel, what prevents the ground wires (from each incoming branch) from being energized by the current coming from the neutrals? What prevents the electrons from “backing up†along thru the grounds? Does it have to do with the neutral / ground bus bar’s close proximity to the grounding rod and it’s easy path to earth ground?
Thanks for any insight.
Replies
I am not sure what you mean by "energized" and "electronics backing up".
We commonly say that the "hot wire" has 120 volts on it, for example.
ABSOLUTELY WRONG!!!!
But is is common and acceptable shorthand when you understand what it means. What is means is that there is 120 volts of potential difference between that wire and the neutral.
There are companies that do repair work on "engerized" high tension (50 kv and up) power lines that use helicopters. They work from a platform hanging below the helio and the first they that they do is "ground" the platform to the high tension line. So now there is no potential difference and no current will flow.
For those electroncis to back up into the wire you need 2 things. A voltage potential, which is the "driving source" between it an another conductor and a load between those 2 conductors.
Thanks for the reply Bill,
I was using the verbiage “energized” and “electrons backing up” from previous chats I have read to mean that the wire is carrying current (120v).
You mention that in order to have current you need 2 things; voltage potential and a load. In relation to my question, there is no load in the ground wires, but is there anything that can happen to create potential? I guess without a load, there is no chance of potential, right?
I have head the term voltage potential used many times and have heard many explanations. I have read your posts on previous subjects and you seem to have a knack for explaining things. How do you describe / explain voltage potential?
Ryan
a rough analogy would be:
- the wire is a pipe
- voltage is the pressure of the liquid in the pipe, such as 100 psi
- current is the flow through the pipe, such as 5 gal/min
- the ground wire is like a container around the pipe to catch any leaks
the ground connected to the neutral at the panel is like funneling any leaky liquid to the ultimate destination of the pipe.
Marine Engineer
" I guess without a load, there is no chance of potential, right?"
No.
Again the potential is between too points.
Yes you will have 120 volts of potential between the hot side and the ground in residential wiring.
And whenever you have current flow you will have a small voltage drop. So at the load end of circuit loaded to maximum there will be a small difference (potential) of 2 volts are less between the neutral and the ground wire.
" I have read your posts on previous subjects and you seem to have a knack for explaining things. How do you describe / explain voltage potential?"
I don't know if you know anything about hydraulics.
The comparison to water pressure is the same as voltage. But you say that we don't have a neutral or other point that we measure pressure against.
Yes we do it is atmospheric pressure. Most pressure gauages read in waht is called gauge pressure or pressure relative to the atmosphere. The pressure gauge uses a diapham with water pressure on one side and air pressure on the other and the difference is what the guage reads.
Lets say that you have filter that you think might be restricting the flow because there is too much pressure drop. Well you put a pressure gauge on each side and measure it and take the difference. Or you could use a differential guage that allows the one pressure on one side of the diaphram and the other pressure on the other side.
The water pressure is just like voltage potential. It is the force that will drive the current (flow) through a load (orfice).
It is always measured with respect to another point. In electroncis as in working with industrial processes there are many different points that the voltage or pressure needs to be measured againsts.
But in house wiring and as in house plumbing 99% of the time that other point is the neutral or atmosphere.
When you hook the bus bar to the copper water pipe, the rebar in the foundation, or a ground rod you are electrically tieing your metal to earth ground. The earth is modelled as an infinite source of electrons capable of sourcing or sinking any number of electrons without changing its potential.
Potential, opposites attract and likes repel. When one puts a whole bunch of electrons on a piece of metal insulated from the rest of the world the electrons start repelling each other. You keep adding more electrons and the higher the potential gets. Now you put a load between this source of electrons with a high potential and the electrons will do work to get to ground. We measure that work in Watts. Lighting a 100W bulb is work. Current is a measure of electrons per second flowing through the bulb and is called amps.
So on a surface at 100V potential, electrons are trying to get away from each other with a force of 100v in comparison to the zero volts potential electrons are trying to get away from each other on the earth ground surface.
When you cut the neutral and melt your cutters its because you are trying to stop the flow of current and your cutters are now like the electrodes in a arc welder.
There's another wire in the story. The bare wire. No current is supposed to flow in the bare. So if you cut it, it does no welding on your wires. It's only there as a safety factor.
In a three wire cord the bare is connected to say the metal housing on your electric drill. It creates a short between you and earth ground. Likely, you are standing on earth ground so there is no potential difference between you and earth ground. Now if the hot wire in your drill breaks and comes into contact with the housing, the electricty takes the path of least resistance back to ground. It there's no bare wire that path is through your body.
This probable as clear as mud.
The real earth is far from being a good conductor or an infinite source or sink for electrons. Its resistance is high enough that when a high voltage line touches the ground, you get deadly voltage gradients for several yards around the contact point. If that happens and you survive, don't walk or run away, hop away on one foot, or with your feet together.
Ground rods are typically a few dozen ohms to true earth ground. Theoretically, code lets you drive one rod if it measures under 25 ohms, and a second is required if it's over 25. A dead short of 120 volts to a 25 ohm ground rod would pull 120/25 = 4.8 amps, not even enough to trip a residential branch circuit breaker.
Ground rods have nothing to do with conducting significant amounts of current (except for lightning strikes), or with tripping breakers. They exist to keep the voltage between the somewhat conductive dirt and the exposed metal parts of your electrical system as close to zero as possible, so you don't get shocks.
There's no reliable way to make absolutely sure that everything connected to the secondary of a transformer on a power pole -- yours and your neighbors' electrical systems -- stays floating with respect to true earth. Something 's going to accidentally touch ground someplace, and if it's a hot, then all the exposed metal would be 120 volts from earth ground, and one neighbor could get electrocuted by a wire touching a pipe in somebody else's house.
With grounded electrical systems, a hot wire stuck in the dirt sets up voltage gradients around it, but it's competing with all the residential and utility company ground rods that have a much lower impedance to the ground, so the dangerous region is much much smaller.
-- J.S.