Shorting neutral to Earth confusion

  • #1

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what will happen if i short neutral ac outlet wire with earth? please see the figure if you have any confusion.
 

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  • #2
davenn
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neutral and earth are usually connected at the house entry buss board anyway

Dave
 
  • #3
OldEngr63
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The answer is nothing, provided that everything was properly wired and that the power source generator was grounded. This covers most cases, but certainly not all of them.

With a Y-connected generator, it is easy to ground the center of the generator, and it is common practice to do so. With a delta-connected generator, there is no center point, and consequently such systems often have not grounded neutral (the happens in the US Navy in places).

If in doubt, measure the potential between the two wires before you short them.
 
  • #4
nsaspook
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With a ground to neutral connection at more than one place (at the main power panel) on a normal house AC system the risk of the ground connection becoming a current carrying connection increases. The voltage difference between ground and neutral might be several volts at a distant power plug on a circuit that draws significant current (due to voltage drops from line resistance). If the connection is made at the distant device a GFCI device will trip due the unsafe condition if you are lucky. High currents passing in grounding circuits can be dangerous because they are designed to handle fault currents that only last a short time and are not normally current carrying conductors.

http://en.wikipedia.org/wiki/Residual-current_device
 
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  • #5
NascentOxygen
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Sometimes neutral and active get swapped around, accidently, as can happen when someone makes up their own extension cord, or replaces a broken plug, etc., and in the process gets the wiring reversed through carelessness. Many appliances [seem to] operate satisfactorily despite this, though they can be an electrical hazard to the user.

You can picture the scene were you to unknowingly connect the lead marked "neutral" to ground in the event of it being one of those botched wiring jobs. :eek:
 
  • #6
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there is a concern it that if there is a bad connection (Break) in the Neutral somewhere the Neutral would become live creating a short circuit (hopefully blowing the fuse )
 
  • #7
Averagesupernova
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there is a concern it that if there is a bad connection (Break) in the Neutral somewhere the Neutral would become live creating a short circuit (hopefully blowing the fuse )
How do you figure? Where did you come up with this misinformation?
 
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  • #8
SteamKing
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It's like grounding a ground wire.
 
  • #9
Baluncore
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A “Multiply Earthed Neutral” MEN system is often used to distribute three phase power. The local transformer has a star (or Y) wound secondary. The centre connection is called neutral and is earthed at the transformer, and usually at every second pole. So it takes 4 wires to supply customers in one street. Single phase power is provided to a customer as a two wire line. One is neutral, the other is active. When they enter the customers meter/fuse box they meet the customers buried earth line which gives the three wires available at outlet sockets. There is only one connection (it is in the fusebox) where the earth is joined to the neutral. At no other point should they connect.

Earth leakage detection, AKA residual current detection, compares the current flowing in the neutral and active wires. It does that by winding them together on a magnetic core. Their currents should be equal and opposite so there should be no magnetic field in the core. If a magnetic field is detected then the circuit is quickly broken automatically.

Any current that returns through the earth rather than the neutral will trip the system because the neutral and active currents are no longer equal and opposite. That will happen if you connect an earth to a neutral on a circuit that is drawing current since part of that current will flow through the earth wire. If no current is being drawn while the earth to neutral connection is made it will not trip.

For a street with several single phase customers, each will have their own grounded earth and a common neutral for the street. But their active will be taken alternately from the three phases available. That will tend to balance the load on the phases in the street. So your two neighbours will have a different active phase. Your neighbours neutral, active and earth are all different to yours so they cannot be paralleled.
 
  • #10
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Where I live we have a multiple earth neutral electricity system. With that if you short the neutral to earth some of the current will return to the Neutral through the earth ( through the earth wire attached to the appliance and then through the link between the neutral and earth bus bars in the switch board ) so that if there is a RCD on the circuit it will trip as the current in the Neutral and phrase will not be balanced. If there is no RCD and no other faults it will keep operating with some of the current returning through the Neutral and some through the earth wire. There are differing electricity supply systems , I don't know about where you live.
 
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  • #11
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How do you figure? Where did you come up with this misinformation?
Yeah you are right I'm wrong , for the case of the appliance to become live there would have to be both a bad earth and Neutral and it wouldn't blow any fuse.
 
  • #12
Evo
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Stop me if I am wrong here. But in English the term is GROUND not Earth. I remember years ago my Toyota car manual said to EARTH one end of a connection. They meant GROUND one end.

I think we should be correcting these errors if they are born originally from translation errors.

I look at this

Quote:
Originally Posted by subtech View Post
I've no comments just yet, but earth, ground, and common are about the three most misunderstood words in all of the electrical world.
This discussion should be an eye opener.
I feel the same. That's the main reason why I opened this topic.
http://forum.allaboutcircuits.com/showthread.php?t=6050

and http://wiki.answers.com/Q/What_is_the_difference_between_earth_and_ground

http://ecmweb.com/nec/grounding-bonding-definitions

My dad was an ee, and he never taught me to use earth instead of ground. (he's been dead for over thirty years, so I can't ask him) Is this just an acceptance of the misuse of the translation, or is this something that just wasn't in common use 45 years ago, that has a different meaning now, (earth doesn't mean ground)?

I find it confusing. Please help me out here. Is it correctly grounding, or is earthing something different from grounding now? And if it is different, what is the difference?

I'm beginning to think this stems first from bad translation then becomes literal... Any old folk out there that know what I am saying? I know my dad's old books never said earth, I read them all, he made me do all of the exercises.

Thanks!

well, lost all of what i just posted thasnks to chrome crashing again.

Anyway.

Difference in Terminology:
In USA term Grounding is used but in UK term Earthing is used.
So it is the result of an old translation error.

http://electricalnotes.wordpress.com/2012/09/07/difference-between-bonding-grounding-and-earthing/
 
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  • #13
Baluncore
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Evo said:
But in English the term is GROUND not Earth. I remember years ago my Toyota car manual said to EARTH one end of a connection. They meant GROUND one end.
What Toyota really meant was “connect it to the chassis”.

I have always interpreted the meaning of Ground and Earth as being synonyms.
My guess is that the terms arose about 100 years ago when radio (or wireless) used long wavelengths (or Low Band). At those wavelengths a dipole was not possible, so the local Earth (or ground) had to be used with the aerial (or antenna).
 
  • #15
russ_watters
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Stop me if I am wrong here. But in English the term is GROUND not Earth. I remember years ago my Toyota car manual said to EARTH one end of a connection. They meant GROUND one end.
Yes, but I believe in Britishese, the term is "earth".
 
  • #16
Evo
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What Toyota really meant was “connect it to the chassis”.
Yes, that was the case.


Ground is ground. It's a question of "what" ground. Here are the symbols:
Thanks!

Yes, but I believe in Britishese, the term is "earth".
Yes, and they call cookies "biscuits" and french fries are "chips".

Thanks all of you, I was wondering if terminology had changed or if it was just that 40 years ago I was only exposed to American books and with the internet, terminology became mixed, it seems the latter is responsible for a lot of the mixed terminology I see now. This is what happens when you stop learning and then pick up after 40 years.
 
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  • #17
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Naspook said, "If the connection is made at the distant device a GFCI device will trip due the unsafe condition if you are lucky."
As others explained, that could happen if part of the return current went through ground rather than neutral.

But the opposite is scarier: If an unlucky person touched a life wire at the remote site, current would flow through them to ground (or earth if you like). But as ground and neutral are connected there, the harmful current may return through the extra earth-neutral connection and through the neutral wire. After all, a solid copper neutral wire likely offers a path of less resistance than traveling through the soil. And the remote grounding rod might have a better connection. That means that the GFCI will detect matching opposite currents on life vs. neutral, and fail to trip.

Does this scenario make sense? If yes, then I'd be safer disconnecting the remote neutral-ground short.
 
  • #18
rbelli1
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If someone disconnects the ground while working on the wiring they could end up getting a voltage fed from your incorrectly wired outlet. Generally the green wire is always at 0V potential. Making it easier for that to be untrue is always a bad thing.

BoB
 
  • #19
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Thanks rbelli1. I'd hope that the person working on the wiring would know what they're doing, and turn off power anyway. With the GFCI I'm primarily aiming to protect the normal user from damaged or worn appliances, maybe held in wet hands. BTW, there's no ground wire at the remote barn -- it's all old 2 conductor wiring & 2 prong outlets.
 
  • #20
rbelli1
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In a normally grounded system there could be faults that cause multiple branches to have one ground segment. For example if your panel has a left and right hand ground bar and they are isolated types. Pulling the jumper between them could have one half of the installation with a floating ground. Any faults to ground on that side will cause a dangerous voltage on the ground. However I agree in that case the whole panel should be de-energized.

This clearly is not the case you are talking about. You are starting a totally new ground domain. You should consult your local codes and a qualified electrician.

BoB
 
  • #21
The answer is nothing, provided that everything was properly wired and that the power source generator was grounded. This covers most cases, but certainly not all of them.

With a Y-connected generator, it is easy to ground the center of the generator, and it is common practice to do so. With a delta-connected generator, there is no center point, and consequently such systems often have not grounded neutral (the happens in the US Navy in places).

If in doubt, measure the potential between the two wires before you short them.
Do you mean Us Navy ships have delta connected gen? As i understand ships normally have Y gen with isolated neutral. High voltage 6.6 kv ones are earthed with a high Resistance.
 
  • #22
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Theoretically nothing will happen if you connect the neutral to ground in a balanced system. The reality is a lot more complicated.

There are lots of good things that can happen when connecting the neutral to the ground. There are also lots of bad things that can happen. Most of these involve low probability events (lightning, electrical shorts, voltage spikes, etc.). Therefore the rules for grounding are a) very strict, and b) vary from jurisdiction to jurisdiction. (Which low probability events are most likely in any particular case affect the rules.)

Where I'm heading with this is that local electricians will know the rules for your area. So unless you are either one of those or are a P.E., don't mess with the grounding. A background in electronics or even an E.E. degree is not enough.
 
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  • #23
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Thanks all of you, I was wondering if terminology had changed or if it was just that 40 years ago I was only exposed to American books and with the internet, terminology became mixed, it seems the latter is responsible for a lot of the mixed terminology I see now. This is what happens when you stop learning and then pick up after 40 years.
I know there is a move to standardize both the terminology and equipment (wire colors, etc.) internationally. It's not my field, so I don't know how far along it is or what the changes are.
 
  • #24
jim hardy
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@Evo

The term "Ground" is very commonly used to mean "Circuit Common" which may or may not be tied to earth.
As in your Toyota, where it's insulated from earth by the tires. Or in an airplane or in the ISS. Aerospace industry tends to call it "Vehicle Skin" instead of "Ground".

"Earth" is used by the British. I first ran across the term in a Royal Enfield motorcycle manual for 1956 model year and thought it quaint, like their "Valve" for vacuum tube. But they used it to mean chassis. That planted a seed of thought because of the tires which separate chassis from earth..

Over the years i've seen many, many people including engineers confused by the term "Ground" . They think electricity has some affinity for the earth, which it does not.
I think that confusion stems from the water analogy used to teach basic electrical circuits.
We are imprinted by childhood experience that water comes out of the ground and soaks back into it. We transfer that to electric current which is a mistake. Of course that mistake gets reinforced by our childhood observations of lightning striking the ground.

So, the term "Ground" has dual use.
It means "Earth" in the electrical code . It means "Circuit Common" in most electronic equipment.
By itself then, "Ground" is an ambiguous term.
I always try to avoid that ambiguity by using the more specific terms "Earth" for something that's wired to the earth , and "Circuit Common" for the point in a circuit where all currents are collected for return to the power supply.
"Circuit Common" may or may not be "Grounded", or more precisely it may or may not be "Earthed" .

I've been a broken record in lots of PF threads encouraging use of terms Earth and Circuit Common instead of Ground.
By improving our language we reason better.

old jim
 
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  • #25
jim hardy
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IEEE 142, aka "The Green Book" is an excellent source for a clear explanation of "Grounding" in power systems.
A search turns up lots of excerpts.
I feel a 1 hour course on it should be in all EE curricula.
 

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