3 very easy questions,please help I am really confused

[Kaaud]

Question 1I dont know how can this be a closed circuit?? How will current flow between the two earthing bars??
If this works,what is the need for neutral wires?? why dont we just dig a big earth bar at each building to close the circuit??

Question 2
As u can see,the neutral is earthed,the neutral carries the same current as line (hot wire),why doesnt all the current flow in the earth,knowing that the earth has less resistance than the circuit.

question 3
As u can see,the second leg of the transformer is earthed?? Does this really happen practically?? we just earth a leg of a transformer instead of connecting it to the neutral??

Help me

 

[haskindm]

Current will flow through the earth, but it is a high resistance path. While conductor resistance in a properly designed circuit may often be measured in milliohms, earth resistance is, at best, measured in ohms. If a low resistance path is available (ie a neutral conductor), the majority of the current will flow on it, but electricity takes all available paths so there will be neutral current traveling back to source through the earth. Can this cause problems? Yes, but it is the system that we have. That is one reason that bonding around a swimming pool is so important, all metallic objects must be held to the same potential.

 

[rattus]

Question 1I dont know how can this be a closed circuit?? How will current flow between the two earthing bars??
If this works,what is the need for neutral wires?? why dont we just dig a big earth bar at each building to close the circuit??

Question 2
As u can see,the neutral is earthed,the neutral carries the same current as line (hot wire),why doesnt all the current flow in the earth,knowing that the earth has less resistance than the circuit.

question 3
As u can see,the second leg of the transformer is earthed?? Does this really happen practically?? we just earth a leg of a transformer instead of connecting it to the neutral??

Help me

Your diagrams are missing a primary neutral which must be connected in (A) and (C). You cannot use the earth as a current carrying conductor.

However, in the early days, some local telephone loops used a ground return path, but we are talking of milliamps instead of amps.

 

[don_resqcapt19]

A1) The earth is a high impedance path and that circuit will not work well in most cases. It would also be a code violation.
A2) The current does not want to get to earth...it only wants to get back to its source. The earth will not have a lower inpedance than the circuit. Note that the source on the secondary of the transformer is not the same source as on the primary. Transformers are the most common source for aSDS (seperatly derived system). Current from the secondary will not return to the primary.
A3)The only way to create a grounded system is to connect one leg of the transformer to a grounding electrode system (earth). This connection is required for most transformers.
Don

 

[charlie b]

A1) The earth is a high impedance path and that circuit will not work well in most cases.

Isn?t it more accurate to say that the Earth is a very low impedance path, but there is a high impedance in the connections between the power system and the Earth? That is, it is the interface between the ground rod and the surrounding ground that has high impedance (i.e., on the order of ohms).

The way I look at it, if you put two resistors in parallel, the net resistance is lower than either of the two. If you put a billion resistors in parallel, the net resistance becomes very low indeed. I think that there are billions of parallel paths between any two ground rods. These include paths in a straight line from one to the other, and paths that go through the center of the planet, and paths that go from my house to my neighbor?s house (in Seattle) via Mike Holt?s offices (in Florida). A box of dirt may have a high impedance, but a planet?s worth of dirt will not.

Am I off-base in this way of thinking?

 

[Mike01]

hmm got me thinking

hmm got me thinking

what about S.W.E.R. systems? how would they operate? don't they use the earth as the for a return path?

 

[Kaaud]

So if earth has higher impedance than the circuit it wont be much help at short circuits??

Cause the short circuit will be much similar to my second diagram but with zero ohm load??

How can earthing protect the single phase circuit then?

And really thx for replying and approving my first post,I really appreciate it,cause this forum looks like a gold mine!

 

[Kaaud]

Your diagrams are missing a primary neutral which must be connected in (A) and (C). You cannot use the earth as a current carrying conductor.

However, in the early days, some local telephone loops used a ground return path, but we are talking of milliamps instead of amps.

so what will happen if I earthed the second leg of a 40 KVA transformer as shown in the diagram,what are the problems other than code violation

 

[Kaaud]

A3)The only way to create a grounded system is to connect one leg of the transformer to a grounding electrode system (earth). This connection is required for most transformers.
Don

You mean connect one leg to neutral and earth,not just earth right??

 

[Kaaud]

Isn’t it more accurate to say that the Earth is a very low impedance path, but there is a high impedance in the connections between the power system and the Earth? That is, it is the interface between the ground rod and the surrounding ground that has high impedance (i.e., on the order of ohms).

The way I look at it, if you put two resistors in parallel, the net resistance is lower than either of the two. If you put a billion resistors in parallel, the net resistance becomes very low indeed. I think that there are billions of parallel paths between any two ground rods. These include paths in a straight line from one to the other, and paths that go through the center of the planet, and paths that go from my house to my neighbor’s house (in Seattle) via Mike Holt’s offices (in Florida). A box of dirt may have a high impedance, but a planet’s worth of dirt will not.

Am I off-base in this way of thinking?

I thought that current doesnt flow under earth between two states,I thought that the circuit exchange electrons in and out between earth.

Notelz guys try to reply to my last 4 posts

 

[winnie]

The Earth is a conductor. Like any conductor you can characterize it in terms of voltage drop at the desired load current.

If the voltage is high enough and the load current is low enough, then the voltage drop through the earth will be acceptable and the load will be properly powered.

A method known as 'single wire earth return' is still used for power distribution in rural areas. The distribution transformer is connected between a distribution conductor and earth. Say the distribution voltage is 7200V, and the grounding electrode impedance is 10 ohms. With 3A flowing on the primary side, you have a 30V drop in the grounding electrode (thus less than 0.5% voltage drop), but you are delivering some 20KW to the load, more than enough for a few homes.

In your electrical distribution diagrams, circuits are drawn connected to 'ground'. However it is very likely that this represents a connection to a 'multi-earth-neutral', meaning a conductor that is continuous in the distribution network, but which is connected to earth at many locations.

The 'earthing' of the secondary is generally accomplished by connecting to the multi-earth-neutral, but it could also be accomplished by connection to a grounding electrode.

Finally, on Charlie's point, approximating the earth as a perfect conductor, with the impedance concentrated at the grounding electrode is a very good approximation. Imagine a series of concentric shells around the grounding electrode. For current to flow from the grounding electrode to the greater earth, it has to pass through each of these shells in turn, and as with any series circuit, the resistance adds. But each larger shell has greater and greater cross section, and thus lower and lower resistance. Very quickly the additional resistance of each shell is less than a similar length of copper grounding electrode.

-Jon

 

[Kaaud]

In your electrical distribution diagrams, circuits are drawn connected to 'ground'. However it is very likely that this represents a connection to a 'multi-earth-neutral', meaning a conductor that is continuous in the distribution network, but which is connected to earth at many locations.

Sir u r a genius,because somebody told me the same thing,and I think it is right.

thx alot.

 

[kc8dxx]

IMHO the book symbology is a bit misleading...

IMHO the book symbology is a bit misleading...

kaaud, As I look at the diagram from the book, it appears to me that the author used a ground symbol as 1) a symbol to indicate a connection to a grounded (earthed) current-carrying conductor back to the source, and as 2) a symbol indicating an earthing rod. IMHO the author left out the current-carrying conductor lines in both diagrams, which makes the diagrams potentially confusing. IMHO the symbol on the primary of circuits A and C indicate a connection to a grounded (earthed) current-carrying conductor, which runs back to the source. IMHO the symbol on the secondary of circuits A, B, and C indicate an earthing rod. Practically speaking, yes, those earthed connections do exist (see NEC article 250 section X).

Charlie, While I agree with what you say in theory, IMHO the earth is not modelled so simply as a set of parallel resistors. I believe a better analogy would be that the earth can be modelled as a set of LCR impedances, where the LCR values vary depending on the composition of the earth, the distance away from the current path, and the frequency. Part of the consequences of this is that in lightning protection for a communication facility (or my amateur radio station antenna), one cannot simply put a ground rod in and think you're done. Professional setups have extensive ground rod systems installed, spaced out and interconnected in a methodical manner, all to entice the lightning strike current back to earth instead of down the coax line to the transmitter. Of course another consequence is that my wife and neighbors think I'm off my rocker for burying copper wire to the back yard, but that's a story for another day.

 

[winnie]

So if earth has higher impedance than the circuit it wont be much help at short circuits??

The earth is no help at all for short circuits at normal usage voltages. If you hammer an electrode into the ground, and connect a 120V lead to it, then the current flow will be far too low to trip a breaker. The voltage on the electrode relative to the earth you stand on will be enough to deliver a significant shock.

At distribution voltages, earth resistance is low enough that a ground fault will cause signficant current flow.

How can earthing protect the single phase circuit then?

Earthing provides protection of the insulation system from transient high voltage; eg caused by small static charges. It does not provide short circuit protection.

_Bonding_ of conductive metal provides short circuit protection. Since the conductive metal (pipes, rails, building steel, etc.) is often physically in contact with the earth, bonding is often closely tied to earthing. But bonding provides short circuit protection even with no earth contact, for example in an airplane.

so what will happen if I earthed the second leg of a 40 KVA transformer as shown in the diagram,what are the problems other than code violation

It would not be a code violation to use a grounding electrode with no connection to the primary neutral. This is the case with an isolated generator plant; the generator _neutral_ is grounded via an electrode.

You mean connect one leg to neutral and earth,not just earth right??

A grounded system simply requires on leg of the derived system to be connected to earth. If the power source is a distribution network with a multi-earth-neutral, then the convenient ground would be that MEN conductor. But if the power source were a SWER system, or a generator, then the ground is simply a grounding electrode. Connection to the _primary_ neutral is not necessary for grounding of the _secondary_ circuit.

I thought that current doesnt flow under earth between two states,I thought that the circuit exchange electrons in and out between earth./QUOTE]

The earth is both a conductor (permitting current to flow between two sites) and also an extremely large capacitor; so a certain amount of current just flows into and out of the earth, with no closed galvanic circuit required.

-Jon

 

[Pierre C Belarge]

The earth, taken as the "planet earth" is considered a great conductor. Parts of the earth, such as local to the services usually are not nearly as good a conductor as the "planet earth".

As has been mentioned, the higher the voltage, the more current will flow through the earth.
That is the reason in the USA and other areas that follow the NEC, do not permit the earth as a conductor. Yet the utility companies can and do.

Lastly, the earth connection to "system" wiring is not intended for the facilitating of overcurrent devices in premises wiring.

 

[Kaaud]

My understanding summary,plz comment

My understanding summary,plz comment

1-The first diagram
it is used at signal voltage (under 50V in electronic circuits),the circuit exchange electrons with earth,or current flow between earth rods??

2-About using earth rod as a return bath for transformers,it is used in rural areas (SWER), and the current finds its way back through earth.

However the books diagram doesnt mean this.

3-About earthing neutral,earth connection has higher impedance than the circuit,and the protection is done when there is high static voltage,where significant current flows through earth,right??

Or/And is it used to protect persons in contact with the circuit?

I still see it can be used against shorts?? same principle,right?

 

[haskindm]

Read Artcle 250.4(A)(1) in the NEC which gives the reasons that electrical systems are grounded. There is nothing in there about protecting against short circuits, operating overcurrent devices, or protecting people. There is a great deal of misunderstanding regarding grounding and its puposes. Grounding may serve many purposes; lightning protection, static charge disipation, voltage stabilization, etc. Providing a return path for voltage at utilization levels is NOT a purpose of grounding.

 

[Kaaud]

Read Artcle 250.4(A)(1) in the NEC which gives the reasons that electrical systems are grounded. There is nothing in there about protecting against short circuits, operating overcurrent devices, or protecting people. There is a great deal of misunderstanding regarding grounding and its puposes. Grounding may serve many purposes; lightning protection, static charge disipation, voltage stabilization, etc. Providing a return path for voltage at utilization levels is NOT a purpose of grounding.

Can u give me the link plz?

 

[cripple]

3 very easy questions,please help I am really confused

Utilizes are using the earth as a parallel path back to the source, which is causing stray current to flow in the earth. The NESC (National Electrical Safety Code) is what the utilities and it requires a install a neutral conductor (static wire) and are required to grounded (earthed) it four times per mile. The currents flowing in the earth are call stray currents, which are causing hazard step potentials, as a result of voltage gradients. The NEC is trying to address the hazard by requiring not permitting use the earthy as return path or as an effective ground-fault current path. And by requiring the equipotential bonding grid in swimming pools and equipotential planes in agricultural buildings.

 

[charlie b]

Can u give me the link plz?

If you mean a link to the NEC, you can start here:
http://www.nfpa.org/freecodes/free_access_agreement.asp?id=7005SB

This is a way to gain free access to the text of the NEC, but it is not a very convenient way. You cannot search, or copy, or print, but you can read. You have to first agree to their rules.

If you want to have a more convenient method of reviewing the NEC, you will have to buy it for yourself.