Difference between AC and DC current

In summary: Basically, the neutral wire is always grounded, but the hot wire can be either live or neutral. If it's live, then it is receiving power from the AC source, and if it's neutral, then the AC source isn't supplying power to it.
  • #1
QuantumTheory
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0
What's the difference?
 
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  • #2
alternating current varies with time, sinusoidally... where as DC remains steady_ hence the names.
 
  • #3
I'll elaborate here.

Exequor was referring to the flow of electrons. In AC, the electrons change direction sinusoidally (in the UK, for mains electricity this frequency will be 50Hz, in the US it's 60Hz I believe). In DC the electrons will carry on flowing around the circuit without changing direction.

In terms of practical differences, AC is used to transmit power over long distances for a few reasons. Firstly, electricity comes out of the generator at the power station as AC. Secondly, it's easier to 'step' up and down the voltage of AC electricity to convert high voltage/low current to high curren/low voltage. To transmit over long distances, AC offers the advantage because resistive losses are minimised with the use of high voltages.
 
  • #4
Well said!
 
  • #5
ok, but how does the current change direction in AC, if it were to change direction as you say the 2 wires would alternate between pos and neg polarity and the direction changes, so if i had a loop made of some AC cable, the electrons would flow clockwise for half of the 60hz, and counter clockwise for the other half of the 60hz. But if this was true then touching the pos or neg wire and a ground will result in a shock pulsing at 30hz, because ur only gettin one of the direcions, and even if it wasent pulsing at 30hzm you just plain don't get a shock if you grab the ground and the white lead (the non hot one i think), but ur fine if u touch the ground and the black(hot lead) u get a shock, what gives?
 
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  • #6
Not sure what you mean.

AC does mean that the electrons change direction constantly. With AC, we don't think of a wire being positive or negative, because of this constant change. If you touch either of the wires and ground, then you will probably get a shock. You won't just be 'getting' one of the directions, - the flow of current is constantly alternating.

I'm not being rude, but can you try and rephrase your question? I don't understand you.
 
  • #7
In DC there is a constant polarity over time, where as in AC the polarity changes with time. I don't know how this relates to shocking in AC because at any give point there will be current flowing in one of two directions. If we go back to a simple AC generator it would be seen that if a magnet is spining and the coil is stationary (or vice versa) current would be induced in one direction for a 1/2 cycle and then in the other direction for another 1/2 cycle because the poles of the magnet would have swapped positions. So at 50Hz if you touch the live wire and you are grounded you should get a shock.
 
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  • #8
The only alternating wire is the live (hot) wire. It alternates between 240v and -240v 50 times a second (50Hz) here in the UK. The neutral (non-hot) wire is effectively tied to ground (0v) which is why you won't get a shock if you hold the neutral (non-hot) and groud wires (they're both, theoretically, at the same potential).

When the live (hot) wire is at a positive voltage, electrons will flow from the neutral (non-hot) to the live (hot), but when the live (hot) is at a negative voltage electrons will flow from the live (hot) to the neutral (non-hot).
 
  • #9
im still u clear why you only get shocked when you touch the live(hot wire) and other the ground or the neutral. If u touch the neutral and the ground nothing will happen. I asked my grandfather this(he was an electrician) and he can't really explain AC to me, but he did verify that you do not get a shock from the neutral wire and the ground wire
 
  • #10
Because both the neutral and the ground wire are at the same voltage, 0v, so electrons (current) have no need to flow. No shock.

The hot wire on the other hand is not at the same voltage, it is either higher or lower than the ground (changes 60 times a second). Therefore the electrons will flow from the wire which is at the lowest voltage, to the one which is at the highest voltage, and if they have to flow through you in order to do that, then they will!
 
  • #11
Colt,

In America, if one looks into the Power panel, one would most likely see all the ground wires (generally bare, sometimes green) tied into a buss on one side, all the white (neutral) wires tied into a nearby bus and the black wires (hot) come out of the fuses/breakers. There will be a connection between the white wires/bus and the ground bus, if it isn't a single bus.

This is the single-point ground reference for the neutral line in the building. The reason why one can touch the white wire and not get shocked is, as cosmik has said, because the neutral wire is electrically grounded.

However, do not count on the white wire always being neutral... the average homeowner doesn't necessarily know which wire is which and there are miswired electrical outlets around, so one should treat all wires as live until proven otherwise.
 
  • #12
Regardless of country, a three-wire AC system consists of an active wire, a return wire, and a neutral wire. The active wire is "live" at all times, carrying a potential that varies by 240volts or 110volts (rms) above and below the reference level (ground). This reversal in potential means that the electrons will flow in alternate directions during each half of the cycle, hence the name "alternating current". Work is done by the passage of the electrons, regardless of the direction of flow, so this point is largely irrelevant. As to when you get "shocked" ...

Electrons will only flow if there is a return circuit to the source, and this is the purpose of the return wire. When you close the switch in your circuit (light bulb, freezer, whatever) the return wire will also be "live" as it will be connected to the active wire through the energised device.

The neutral wire is normally connected to Earth (usually at one point in the wiring system) and provides a reference level for those devices that need it. (Note that those devices with only two wires connecting to them are referred to as "double insulated" and do not have a neutral connection). The neutral should not carry current - if it does, this indicates a fault and will normally cause your Earth leakage protection devices to trip. This is what happens when you cut through your cabling with a power saw or put your knife into the toaster while it is on ... IF you have Earth leakage devices fitted ... DO NOT try this at home ... it is supposed to save your life but...

If you have the switch on, then both active and return wires are "hot" or "live" - if you touch either of these at the same time as the neutral or some other grounded surface then you will get a shock (and hopefully cause the Earth leakage to trip in some fraction of a heartbeat and hence save your life). This obviously also applies if the switch is not on and you touch active and neutral simultaneously.

If the switch is not on then the return wire is not live. You can touch both return and neutral and live ... but why tempt fate? If you touch active and return then you bypass the switch and become the electrical device ... in this case the Earth leakage devices in your house may not trip as there is no excessive current on the neutral line - you are now relying on the fuse or circuit breaker in the active line to trip but these are slower than Earth leakage trips and there is a greater chance that they will not save your life.

Summary ... switch on -> 240v, 240v, 0v ... switch off -> 240v, 0v, 0v ... touch two wires of different potentials and life gets electric.
 
  • #13
Xodar, I live in the U.S. so I am not 100% sure of how things are in other countries electrical-wise but I think you have posted quite a bit of mis-information.

You talk about the neutral not carrying any current. In the U.S. the neutral carries the same current as the hot wire in 120 volt appliances. In 240 volt appliances touching any wire except the neutral will give you a shock regardless of how the switch is thrown. I could go on...
 
  • #14
I am appropriately chastised ... although I think it is miscommunication rather than "misinformation".

It does appear that North America is different to the rest of the world (again) in having two live conductors (http://en.wikipedia.org/wiki/Split_phase). This is based on a split phase system where the 240v supplied to the house is split with a centre tap on the transformer, giving a central connection connected to ground (and called neutral) and two 120 volt lines. As you say, a connection for a 240volt appliance then requires both live wires (and is a very dangerous beast). I expect the 120 volt devices are far more common, and these I expect conform to the description that I suggested of one live wire and a return (neutral).

Those parts of the world that use 240volts for everything (UK, EU, Aus, etc.) have only one live wire (240v) to the socket, and use the "neutral" (what I originally called the "return") to complete the circuit once the appliance is plugged in and switched on. Only then are both lines "hot" or live. (http://en.wikipedia.org/wiki/Domestic_AC_power_plugs_&_sockets). In these plugs the third wire is the earth, which is distinct from the neutral at this end of the wiring system. Because the Earth wire does not normally carry any current in these systems, it is possible to Earth appliances by connecting their frame to this third wird (no current -> no potential drop -> ground potential throughout the house).

Other potential for terminology confusion: the Earth Leakage Trip that I mentioned is apparently called a GFCI in the US (Ground Fault Circuit Interruptor).

I hope this satisfies any translation concerns...
 
  • #15
I wouldn't say that a 240 volt appliance in the U.S. is any more dangerous than anywhere else.

Can you please explain in detail why you think that touching a wire on a 240 volt system will give you a shock when the appliance switch is on but not shock you when the switch is off?

Calling the return wire 'neutral' in parts of the world where only 240 is used gets the point across to those of us on the split phase system, but it is actually incorrect. It is so called neutral because it is in the center of the transformer. It is 'in the middle'. It is correctly called the return on these systems.
 
  • #16
Xodar – You do have some misconceptions. The power distribution system in Australia (ignore frequency and voltage standards) is very similar to that of the US. Both are slightly different than England, all three different than France, and all four different from Japan. The difference lies in the means of referencing the neutral to ground (Earthing); it is variations of isolated, impedance grounded, single point grounded, or multi-point grounded. The differences arise from opinions on the means of efficient and uninterrupted power transmission, not safety. It is at the point of use wherein safety concerns are prioritized. For purposes of safety, in Australia a person touching a live wire and ground would get a 240vac shock, in the US the person would get a 120vac shock. If the person in the US touched both live wires he would receive (as will the Australian) a 240vac shock. Both countries require all exposed metal to be earthed or the internals to be doubly insolated. How the neutral is derived, whether it is connected to an appliance or not, has no bearing on safety.

In my opinion, the 240vac system as in Australia is slightly superior in the sense it is less confusing to a layperson when attempting to do his own electrical work at home. We have single and double pole breakers, shared neutral systems wherein the GFCI or your Earth Leak Trigger cannot function, and the 120vac branches may require larger conductors.


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  • #17
Actually, in the US if one touched both the hot wire and neutral on a 120V circuit, then that individual will receive a 120v shock. Only within the power box or 440V circuits will they receive a 440VAC shock.

I am familiar with American electrical systems, so I will talk specifically about our electrical distribution. The power coming in form the service (generally a pole, but more and more often in urban settings it's an underground service) comes as three wires and a ground. The wires are Black, Red and White, generally, where black and red are "hot" legs of a 240VAC service, the white is a "neutral" point taken off a center-tap on the power transformer to provide a reference between the two hot wires. The ground is a cable that provides an Earth connection for the entire mess.

As someone mentioned earlier, code requires all exposed metal components in an electrical system to be tied directly to ground, including metal boxes and conduits.

In the main circuit box (circuit breaker box, fuse box, etc.), the white neutral wire will be tied to the ground. The red cable will go to one side of the breaker mains and the black to the other. Each breaker/fuse will tie into one or the other hot main (a 240v breaker will tie one leg into each) and provide overcurrent protection for the circuit wired into it. One can also get Ground Fault Circuit Interrupt (GFCI) breakers for the main box, though at cost.

All circuit wires will have their white wires tied into the neutral buss, all green or bare ground wires will be tied into the ground buss, each red or black hot wire will go into its respective circuit breaker.

When wiring switches, the switching will be done on the hot wire... neutral and ground wires should not be interrupted.

The tie point between neutral and ground in the main box should be the only placed in the house where these two wires are connected (sub-breaker boxes will have separate neutral, ground connections)... this is to protect the user. Any device which has a three prong plug attached will ground its case. That way, if there is a short from the hot or neutral to case, the case will not become "hot" and create a potential hazard to the user. Even with circuit breaker protection, a short circuit will allow current to flow in the amount of time it takes for the breaker to trip, and with a properly grounded case, the current will go directly to Earth and the user would be protected from shock.

If there are any questions, read this and if that doesn't adequately cover it, ask away.
 
  • #18
Sometime ago I asked about phases in this forum and I think the whole thing was mis-understood. When I was talking about a 2 phase system I was not referring to this split phase that you guys mentioned earlier. I live in the Caribbean and or a residential connection you get 240V from the pole, a live (240V) and a neutral which go to you circuit breakers (I'm omitting the electricity meter). You also have to setup an earthing pole outside of your house with the cable going to you main box. For every outlet in your house you have three connections, the live, the neutral (return) and the ground (earth). If you hold the live and any other one, or the live alone you get shocked, since your feet will be grounded.

Now back to phases, I think that prior to this I was told that there was nothing as 2 phase and 3 phase, but for commercial applications here the buildings get 3 wires from the pole (2 live and a neutral). I don't know for sure if they just spit one phase and send it as two separate live connections (split phase) but I was taught in Physics:electricity that in an electric (AC) generator if you have 3 coils place 60 degrees apart with the magnet spinning at the middle then the angle at which the induction takes place would result in the electricity having 3 phases (and this can be shown on a graph). So could there be more than one phase?

Btw, I wanted to clarify this. If you hold the live and the neutral but you body is in the air (you are suspended with no strings, etc._ its hypothetical), would you receive a shock? Or would the current just flow from live to neutral?
 
  • #19
exequor said:
Btw, I wanted to clarify this. If you hold the live and the neutral but you body is in the air (you are suspended with no strings, etc._ its hypothetical), would you receive a shock? Or would the current just flow from live to neutral?

Well, you tell me. The current would indeed flow from live to neutral. Where is it flowing through? Your body perhaps? :wink:
 
  • #20
No, wrong question. the question should have been if you hold onto the live wire (with both hands or one) and you are not grounded, would you get shocked?
 
  • #21
My advice was, I believe, don't touch anything. I actually said that if you touch two wires of different potential, you will get a shock. If you touch one wire then it depends on the potential of that wire and anything else you are touching - you already know this.

My mention of the "switch being on" was a (poor) attempt to say "when the circuit is complete". In UK/Aus we do not have plugs without switches, so the process of energising any circuit requires that a device be plugged in and the switch be turned on ... hence the switch reference. The socket will be wired as [240v ... Neutral (0v) ... Gnd].

When the circuit is complete, you will have a return current and both wires will be "hot". When the switch is off (circuit is incomplete) there is no return current and only the 240volt wire is "hot". Notice that the Neutral line will NOT be at 240volts when "hot", i.e. when carrying the return current. The majority of the 240 volts is dropped across the appliance. The voltage at any point on the Neutral line will be the product of the return current and the resistance of the neutral wire back to the Ground point. The low voltage doesn't make it much less dangerous, as we all know it is the current that will kill you, but at least it is not dangerous when the circuit is incomplete.

In the US 240 volt plug, I believe you have [120v...Neutral...-120v] and the return current is carried by the second 120volt line. Even when you do not have an appliance plugged in you have two "hot" wires, courtesy of their potential.

You are correct in saying that the term "neutral" is related to being in the centre of the transformer, but incorrect in suggesting this doesn't apply to 'conventional' single phase 240 volt systems. The 240volt line is one phase of a three phase system, and the "neutral" is derived from the centre-point of the three phases. It is at the "centre" of a three phase transformer. So this usage of the term is no less correct. I expect you will find that you have a different "neutral" on the primary side of the centre tapped secondary, and that it would be related to the three phase system distributed through your neighbourhood.

In the US case this means that the supply inside the house is isolated from the street supply, which might be an advantage although I can't think why at the moment.
 
  • #22
Dngrsone said:
...The power coming in form the service (generally a pole, but more and more often in urban settings it's an underground service) comes as three wires and a ground. The wires are Black, Red and White, generally, where black and red are "hot" legs of a 240VAC service, the white is a "neutral" point taken off a center-tap on the power transformer to provide a reference between the two hot wires. The ground is a cable that provides an Earth connection for the entire mess.
Unless it’s due to a weird local electrical code you will be hard pressed to find a 4-conductor cable from the utility xformer to the residence, as the grounding conductor doesn't exist. The utility provides a neutral conductor and two "hot" conductors only. With an additional grounding conductor you would only provide a parallel path for neutral current since the safety ground and neutral conductor are bonded at the residences service entrance.

"Hot” leg conductors need not be black or red, they can be any color except white, gray, or green. My 14-year-old home’s main breaker is supplied by the utility with two black “hot” conductors and one un-insulated neutral conductor.
Dngrsone said:
... and with a properly grounded case, the current will go directly to Earth and the user would be protected from shock…
Your example (properly grounded case) evidences a common misconception. Some smaller amount of current will go to go directly to Earth and then to the transformer center tap but the low impedance return path to the utility transformer is obviously the neutral conductor, not the parallel path through the dirt.


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  • #23
exequor said:
No, wrong question. the question should have been if you hold onto the live wire (with both hands or one) and you are not grounded, would you get shocked?

Not unless you can sense a nano-ampere current flow or don't take a pee.

I think the Myth Busters disproved the "pee" aspect though I wouldn't try it.

You have to have a complete circuit path, source-load-source, to have current flow. Air is generally a good insulator, an on-off switch depends on it, and your PC circuit boards depend on it. I suspect you would also be perfectly safe standing on the ground if ground were a Saharan sand dune.

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  • #24
Xodar said:
My advice was, I believe, don't touch anything. I actually said that if you touch two wires of different potential, you will get a shock. If you touch one wire then it depends on the potential of that wire and anything else you are touching - you already know this.

My mention of the "switch being on" was a (poor) attempt to say "when the circuit is complete". In UK/Aus we do not have plugs without switches, so the process of energising any circuit requires that a device be plugged in and the switch be turned on ... hence the switch reference. The socket will be wired as [240v ... Neutral (0v) ... Gnd].

When the circuit is complete, you will have a return current and both wires will be "hot". When the switch is off (circuit is incomplete) there is no return current and only the 240volt wire is "hot". Notice that the Neutral line will NOT be at 240volts when "hot", i.e. when carrying the return current. The majority of the 240 volts is dropped across the appliance. The voltage at any point on the Neutral line will be the product of the return current and the resistance of the neutral wire back to the Ground point. The low voltage doesn't make it much less dangerous, as we all know it is the current that will kill you, but at least it is not dangerous when the circuit is incomplete.

That doesn't make any sense. You have to have voltage to push the current through the relatively high resistance of the human body. You say that the return wire will be 'hot' when the circuit is complete but yet it has a low voltage. Pick one or the other, you can't have both.


Xodar said:
In the US 240 volt plug, I believe you have [120v...Neutral...-120v] and the return current is carried by the second 120volt line. Even when you do not have an appliance plugged in you have two "hot" wires, courtesy of their potential.

True, but technically it's not a neutral. It is a ground. A true 240 volt appliance such as a water heater, (yeah I know it doesn't actually plug in) has three wires and the third is considered a ground. The ground is not meant to carry ANY current except during a fault (malfunction/short circuit in the heater). A clothes dryer on the other hand requires 4 wires. The 2 hots, a ground and neutral. The neutral is there because the electric motor in the dryer runs on 120 volts. At one time the code allowed the neutral and ground to be the same wire in clothes dryer and similar appliances, but not anymore.

Xodar said:
You are correct in saying that the term "neutral" is related to being in the centre of the transformer, but incorrect in suggesting this doesn't apply to 'conventional' single phase 240 volt systems. The 240volt line is one phase of a three phase system, and the "neutral" is derived from the centre-point of the three phases. It is at the "centre" of a three phase transformer. So this usage of the term is no less correct. I expect you will find that you have a different "neutral" on the primary side of the centre tapped secondary, and that it would be related to the three phase system distributed through your neighbourhood.

Not sure. I've often wondered if on the primary side of transformers things are configured in delta or wye. If delta, it is unlikely there is a neutral at all. Incidentally, there are many places that three phase is simply not run. Where I live you need to drive several miles before you see three phase power. A single pair of wires comes to where I live and there are at least 4 subscribers on this pair.

Xodar said:
In the US case this means that the supply inside the house is isolated from the street supply, which might be an advantage although I can't think why at the moment.

I don't pay that close of attention, but I know I have seen several instances where the neutral/ground wires off of the secondary are tied to one of the primary wires. Hmmmm. This would mean that it is unlikely that the primary system is delta at least in those cases. I may have answered my own question.
 
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  • #25
Xodar said:
…My mention of the "switch being on" was a (poor) attempt to say "when the circuit is complete". In UK/Aus we do not have plugs without switches, so the process of energising any circuit requires that a device be plugged in and the switch be turned on ... hence the switch reference.
Ditto in the US, at least I can’t think of anything I have that is not switched if I ignore fixed equipment or my hand-made devices. The difference is our 240vac appliances are double pole switches while the 120vac devices are single pole. I can’t think of any residential-use reason to break the neutral connection in an Earth referenced system.
Xodar said:
You are correct in saying that the term "neutral" is related to being in the centre of the transformer, but incorrect in suggesting this doesn't apply to 'conventional' single phase 240 volt systems…
I will have to insist the US system is the conventional system with many world-wide, derivatives. :biggrin:
Xodar said:
The 240volt line is one phase of a three phase system, and the "neutral" is derived from the centre-point of the three phases.
This is not the norm for residential wiring in the US. Recently residences are being created from shut-down manufacturing facilities or other commercial buildings wherein the electrical power may have been maintained as is. In this case a three-phase 208vac “Y” system is common.
Xodar said:
…It is at the "centre" of a three phase transformer. So this usage of the term is no less correct…
It is an incorrect representation of the US residential electrical (not commercial) electrical system. We are supplied by a Delta configured transformer, therefore there is no center point of the three phases
Xodar said:
…I expect you will find that you have a different "neutral" on the primary side of the centre tapped secondary, and that it would be related to the three phase system distributed through your neighbourhood.
We have no neutral on the primary side and I believe that would be true in Australia also. It is more likely you have a delta to Y configuration whereas we have a delta to delta or delta to one or two phases.
Xodar said:
In the US case this means that the supply inside the house is isolated from the street supply, which might be an advantage although I can't think why at the moment.
I’m not sure what you mean by isolated but the utility transformer’s center tap, one for each separately derived phase (electrically isolated), the neutral is grounded in situ. The neutral is also bonded to the residence's safety grounding conductors.

Averagesupernova said:
That doesn't make any sense. You have to have voltage to push the current through the relatively high resistance of the human body. You say that the return wire will be 'hot' when the circuit is complete but yet it has a low voltage. Pick one or the other, you can't have both.
That is certainly true. A 14 gauge neutral carrying 10 amps with connection and wire impedance may rise to 3vac give or take; that is hardly “shocking”.

Averagesupernova said:
Not sure. I've often wondered if on the primary side of transformers things are configured in delta or wye. If delta, it is unlikely there is a neutral at all. Incidentally, there are many places that three phase is simply not run. Where I live you need to drive several miles before you see three phase power. A single pair of wires comes to where I live and there are at least 4 subscribers on this pair.
It will be done the most cost effective way. This may mean one delta primary phase is transformed to a lower voltage and finds its way to a local neighbor hood via two hots and a neutral. The other two phases go off to the next split. I’m certain all variations are being done. I doubt that any country transmits power requiring a neutral conductor on the KV lines. If grounded, it would be certainly attractive to lightning and copper is expensive.

My uncle had a home in the outskirts of a city and his neighbor’s home was about 10 feet from his. The main breaker in his exterior main panel fed his neighbors main panel. The neighbor was more or less a sub-panel. Don’t ask for specifics, it has been many years but I recall him griping about shutting him down when they serviced his neighbor.


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  • #26
GENIERE said:
Unless it’s due to a weird local electrical code you will be hard pressed to find a 4-conductor cable from the utility xformer to the residence, as the grounding conductor doesn't exist. The utility provides a neutral conductor and two "hot" conductors only. With an additional grounding conductor you would only provide a parallel path for neutral current since the safety ground and neutral conductor are bonded at the residences service entrance.

The ground being the suspension wire... every residence I've lived in has had three cables and the suspension wire that the cables are wrapped around is grounded. My apologies for not clarifying that.


exequor said:
No, wrong question. the question should have been if you hold onto the live wire (with both hands or one) and you are not grounded, would you get shocked?

Good question. In a nutshell, no. There was a good show on either Discovery or Learning Channel a couple months back that focused on the maintenance that has to be done on high-tension power lines... the workers for the most part have to work on live power lines and the hard part is getting them to the same potential as the wire they are working on. Once they are on the wire, they are relatively safe (bearing in mind they are hundreds of feet off the ground) unless they come in contact with another wire or any object of different potential.

IIRC domestic American phases are three phase, 60Hz in a delta transformer configuration... the neutral doesn't come into play until one gets to the residential subgrids... a neighborhood may only see two phases, depending on the load and such. Of course, there are industrial sites that require all three phases.

Now, on a ship that creates its own power, the system is three phase wye configuration... the hull has to be set as the ground, but it's floating (literally and figuratively). So if one were to measure the ac voltage between, say neutral and ground in the shipboard system, one could find a good 85vac potential there because the neutral is not necessarily tied to the ground (in this case, ships hull). It's enough to drive electrical engineers nuts. :rolleyes:


This is a very entertaining discussion, thanks, guys. :smile:
 
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  • #27
Lets separate and clarify two different and important ideas here:

(1) The difference between A.C. and D.C. currents (and voltages).

(2) The difference between the 'hot' and 'neutral' lines in house-wiring.

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(1) All generated currents are driven by 'voltages'. To take the analogy of 'water' flowing in pipes, 'voltage' is equivalent to 'pressure' in a waterhose. If the hose is blocked, (no place for the water to flow) there is no current, even if there is pressure. If the hose is disconnected, there is no current, even if there is water in the hose, because there is no pressure.

Now let's ignore the fact that electricity has to be connected in a 'loop' for the moment in order for current to flow (this is not strictly true, but is only a useful and sometimes dangerous approximation). I have water in the hose, and it has two ends. It is obvious that I can have alternating current (direction of flow) in the hose by two entirely different methods:
(a) I can blow in one end, moving the water one way, then go to the other end and blow, and move it backwards. (Or suck on either end.)
(b) I can stay where I am and still have alternating current by just blowing *and* sucking on one end. This we can call the 'power' end, the one where we are applying the pressure (voltage) to get work done.

(2) Now you should be able to guess why you can have alternating current, but only one 'live' wire with an actual 'voltage' or pressure to get the electrons moving.

Just to complicate things a bit, a real house has a 240 volt service (even if your plugs are 120 volts of pressure each). The Secondary winding (the one connected to the house fusebox has THREE wires, the two ends of the (240 volt) coil and a centre-tap.
The centre-tap is grounded through the fusebox by an iron rod into the ground. Each of the two ends of the coil is 'LIVE', and pressure comes out both ends by turns. They are 180 degrees out of phase. But there is only 120 volts across each half of the coil, and since the middle (centre-tap) is grounded, there is no voltage possible off that wire (between it and ground). Remember, you can't push (even in electronics) without pushing against something for leverage, and you can't have a voltage-drop (pressure) across a short.
At the Fusebox, the electrician has the choice of either hooking up to just one side (and the centre ground) for 120 volts, or taking a lead from both ends of the transformer and getting TWO live wires with 240 volts difference between them.
So although both 120 v.A.C. outlets and 240 V.A.C. outlets have 'three' wires, this is an appearance only: the 120 V.A.C. outlet actually has TWO of the wires running straight to the centre-tap (ground) So it is just an extra grounded return for safety and has no voltage on either of two of the lines, since if they are hooked up properly, they go to the same grounding bar inside the fusebox.

The 240 V.A.C. socket (for your stove/dryer) however really HAS TWO live wires and (two) grounds, and is far more dangerous than a normal socket.

WARNING: As a secondary note, sometimes electricians (sometimes idiots) will hook up two or four sockets in a modern kitchen, with each outlet (not pair) on its own line to the fusebox. Sometimes, these will run to opposite sides of the panel! This means that although there is only 120 volts to ground from each socket, there can be 240 VOLTS between OUTLETS, say between the toaster you are poking with a knife, and the tracklight you are leaning on with the other hand that's been poorly grounded.

Thus the houseline produces alternating current by applying positive pressure and negative pressure (vacuum) to one end of the circuit, while the resistance (load) between the source of pressure/vacuum (push/pull) and ground determines the amount of current which will actually flow in both directions. The neutral or ground lines just provide an easy path for the return of the electrons you have borrowed to do work.
 
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  • #28
Rogue Physicist said:
WARNING: As a secondary note, sometimes electricians (sometimes idiots) will hook up two or four sockets in a modern kitchen, with each outlet (not pair) on its own line to the fusebox. Sometimes, these will run to opposite sides of the panel! This means that although there is only 120 volts to ground from each socket, there can be 240 VOLTS between OUTLETS, say between the toaster you are poking with a knife, and the tracklight you are leaning on with the other hand that's been poorly grounded.

Thus the houseline produces alternating current by applying positive pressure and negative pressure (vacuum) to one end of the circuit, while the resistance (load) between the source of pressure/vacuum (push/pull) and ground determines the amount of current which will actually flow in both directions. The neutral or ground lines just provide an easy path for the return of the electrons you have borrowed to do work.

It is common practice to do this. In fact it is a REQUIREMENT when splitting outlets to have each half wired on each side of the line with a double pole breaker. The reason for this is because they share a neutral wire and they also want both halves of the outlet to go dead when one is overloaded. With a double pole breaker when one leg exceeds the current both legs disconnect. Your scenario is really no more dangerous than some track lighting or whatever plugged into a different outlet which happens to be on a different circuit while you are poking your toaster. Incidentally, anyone who sticks a knife in a toaster while it is plugged in deserves whatever they get.

Incidentally, you cannot share neutrals with GFCI outlets so in a kitchen with GFCI outlets I cannot imagine this would happen. But my point still remains that any outlet that has the upper and lower halves split and run on different circuits is REQUIRED to have each half on opposite sides of the line.
 
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  • #29
brewnog said:
In AC, the electrons change direction sinusoidally (in the UK, for mains electricity this frequency will be 50Hz, in the US it's 60Hz I believe). In DC the electrons will carry on flowing around the circuit without changing direction.

I would like to clear the concept here. Electron flow and current is not exactly the same thing. In every metal, the free electrons are in continuous motion irrespective of whether a potential is applied across it or not. But this motion being highly random, if we consider any cross section of the conductor the number of electrons moving across it in one direction would be equal to the number of electrons moving across it in the opposite direction resulting in zero net current.

However, if a potential is applied across the conductor, the electric field will provide a drift to the random motion of electrons in the opposite direction of electric field. Thus the number of electrons crossing a cross section opposite to the electric field will exceed the number of electrons crossing it in the direction of the electric field constituting a net electric current.

In AC the applied potential changes magnitude and direction sinusoidally and hence the electric current. In DC the potential and hence current will be in the same direction. In any case the electrons are in motion in both the directions.
 
  • #30
I think he means that if an appliance is turned on, it acts as a resistor connected to the hot power line and so can deliver an electric shock. Likewise, if an appliance is plugged in and turned on, the 'neutral' (U.S.'return') is 'hot' too, if not properly grounded or some distance from ground. So if you put a knife in an English (240v) toaster and and grab the radiator with your other hand, zap.
 
  • #31
Nam_Sapper said:
I think he means that if an appliance is turned on, it acts as a resistor connected to the hot power line and so can deliver an electric shock. Likewise, if an appliance is plugged in and turned on, the 'neutral' (U.S.'return') is 'hot' too, if not properly grounded or some distance from ground. So if you put a knife in an English (240v) toaster and and grab the radiator with your other hand, zap.

The US National Electrical Code requires that the neutral conductor be grounded at the service entrance (residential use). It is NOT hot with respect to Earth except for the small voltage drop across it due to its impedance. This is UNIVERSALLY true regardless of which country you reside in; a current carrying conductor will never be at the same potential as the safety-grounding conductor if it is carrying current except at the bonding point.

There are specific circumstances where it is necessary to isolate the system from ground as in operating suites, gas stations, grain silos; anyplace where a spark to ground my ignite an explosive atmosphere i.e., anesthetics, gasoline fumes, grain dust. This is usually accomplished by using an isolation transformer whose secondary windings produce the desired voltage(s) and are maintained to assure both current carrying conductors have at least 1megohm of impedance to ground. The floor of the operating suite must be conductive to ground (forget the maximum impedance) to dissipate static charges. You can, I have done so, grasp one of the conductors in an operating suite (one that uses explosive anethetics) and intentionally Earth one’s self. There are micro amps of current flow through the body but no perceptual shock. Yes I do know that micro amps may cause cardiac fibrillation but why that does NOT happen is a function of current density, not total current flow.

...
 
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Related to Difference between AC and DC current

What is the difference between AC and DC current?

AC (alternating current) and DC (direct current) are two types of electrical current. AC current constantly changes direction, while DC current flows in one direction.

Which type of current is used in household appliances?

Most household appliances use AC current because it is more efficient for long-distance transmission and can easily be converted to different voltages.

Why is AC current used for power transmission?

AC current can be easily transformed to different voltages using a transformer, making it more efficient for long-distance transmission. It also experiences less power loss during transmission compared to DC current.

What are the advantages of DC current?

DC current is more stable and reliable compared to AC current. It is also easier to control and does not experience energy loss due to capacitance and inductance.

How are AC and DC currents produced?

AC current is produced by rotating a coil of wire in a magnetic field, while DC current is produced by converting AC current using a rectifier or battery.

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