Difference between AC and DC current

  • #26
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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.

----------------------------------------------------
(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
Averagesupernova
Science Advisor
Gold Member
3,737
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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
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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
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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
GENIERE
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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