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Thread: [Alternating Current]Why does current flow from live wire where as AC is supposed to change its polarity frm time to time

  1. #1 [Alternating Current]Why does current flow from live wire where as AC is supposed to change its polarity frm time to time 
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    Why doesn't it shock if I touch the neutral wire as ac is supposed to be changing i.e it changes polarity from time to time....which is known as frequency of the ac.According to it....the live and neutral polarities must change as per their frequency....which means neutral wire changes to live and vice versa....and if I touch a wire and I'm touching the ground.....I should experience electric shock like...60 times per sec(assume ac freq as 60Hz).....But this is not happening....why ???Can anyone please explain me why ?According to AC.....we can see that from the sinusoidal graph....the voltage changes from positive to negative with some frequency but we experience current flow with only one wire.....why do they discard the neutral ��?Also xplain me exactly about these live and neutral terms clearly.['+' means positive potential(higher potential) and '-' means negative potential(lower potential)....cureent is supposed to flow from higher potential to lower potential na.....]But in AC the '+' and '-' interchange at the rate of 60 times per sec...


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    Universal Mind John Galt's Avatar
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    Interesting. I have never thought about this before and do not specifically recall ever being told about it. Someone will be along with the correct answer shortly. In the meantime, could the cycling be restricted to the live wire, with respect to neutral, which is linked to the Earth? Though that leaves me wondering why we would need the Earth wire.

    My ignorance in this area does at least go some way to explaining why I used to get electric shocks several times a day.


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    There is no such thing as the voltage of a single wire. Voltage is a potential difference and has to be defined between two points. So as long as the wire you are holding onto does not change its potential with reference to the ground you are standing on, or some other wire you are touching, no current will flow through you.

    You can touch the "hot" wire without getting shocked too, as long as you are not also touching a ground. Birds do it all the time when they land on a wire.

    The "neutral" wire is one that is connected to ground back at the distribution panel. It can sometimes have some voltage on it relative to ground due to the current flowing through it and resistance of the wire, or possibly a high resistance connection. I do not recommend touching the neutral wire.

    In an a-c circuit the voltage does change from positive to negative. That's why the current changes in direction.
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    The power before it gets to the house is 220. The transformer at the pole is center tapped.
    This splits the sine wave into a positive and negative supply with two 110 loops.
    The transformer center tap (neutral) is connected to ground at the pole and the center tap (neutral) in the house is connected to the ground. The earth forms the return loop for both halves of the transformer to the house wiring.
    Since the halves of the 220 are 180 degrees out of phase a properly balanced loading of the 110 house circuits will cancel and you get minimal current flow through the neutral wires.
    But as Harold says bad wiring or unbalanced loads can result in it carrying current and being hot. In some circuits in houses the white wire is used as a hot too, the power to a light often goes through the switch and the loop from the metal box the light is in to the switch and back is made with a normal piece of wire so from the switch to the light the white wire is a hot instead of a neutral.

    The green or bare copper bonding wire (ground,earth, third prong) is there to connect everything you might touch from becoming hot. If a current carrying wire in your toaster shorted to the metal frame of the toaster (even when it is turned off) the ground wire would complete the short circuit and blow the fuse. It also is supposed to give a lower resistance path to ground in case you are holding the toaster when it decides to try electrocuting you. It is suppose to be connected to everything that could become hot, light switches, appliances, utility boxes, electric motors, etc.
    It is a safety item.
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    Just wondering.....If the Earth was attached by a strong wire to the moon, then would our world still be ground for all things electrical?
    All that belongs to human understanding, in this deep ignorance and obscurity, is to be skeptical, or at least cautious; and not to admit of any hypothesis, whatsoever; much less, of any which is supported by no appearance of probability...Hume
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    Quote Originally Posted by mysciencedays View Post
    Why doesn't it shock if I touch the neutral wire as ac is supposed to be changing i.e it changes polarity from time to time....which is known as frequency of the ac.
    In most installations (like homes) the neutral is close to ground potential. (In fact, is connected to it at the service entry.) In other words, it's the same as the ground lead, which is very close to the voltage of everything in your home. So no shock.
    According to AC.....we can see that from the sinusoidal graph....the voltage changes from positive to negative with some frequency but we experience current flow with only one wire.....why do they discard the neutral ��?
    They don't. It is where the current is returned. The "hot" swings from +160 to -160 volts (which is 120 volts AC) while the neutral remains close to zero volts with respect to ground.
    Also xplain me exactly about these live and neutral terms clearly.['+' means positive potential(higher potential) and '-' means negative potential(lower potential)....cureent is supposed to flow from higher potential to lower potential na.....]But in AC the '+' and '-' interchange at the rate of 60 times per sec...
    Yep - hence no polarity markings on AC.
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    Quote Originally Posted by John Galt View Post
    Though that leaves me wondering why we would need the Earth wire.
    This part is a little harder to explain. Your house's electrical system will work fine without being connected to earth ground as long as there aren't any faults on the system. Here's what Wikipedia says
    According to the NEC, the purpose of earthing an electrical system is to limit the voltage to earth imposed by lightning events and contact with higher voltage lines, and also to stabilize the voltage to earth during normal operation.
    Ground (electricity) - Wikipedia, the free encyclopedia
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    Universal Mind John Galt's Avatar
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    Thank you Harold. I never could get my head around analog circuitry, but was a dab hand at troubleshooting quite complex - for their day - digital circuits. Give me a pair of logic probes and a circuit diagram and I could tell you which J-K flip-flop, or NE555 timer had cratered, quicker than Bill Gates could make $10,000.
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    Cooking Something Good MacGyver1968's Avatar
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    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by John Galt View Post
    Though that leaves me wondering why we would need the Earth wire.
    This part is a little harder to explain. Your house's electrical system will work fine without being connected to earth ground as long as there aren't any faults on the system. Here's what Wikipedia says
    According to the NEC, the purpose of earthing an electrical system is to limit the voltage to earth imposed by lightning events and contact with higher voltage lines, and also to stabilize the voltage to earth during normal operation.
    Ground (electricity) - Wikipedia, the free encyclopedia
    To expand on this....earth ground is connected to chassis to prevent accidental energizing if there is a fault. Let me give you an example. We have a grill like this one at work:


    It was dirty as hell, so I gave it a good cleaning. During the cleaning...some of the water got inside and bridged the gap between a live voltage source and the chassis. Without the earth ground, you would get shocked if you touched any of the metal outside parts. With the earth ground...that current gets safely sent to ground. Fortunately we have GFI, or ground fault interrupt plugs, that pop if any current goes to earth ground.
    Fixin' shit that ain't broke.
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    Quote Originally Posted by MacGyver1968 View Post
    During the cleaning...some of the water got inside and bridged the gap between a live voltage source and the chassis. Without the earth ground, you would get shocked if you touched any of the metal outside parts. With the earth ground...that current gets safely sent to ground. Fortunately we have GFI, or ground fault interrupt plugs, that pop if any current goes to earth ground.
    Well, it's not quite that simple because if the whole electrical system was ungrounded, then no current would flow through you to ground when you touched the "hot" grill chassis. It would be like the bird landing on a wire. But if you didn't have your system grounded, you could get a ground fault somewhere in the system and never know it because it wouldn't trip any breakers or anything. Then it would be lurking there waiting to bite you when you washed the grill and caused a second short.
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    Quote Originally Posted by Harold14370 View Post
    Well, it's not quite that simple because if the whole electrical system was ungrounded, then no current would flow through you to ground when you touched the "hot" grill chassis. It would be like the bird landing on a wire.
    Birds can land on wires because they are not touching anything that is grounded. People often stand on the ground which is (by definition) at ground potential, and they often touch other things (like sinks) that are also at ground potential. Thus whenever you touch a hot line you have a significant chance of getting shocked. You don't need a second short to cause a hazard.

    All large electrical systems are grounded either intentionally (via an explicit ground point, usually at the entry to the property) or unintentionally (due to the small coupling to ground via parasitic capacitance/resistance.) You can avoid a shock hazard with a very small "ungrounded" system (such as in an electric vehicle or home power system) but cannot avoid it on any large system. We have, unfortunately, learned this the hard way.
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    The systems are grounded because they are required to by code, but what if they weren't. If the soil is very dry, the copper rod sometimes doesn't work all that well. Parasitic capacitance? - meh. That kind of depends on how close to ground the wires are routed, and the "hot" side would be just as likely to be coupled to ground as the neutral.
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    In the USA 'they' ( Unknown) decided that the 240 volt system was just too dangerous for domestic use.. so you in the US of A have a 110 volt domestic voltage.. Apparently you are more likely to survive a Zap than we.. Please be applying a little common sense when messing about with sparky..
    ~ Him's will get you.. ~ ZAP... BANG.~ Oh damn..plonk.
    On and in regard to AC verses DC.
    ~ From the power station to the local Grid the method of transmission is DC ( Direct Current ) It's only converted to AC ( Alternating Current) for domestic users..
    Some industrial plants require a 440 volt feed and a three phase system..Same stoves and ovens have that here..
    Talking with electrical engineers and electricians whom are qualified is vital for a proper understanding.. and always remember that ...
    Zap...Bang is no joke.. you gota know what you are messing with.. Stick to code..
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    The main function of the third wire; the 'earth' connection, is actually to blow the supply fuse or trip the breaker if the chassis or metal work becomes live.

    Normally current from the live supply goes through the device and back via the neutral. However, if the live voltage feeding a device comes into contact with the chassis, there is no indication that it has done so, and anybody touching the chassis and earth would receive a shock. To prevent this, a third wire, the 'earth' wire is connected to any metal parts of the device and forms a very low resistance circuit which allows maximum current to flow from the supply - much more than the device itself would use - and this blows the fuse or trips the breaker, disconnecting the dangerous device from the supply.

    The 'earth' wire is really a second neutral wire, connected to what should be the non-electrified metal parts of a device.

    Here in the UK a wiring convention connects the neutral and earth wires together at the supply source.


    In UK single phase AC supplies, the supply wire alternates between positive voltage and negative voltage. The neutral stays neutral, it does not supply the opposite potential to the supply wire.


    OB
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    Quote Originally Posted by One beer View Post
    Here in the UK a wiring convention connects the neutral and earth wires together at the supply source.


    In UK single phase AC supplies, the supply wire alternates between positive voltage and negative voltage. The neutral stays neutral, it does not supply the opposite potential to the supply wire.


    OB
    US and UK are not really that much different, except that in the US, the 240 volt secondary of the distribution transformer is center tapped, with the center tap grounded at the service entrance. Each 120 volt circuit has one conductor connected to neutral and the other to one of the "hot" sides of the transformer. The two "hot" voltages are commonly referred to as "phases" but it is still just a single phase system. If you supply a circuit using two of the "hot" wires of different "phase", with a 2-pole breaker, you get 240 volts. Neither hot wire is grounded, but you will still have a separate grounding conductor to ground the chassis. The 240 volt circuits might be used for water heaters, electric clothes dryers and such.

    I'm not sure I agree when you say the neutral does not supply the opposite potential to the supply, but maybe it's just a matter of semantics.
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    Quote Originally Posted by Harold14370 View Post
    US and UK are not really that much different, except that in the US, the 240 volt secondary of the distribution transformer is center tapped, with the center tap grounded at the service entrance. Each 120 volt circuit has one conductor connected to neutral and the other to one of the "hot" sides of the transformer. The two "hot" voltages are commonly referred to as "phases" but it is still just a single phase system. If you supply a circuit using two of the "hot" wires of different "phase", with a 2-pole breaker, you get 240 volts. Neither hot wire is grounded, but you will still have a separate grounding conductor to ground the chassis. The 240 volt circuits might be used for water heaters, electric clothes dryers and such.

    I'm not sure I agree when you say the neutral does not supply the opposite potential to the supply, but maybe it's just a matter of semantics.

    Ah, OK, I did not know that about USA electrics. Is there a difference in outlet sockets then depending if they are 110V or 240V? The ones I've seen in hotels in America all look the same. Or are all outlet sockets 110V and maybe water heaters etc. are hard wired 240V from the supply box?

    I take your point about neutral. Our power is supplied from the power stations as three phase connected in a delta, but then converted in local neighbourhood distribution transformers into star configuration with a centre tapped neutral. The houses usually get just one phase each, evenly distributed along the street, and they all feed back to the neutral. The distribution companies try to balance the loads equally, but depending on demand, the neutral can sometimes be 'pulled' too far towards one phase.

    OB
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    AS shown here there are two kinds of dryer receptacles. The older ones have 3 prongs and are non-grounding and the newer ones have a fourth grounding terminal.
    How to Wire a Dryer Cord
    You probably won't see these in the hotel, unless you go to the laundry room.
    Of course, these are made so that you cannot plug a regular 120 volt plug into them. My water heater doesn't have a plug. It's just hard wired.
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    Quote Originally Posted by One beer View Post
    I take your point about neutral. Our power is supplied from the power stations as three phase connected in a delta, but then converted in local neighbourhood distribution transformers into star configuration with a centre tapped neutral. The houses usually get just one phase each, evenly distributed along the street, and they all feed back to the neutral. The distribution companies try to balance the loads equally, but depending on demand, the neutral can sometimes be 'pulled' too far towards one phase.

    OB
    We have the same problem with balancing loads here too, but we even have to try balancing the loads inside the house on the 110 circuits. If the red side of the breaker box is loaded more than the black side (or vice versa) you can get the white carrying a current instead of cancelling out.

    Our high power transformers are also 3 phase and industrial power is often 3 phase because it is cheaper to run electric motors on 3 phase. However you only see the power as single phase in a house, and if you look at the wires from the pole to the meter you only have two wires supplying 240 volt. The split to two 110 branches is done at the breaker panel using the neutral wire to connect through the earth to the center of the transformer at the main (hydro pole).

    PS: I tend to talk of 220 and 110 voltage but it is really supposed to be supplied as 240 and 120. However step down transformers and voltage drops in the transmission lines usually reduce it to 110 and 220.
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    Quote Originally Posted by astromark View Post
    ~ From the power station to the local Grid the method of transmission is DC ( Direct Current ) It's only converted to AC ( Alternating Current) for domestic users..
    99% of transmission in most countries is AC. AC is generated by the utility, is boosted in voltage via transformers, is transmitted via transmission lines, and is dropped in voltage by transformers for use by customers. In a few cases, long transmission lines first convert AC to DC, transmit it at _very_ high voltages, then convert it back to AC on the other end. This is still very rare though.

    The reason for this, BTW, is transformers are a simple and cheap way to boost/drop voltage. It is much more difficult to convert AC to DC, or to boost/drop DC voltages. Thus until recently all power transmission was AC. Recently electronics have improved enough that it's now possible to boost/drop AC voltages, but it is still expensive, and thus only done for long. high power transmission lines where there is significant cost savings from the smaller transmission lines/towers.
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    Quote Originally Posted by dan hunter View Post
    if you look at the wires from the pole to the meter you only have two wires supplying 240 volt. The split to two 110 branches is done at the breaker panel using the neutral wire to connect through the earth to the center of the transformer at the main (hydro pole).
    I think you'll find that there are two insulated wires, and an uninsulated wire that connects the neutral at the service entrance to the center tap of the transformer. Sometimes this neutral wire is a smaller gauge wire than the red and black insulated wires because it only has to carry the unbalanced current.
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    I only wonder why outlets here in the Americas (I'm Canadian) have adapted to using polarized plugs? It suggests that they are partially biasing the AC current in DC. Anyone know of this rationale?

    (I read somewhere from old materials that they were thinking of using polarization prior to things like cable lines so that the DC part of the polarization can carry a signal that could be used to transfer information.)
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    Quote Originally Posted by Scott Mayers View Post
    I only wonder why outlets here in the Americas (I'm Canadian) have adapted to using polarized plugs? It suggests that they are partially biasing the AC current in DC. Anyone know of this rationale?
    Modern plugs are either grounded (three prong) or ungrounded (two prong.) Two prong outlets are polarized so that the "hot" side can be identified. It is more important to insulate the "hot" side than the return side, since the neutral is close to ground.

    Let's take an example - a simple lamp. One side (side A) of the cord goes to the switch, then the center contact of the socket. The other side (B) of the cord goes to the outer part of the socket.

    Case 1 - unpolarized plug, plugged in so A is neutral and B is hot. You go to change the bulb. You first turn off the lamp to be "safe." The neutral is now disconnected. No current flow so the lamp is off. However, the outer edge of the socket and the exposed metal of the bulb thread are all "hot" - they are all connected directly to the hot side. You unscrew the bulb and touch the very edge of the socket, and get zapped.

    Case 2 - polarized plug. Now A has to be hot and B has to be neutral. You go to change the bulb. You first turn off the lamp to be "safe." The hot is now disconnected. No current flow so the lamp is off. The outer edge of the socket, the exposed metal of the bulb thread and even the center contact are all now electrically dead. The only part of the lamp that is "hot" is inside the switch, and that's not accessible. So you are very unlikely to get a shock no matter where you touch it.
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    Thanks billvon. You gave a good example. I also looked it up and found a good diagrammed description here. I thought that they might have been applying a combination AC/DC current for some other potential purpose. It appears that it still remains pure AC but is only used for exactly as you described. I'm guessing that keeping the extra ground (third prong) only acts as extra security for higher current devices. But this also helps to explain what the original poster may have possibly noticed: that for the cold-side (left in the linked diagram), it should not pose a threat since the way the tap is within the transformer, the current will still prefer the short through the whole coil or drained off through the pre-grounded tap. Thus that side outlet would not threaten you even if you were grounded. The hot side would still shock you, however, and more so if you are easily grounded elsewhere. [What Harold was indicating]
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    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by dan hunter View Post
    if you look at the wires from the pole to the meter you only have two wires supplying 240 volt. The split to two 110 branches is done at the breaker panel using the neutral wire to connect through the earth to the center of the transformer at the main (hydro pole).
    I think you'll find that there are two insulated wires, and an uninsulated wire that connects the neutral at the service entrance to the center tap of the transformer. Sometimes this neutral wire is a smaller gauge wire than the red and black insulated wires because it only has to carry the unbalanced current.
    Here it doubles as the support wire so it is a fairly heavy gauge piece of aluminum cable.
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    Quote Originally Posted by dan hunter View Post
    The power before it gets to the house is 220. The transformer at the pole is center tapped.
    This splits the sine wave into a positive and negative supply with two 110 loops.
    The transformer center tap (neutral) is connected to ground at the pole and the center tap (neutral) in the house is connected to the ground. The earth forms the return loop for both halves of the transformer to the house wiring.
    Since the halves of the 220 are 180 degrees out of phase a properly balanced loading of the 110 house circuits will cancel and you get minimal current flow through the neutral wires.
    But as Harold says bad wiring or unbalanced loads can result in it carrying current and being hot. In some circuits in houses the white wire is used as a hot too, the power to a light often goes through the switch and the loop from the metal box the light is in to the switch and back is made with a normal piece of wire so from the switch to the light the white wire is a hot instead of a neutral.

    The green or bare copper bonding wire (ground,earth, third prong) is there to connect everything you might touch from becoming hot. If a current carrying wire in your toaster shorted to the metal frame of the toaster (even when it is turned off) the ground wire would complete the short circuit and blow the fuse. It also is supposed to give a lower resistance path to ground in case you are holding the toaster when it decides to try electrocuting you. It is suppose to be connected to everything that could become hot, light switches, appliances, utility boxes, electric motors, etc.
    It is a safety item.

    This is way off. The voltage in the transmission lines is not 220 Volts. More like 400,000 volts. Otherwise the power loss would be too much by far. The Energy Story - Chapter 7: Electricity Transmission System

    The big step down transformer at the power pole cuts the voltage to the 220 Volts as Dan says, which is then center tapped so it can be either 220 0r 110 for household use, but you cannnot use such low voltages for long distance transmissions.
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    Quote Originally Posted by Mayflow View Post
    .....The voltage in the transmission lines is not 220 Volts. More like 400,000 volts. Otherwise the power loss would be too much by far........

    The big step down transformer at the power pole cuts the voltage to the 220 Volts as Dan says............but you cannnot use such low voltages for long distance transmissions.

    Correct, (although note what billvon says about DC transmission lines).

    Here in the UK we have things called isolating transformers, which are 240-240 Volt transformers in which the secondary is floating, not referenced to anything. Therefore, if one touches either wire, one should not get a shock. These are used to run electric power tools in hazardous environments. I use one if I am using a water sprayed cutting disc for DIY.

    On construction sites and other hazardous areas requiring ad-hoc electrical systems; centre tapped 110V is used. All the power tools run on 110V and the supply is centre tapped so that each wire only carries 55V referenced to the neutral, which is considered to be low enough not to cause a heart attack if touched.

    OB
    Last edited by One beer; September 13th, 2014 at 06:15 AM.
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