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Negative terminal vs Ground. Double definitions?

  1. Dec 3, 2009 #1
    In a regular household plug-in, there are 3 prongs: positive, negative, and ground. But, when doing wiring projects, many times they refer to the negative lead as the ground. Why is it that this lead is sometimes referred to as the ground, while other times, the ground is a completely separate wire altogether?

    This seems like a contradiction that irks me to no end. For example, today in lab, I was to hook up the oscilloscope. There were 3 available ports: a red one, a black one, and one labeled ground. I had assumed the circuit needed to originate from the positive red port and conclude at the negative black port; however, I was told this was incorrect.

    I thought the 'ground' was a just-in-case-of-an-overload route for excess electricity to disperse away from the circuit, as in the case of a household plug-in. But, now it's changed to include the negative terminal, but only "sometimes"? What's the deal?
     
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  3. Dec 3, 2009 #2

    berkeman

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    Welcome to the PF. First, household wiring is AC, not DC. There is an Earth Ground connection, a Neutral connection (which is connected to Earth ground at your fuse/breaker panel -- required by electrical code), and the Hot lead. The Hot lead in a single-phase AC Mains system has an AC voltage swing about the Neutral conductor. The current that powers your single-phase AC Mains devices (refridgerator, etc.) circulates through the Hot/Neutral loop. No power current should flow to ground.

    Second, for many power supplies, their outputs +/- are floating with respect to Earth ground. There is no direct connection in the power supply. Thus, you can have the 3 output terminals on the front of the power supply, +, -, and Earth GND. Again, the power should flow between the + and - terminals. You can connect the Earth GND pin to either + or - (but not both), depending on if you want to have your power supply terminals somehow referenced to Earth GND.
     
  4. Dec 3, 2009 #3

    berkeman

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    On your lab 'scope hookup question, we would need more details about what the circuit was, and what you were supposed to be measuring.
     
  5. Dec 3, 2009 #4

    vk6kro

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    A household plug will have an active, neutral and ground and it will be AC voltage, not DC. So, that is a different story.

    Normally a DC power supply will have a positive and a negative out.

    Sometimes the negative is already joined to the case of the power supply. So it is already grounded.
    In better power supplies, the connection to the case is brought out to a plug and you can connect either the positive or negative terminals to it.
    Or, you can just use them as they are.

    The cases of the various instruments are normally grounded and can be regarded as being joined together via the mains wiring.

    The shielded cable of an oscilloscope lead is always grounded, so you can't connect this to the positive terminal of a power source if the negative side is grounded. This would be a short circuit, because that would connect the two sides of a power source together, causing a large current to flow.

    There are even power supplies that give a symmetrical positive and negative output relative to ground. These are used for opamp circuits and are usually clearly marked as symmetrical supplies. In these, of course, you do not ground either the positive or the negative supply.

    Yet, it is important that there is a ground connection somewhere or the circuit will be very likely to pick up mains interference.
     
  6. Dec 10, 2009 #5
    Hi, I'm new to this forum and fairly new to physics study, so I apologize in advance if this question is very basic. I understand that in a circuit, be it AC or DC, what ultimately matters is the potential difference between - and + sides. In a typical household AC system the circuit is ultimately completed by the neutral lead returning the current to earth ground. Does that mean that I could take a DC power source, like a battery, and route the negative terminal to earth ground and "complete" a circuit, assuming enough potential difference between the negative terminal and earth ground? Thank you in advance.
     
  7. Dec 10, 2009 #6

    berkeman

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    "In a typical household AC system the circuit is ultimately completed by the neutral lead returning the current to earth ground. "

    That is incorrect. Earth ground is not in the picture at all (except for safety issues). The current flows between Hot and Neutral only. Think of the power source and the power consumers as isolated from ground. It's true that Neutral is tied to Earth GND at the breaker panel, but that is just to provide an absolute reference, not for any power return reasons.
     
  8. Dec 10, 2009 #7
    Thank you. So ultimately my question is, where does the neutral wire go? I'm trying to get a picture of the flow of the current in an AC system. I thought that the negative wire eventaully leads to a copper rod buried in the ground, just like the grounding rod in one's home. In essence the grounding rod is a redundant system, in the event of a metal appliance case becoming energized. Is that accurate?

    Also, in a DC system is it dangerous to come in contact with the negative lead, assuming there is enough voltage?
     
  9. Dec 10, 2009 #8
    There is no positive or negative wire in AC since the current changes directions periodically (60 times per second).

    Household AC systems (in the U.S.) are usually comprised of 3 wires: A "hot" black wire, a "neutral" white wire, and a "ground" wire which can be either green or bare (no insulation at all). The ground wire is, in fact, a safety route. But not for "excess voltage," as you stated, rather for a loose hot wire.

    Normally, there is no current running through the ground wire. Suppose you had an electric device that was housed in a metal box that was sitting on an insulated table. Suppose this device ran on 120 volts AC and had no ground wire (only a 2-prong plug). If you were to plug that device in and turn it on, and some part of the circuitry inside that box came loose and came in contact with any part of the metal box, the box would become energized, waiting to find a path to ground. This is just like the hot side of the wall receptacle, which is always hot, waiting to find a path to ground, but much more accessible. If you then come in contact with that metal box, it would be the same as sticking your finger in the hot side of the wall receptacle: if your body is adequately grounded, you will receive a shock. This is the reason for a ground wire: it is connected to a ground rod at the main panel and when you plug your 3-prong plug into the receptacle, the ground wire in your device is connected to the ground rod via the house wiring. Inside your device, the ground wire is connected to the metal case, so that now, if a hot wire comes loose and makes contact with the metal box, the voltage will travel through the ground wire back to the ground rod. Since there is very little resistance, the current will be very high, and the circuit breaker will trip, quickly shutting off the voltage hopefully before anyone has a chance to touch the energized metal box.

    A neutral wire is generally safe to touch (don't go sticking your fingers in any live circuits or power panels to check), UNTIL a circuit is made. At the panel, the white wire is connected to the ground rod, which brings it's potential to that of the earth (which, of course, is generally safe to touch), so there should be 0 volts on the neutral wire (in reference to ground). When you plug a device in the wall and turn it on, you are completing a circuit from the hot wire, through the device (which does some kind of work), and back to ground through the neutral wire. Now that current is running through the neutral wire, it is unsafe to touch. So, you see, the neutral wire is an intentional route for the current to flow from hot to ground, whereas the ground wire is an unintentional route (a safety route) from hot to ground.

    It is entirely possible to have a system (house wiring, for example) with no ground whatsoever (not even a ground rod near the panel). There would only be 2 wires in this system having a potential of 120 volts between them, but since there is no reference to ground, each wire may have a potential of anywhere from 0 to 120 volts to ground. Basically, you would have 2 hot wires with no neutral and no ground. This system would run any 120 volt equipment without damaging it, but without the safety of a grounded system.

    Remember that current is what shocks you, not voltage; when you get shocked, you are feeling the current run through your body (you are completing a circuit with your body). If you were completely insulated from ground you could touch a live wire without getting shocked (although I don't suggest it).

    Hope this helps...
     
  10. Dec 10, 2009 #9
    DC typically doesn't flow very well through the human body, which is why you can touch both ends of a D-cell battery (or both contacts of a 9-volt battery) without getting shocked. Touching one lead or the other is not a problem, in fact there used to be cars that were positively grounded (the frame of the car was connected to the positive terminal, rather than the negative terminal, as is the standard today). The problem is if you complete a circuit with both terminals of the battery and there is enough power capacity (watts) in the battery to push current through your body. For example a 12-volt car battery has much more power than eight AAA batteries in series (you couldn't run your car with the AAA batteries, they would drain almost instantaneously as you tried to start the car). Of course, if you are wet, you will increase the ability for current to flow.

    So, although DC is generally safer than AC (both can be dangerous), AC flows better which is one reason why we use it on the power lines feeding our houses.
     
  11. Dec 10, 2009 #10
    In the case of DC systems, the negative terminal (typically) is generally grounded for simplification in wiring. For example, a car that is negatively grounded (the current U.S. standard) only requires that you run a positive lead to a device (say, the radio). Then you need only to connect the negative terminal of the radio to the nearest metal part in the car. This typically only requires a short pigtail, therefore saving wiring and simplifying trouble-shooting.

    There are other reasons why DC circuits may be grounded or not. This usually involves circuits that deal with (or are trying to block) radio frequencies.
     
  12. Dec 10, 2009 #11

    sophiecentaur

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    The neutral conductor originates in the three phase transformer back in the local substation. The phases are connected as a star, with the centre being the Neutral and the three phases on the outside of the three points of the star. Three pairs of wires (from a phase and the neutral) go out from the substation to feed the consumers so that each phase gets an equal number of consumers / load. When the loads on the 3 phase system are balanced, there will be no net neutral current flowing: the currents in the three Neutral wires should add up to zero at any time. If there is some imbalance in the loads, the actual potential of the neutral may not be exactly zero, because of the finite impedance of the system.
    So, if you use the Neutral wire as a safety earth, you could actually have several volts appearing on it. Connecting the Neutral to Ground (a big fat spike or a water pipe) would just make a lot of current flow, wasting power.

    For safety, they usualy have a local 'Earth' , connected to actual ground -or, when there is an underground armoured cable, to the outer sheath as well. (The neutral 'floats' at anything up to a few volts). The Earth wires from all devices which actually have one are all connected (via the green / yellow wire) back to this Earth. (All other large bits of metal in the building are also connected). This all ensures that when you are standing on a wet kitchen floor in bare feet, you are at the same potential as any piece of metal around you which could have a connection to a high potential.
    The safety Earth network carries no power under normal conditions but, if a fault causes a connection from some live internal part to a metal case, the Earth wire keeps the case at Zero potential. This may cause a fuse to blow if the fault is low enough resistance and a lot of current flows or there can be a small 'residual current' flowing to Earth for years with no harm done. Then, one day, someone is working on the appliance and, unwittingly, disconnects the earth connection to the bit of the case with a wire touching it and ZAA****P !! This is where a RCCB comes into its own because it will measure the current imbalance and switch off - damned nuisance sometimes but it is safe!
    Also, long overhead supply cables can have much higher neutral voltages when the wires are carrying large currents.

    There is another issue about Earthing of Electronic equipment and signal lines. Hum turns up in the most unlikely places and Earthing can sometimes make it worse. That's another topic for Hi Fi buffs and designers.
     
  13. Dec 10, 2009 #12
    Actually, this is not the case: if you look at 3-phase power lines, you'll only see 3 wires, one for each leg of 3 phase power. The transformer feeding your house is a single phase transformer with only 2 high-volt taps (one primary coil) connected to 2 of the 3 available 3-phase wires. The transformer will also have 3 low-volt taps (one secondary coil with an additional tap in the center of the coil) which are fed to your main panel. This center tap is grounded (usually at the panel, but sometimes at the transformer) and becomes the neutral conductor, thus you have 220 volts across the entire secondary coil and 120 volt from the center tap to either of the outer taps.

    The OP is already confused enough about grounding and neutrals, we shouldn't introduce three-phase into the mix yet...
     
  14. Dec 10, 2009 #13

    sophiecentaur

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    Yebbut, it's because of the three phase system that the neutral is as it is, surely.
    But I think you must be referring to non-UK systems because you can't actually see many low power lines in the UK - they are all underground and I'm pretty sure it is as I describe. All overhead lines I can remember seeing in the UK have a fourth wire strung from tower to tower. But these are 11kV (?) or higher. We certainly don't use a centre tapped supply which is what you seem to be describing.
    If you are describing a three-conductor-only system it would be a delta transformer back at the sub. The local single phase transformer must connect across two phases and be isolated from the domestic supply completely. In which case, there would be no inherent Live or Neutral connection. I don't understand how the centre tapped secondary fits in with a single phase 110V supply with a neutral. Are there two possible polarities of power available at each house?
    I think I would appreciate a link to help me spot the difference (I'm confused too). It might be useful to ascertain where the OP comes from.
     
  15. Dec 10, 2009 #14

    vk6kro

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    The North American system uses local transformers across two phases of a 3 phase supply.

    This transformer has a centre tapped 220 volt secondary which is taken to each house.

    The house may use either 110 volt source for normal applications, or they may use the entire 220 volts for higher powered applications.
    There are different sockets for 220 volt output but these seem to be rare for domestic applications. Normally, you only see the familiar 2 vertical pin 110 volt outlets.

    Our high voltage 3 phase systems certainly use 4 wires (in Australia) and a neutral is usually the top wire on the pole.
     
  16. Dec 10, 2009 #15
    I apologize. Not only did I not realize that you were in Australia, I also didn't know that you ran a 4th wire there.
     
  17. Dec 10, 2009 #16

    vk6kro

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    You were talking to Sophiecentaur who is in Britain.
    They, like most of the rest of the world, use the same system as Australia.

    My house actually has 4 wires coming into it. 3 Actives and 1 neutral.
    The 3 actives are 415 volts and 120 degrees out of phase with each other, and at 240 volts 50 Hz AC relative to the neutral.
    So, there is a lot of power available. About 6000 watts per phase.
    I think the neutral is the same one that goes all the way back to the power station.

    The neutral is grounded at each house, but only for safety reasons.
     
  18. Dec 10, 2009 #17

    dlgoff

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    Well, I live in a rural area in the US and you will see two wires strung on poles. One of the wires is the high voltage source and the other one is the return which is grounded at each pole. Power is provided to the customer via a single phase transformer. The secondary having a center tap to ground.
    http://en.wikipedia.org/wiki/Electric_power_transmission#Single_wire_earth_return"
     
    Last edited by a moderator: Apr 24, 2017
  19. Dec 10, 2009 #18
    So, you actually have 415Y/240 volt 3 phase going to each house?

    Do your large appliances (clothes dryer, electric range, electric furnace, A/C, etc.) run on 415 volt 3 phase, or do they only utilize 2 of the 3 "active" legs, hence 415 volts, single phase?

    I assume small electric devices run on 240 volts (1 active leg to neutral)?
     
  20. Dec 10, 2009 #19
    What is the voltage of the live wire? 12,000 volts?

    This is not common in the rural areas where I live.
     
  21. Dec 11, 2009 #20
    The vast majority of houses here just run on single phase 240V. The odd house may have a 415V 3 phase connection, but usually they just use a single phase.

    Also, our high voltage (11,000V+) supply is in delta, ie no neutral. We have delta/wye transformers feeding into the 415V network, and each main swithboard connected is grounded at the property. It's called a MEN, multiple earthed neutral installation.
     
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