swansonj
Guru
[QUOTE 4333827, member: 9609"]what causes these ghost voltages. Is it just magnetic fields from adjacent live wires on another circuit. And if you had a ghost voltage of lets say 5v, could you power something with it or is it just a handfull of electrons (or whatever they're called) that you would never get enough current ?[/QUOTE]
Whatever current you are using in a circuit flows back through the neutral conductor of that circuit. Whatever current you are using in the whole house flows back along the neutral conductor of the supply cable to the house. Whatever current is being used by all the houses on a circuit combined flows back to the substation along the neutral conductor of the distribution cable. And every time a current flows in a neutral, it creates a voltage, through Ohms law, V=IR.
The "neutral conductor" may actually be a combined neutral-and-earth conductor and it may actually be the sheath of the cable, but that doesn't affect the principle at this stage.
The neutral current in your house is simply the load current flowing back through the neutral. When it comes to the distribution cable along the street outside your home, supplying multiple homes, that's usually a three-phase circuit, with the homes split roughly equally between the different phases. The idea of that is the successive neutral currents from successive homes are out of phase with each other and cancel. The cancellation's never perfect, but it does mean that the neutral current in the distribution circuit is usually a lot lower than the sum of the neutral currents from all the individual houses. But it's still there and still creates a voltage.
So the neutral conductor in your home has this small V=IR voltage on it, going all the way back to wherever the neutral and earth are joined together. At that point (to a first approximation) the neutral voltage is zero; the V=IR voltage accumulates along the neutral conductor downstream (on your house side) of that point.
Neutral conductors are required by Regulation to be earthed at the substation. In the good old days, the law also said that was the only point where they were allowed to be earthed (in the obscure jargon we use, TT or TN-S). So the neutral was rising in voltage all the way along the distribution circuit along the street and into your home.
Then, as @subacqua has pointed out, in the 1930s they allowed piloting of protective multiple earthing (TN-C-S) and this has been standard practice in the last few decades. That means the neutral is earthed at, err, multiple positions as well as the substation. So now the voltage rise in the neutral is only (again to a first approximation) in the length from within your house out to the closest earthing point on the neutral in the distribution circuit. And if your house has pme (a separate issue from whether the circuit has pme), the neutral and earth will be bonded together at the meter position as the neutral enters your house, so any voltage rise is only over the length within your house (it is still against the law to have the neutral and Earth connected within your home, though some idiot once wasted his time discovering that 20% of UK homes have accidental neutral-Earth connection and got a scientific paper out of it). But it is still there, and if you've got a joint not done up quite as tightly as it should have been and thus inserting a bit of resistance, that will help make the R a bit higher and hence the V=IR. There are some pretty pictures of where the earths are with pme at http://www.emfs.info/sources/distribution/uk/ (coughs modestly)
That's the main way these voltages arise but @User9609, you're right, induction from other circuits can contribute as well. Not so much, I think, the electric field induction from adjacent circuits at mains voltages, though on transmission lines, there is a seriously problematic safety issue with induction from adjacent live circuits onto a nominally dead circuit that someone may be working on. But in homes near transmission lines, the magnetic field from the transmission line can induce small voltages onto distribution circuits. A mate of mine in California has written several papers about it, e.g. http://www.ncbi.nlm.nih.gov/pubmed/17099403, but as I'm engaged in a genteel academic dispute with him over how seriously to take that issue, I'm not exactly promoting that possibility....
Well, predictably, I can't resist adding my own version of the explanation...
Whatever current you are using in a circuit flows back through the neutral conductor of that circuit. Whatever current you are using in the whole house flows back along the neutral conductor of the supply cable to the house. Whatever current is being used by all the houses on a circuit combined flows back to the substation along the neutral conductor of the distribution cable. And every time a current flows in a neutral, it creates a voltage, through Ohms law, V=IR.
The "neutral conductor" may actually be a combined neutral-and-earth conductor and it may actually be the sheath of the cable, but that doesn't affect the principle at this stage.
The neutral current in your house is simply the load current flowing back through the neutral. When it comes to the distribution cable along the street outside your home, supplying multiple homes, that's usually a three-phase circuit, with the homes split roughly equally between the different phases. The idea of that is the successive neutral currents from successive homes are out of phase with each other and cancel. The cancellation's never perfect, but it does mean that the neutral current in the distribution circuit is usually a lot lower than the sum of the neutral currents from all the individual houses. But it's still there and still creates a voltage.
So the neutral conductor in your home has this small V=IR voltage on it, going all the way back to wherever the neutral and earth are joined together. At that point (to a first approximation) the neutral voltage is zero; the V=IR voltage accumulates along the neutral conductor downstream (on your house side) of that point.
Neutral conductors are required by Regulation to be earthed at the substation. In the good old days, the law also said that was the only point where they were allowed to be earthed (in the obscure jargon we use, TT or TN-S). So the neutral was rising in voltage all the way along the distribution circuit along the street and into your home.
Then, as @subacqua has pointed out, in the 1930s they allowed piloting of protective multiple earthing (TN-C-S) and this has been standard practice in the last few decades. That means the neutral is earthed at, err, multiple positions as well as the substation. So now the voltage rise in the neutral is only (again to a first approximation) in the length from within your house out to the closest earthing point on the neutral in the distribution circuit. And if your house has pme (a separate issue from whether the circuit has pme), the neutral and earth will be bonded together at the meter position as the neutral enters your house, so any voltage rise is only over the length within your house (it is still against the law to have the neutral and Earth connected within your home, though some idiot once wasted his time discovering that 20% of UK homes have accidental neutral-Earth connection and got a scientific paper out of it). But it is still there, and if you've got a joint not done up quite as tightly as it should have been and thus inserting a bit of resistance, that will help make the R a bit higher and hence the V=IR. There are some pretty pictures of where the earths are with pme at http://www.emfs.info/sources/distribution/uk/ (coughs modestly)
That's the main way these voltages arise but @User9609, you're right, induction from other circuits can contribute as well. Not so much, I think, the electric field induction from adjacent circuits at mains voltages, though on transmission lines, there is a seriously problematic safety issue with induction from adjacent live circuits onto a nominally dead circuit that someone may be working on. But in homes near transmission lines, the magnetic field from the transmission line can induce small voltages onto distribution circuits. A mate of mine in California has written several papers about it, e.g. http://www.ncbi.nlm.nih.gov/pubmed/17099403, but as I'm engaged in a genteel academic dispute with him over how seriously to take that issue, I'm not exactly promoting that possibility....
