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Resistance of meter coils

  1. Jan 20, 2007 #1

    Just had a bit of a heated discussion with a friend of mine about electricity meters and I decided to get some info from you chaps so I can promptly add some ammo to my argument next time we meet!

    The claim is that you will get a low resistance reading between phase and neutral meter tails coming out of a property's electricity meter before it reaches the distribution board when the power is off. Apparently the phase and neutral are linked together in the electricity meter through a coil.

    Personally I just don't agree. How could this be possible? Wouldn't it cause a dead short?


    ..is it true that the meter works on a kind of seperated circuit prinicple like a transformer but with the exception being that the phase on one coil is linked to the other so to provide the earth loop to the substation which is needed during a fault scenario for quick supply disconnection :confused:
  2. jcsd
  3. Jan 20, 2007 #2
    The last meter I pulled had no connection to the neutral which I believe is the norm at least in residential 240 volt 3 wire service. A meter several coils. Since it is a watthour meter, the voltage has to be sensed as well as current. There will be a coil across both hot legs and one in series with each leg. The ones in series are VERY low impedance and amount to little more than a turn or so of very heavy copper. I have never checked the resistance across each leg but I would assume it is relatively low. The AC impedance is higher though due to the inductive reactance of the coil.
    Adder, I thought you were an electrician? What gives? How did you miss out on the fact that the meter has no neutral connection?
  4. Jan 20, 2007 #3
    Thank you, that clears things up nicely :approve:

    Never had much to do with them. Spent the beginning of my career at University and then as a board tester and schematic drawing maker. Only re-trained as a domestic spark within the last year or so. Fundamental knowledge gaps mean I'm going back to sit the same course apprentices do even though legally I don't have to.

    The law allows people to go out trading provided they have the two highest qualifications required. It is not a requirement to sit the installation based basic course. I can positively run rings around most electricians when it comes to theory, but the practical parts of what I've been taught are poor, hence why I've opted to go back and do the really basic fundamental stuff, even though I'm not legally obliged to because I'm already qualified enough to keep trading.

    The current trend of encouraging people without the required practical training to go out trading in this country is really starting to worry me. At least I'm doing my bit not to be one of them by going back to do Parts 1 & 2. I've already seen some horrendous wiring on some of the jobs I've been to recently. I saw one recently where the live coloured cable had been used as the neutral and vice versa. I had to refuse to do the job. Scary.
  5. Jan 23, 2007 #4


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    There are 2-wire meters that read 120 from hot leg to neutral. They also have a potential coil. The theory is the same as the 3-wire meter mentioned above, except the there is only one current coil and (as mentioned) the potential coil is 120 volts.
  6. Jan 24, 2007 #5
    I'm not saying your wrong, as I have been caught out by your strange wiring conventions before but...

    In UK 240 is 240 even when it is not. Brown outs are an incredibly rare occurence even in very remote areas, therefore there is no need to measure voltage just current.
    As far as I understood (May have to do a bit of homework) the current flow is measured using a pick-up coil and integrated to give Watts, as Voltage is constant.
    For industrial set-ups they may have a phase (Power Factor) meter that measures how unbalanced your load is. Companies with very inductive PF are charged a weighting as the power company a) is only measuring the resistive part of the power consumption and b) has to deal with the unbalanced star point. Companies are actually encouraged to install PF correctors (Big Capacitor) as even though this wastes power overall it is cheaper than dealing with unbalanced star points.
  7. Jan 24, 2007 #6
    Large capacitors do quite the opposite of wasting power over all. Yes, there are losses in the circulating currents between the capacitor(s) and the inductive loads. However, correcting for this sort of thing LOWERS the current in the transmission lines feeding the customer who has this inductive load. The lowered current in the transmission lines feeding this customer contributes to less loss along these transmission lines.
  8. Jan 25, 2007 #7
    You seem to be implying that if I am running a big motor with a resistive inductive load drawing constant power and then I put a big capacitor across the motor which is a resistive (although small) capacitive load, overall I will draw less power from the supply with no drop in power out from my motor.

    I'm merely a rocket scientist but I'm pretty sure that the more bits in your circuit the more losses you have not less. If you get less loss by adding bits to you circuit something must be making energy.
  9. Jan 25, 2007 #8
    Are you serious? Rocket scientist? No really, I am serious in my previous posting, and, I am correct. Drawing less current does NOT imply pulling less power from the line. Pre capacitor install the current is higher but more out of phase than after the capacitor is installed. If you truly understand power factor then you should understand what I've posted.
    Last edited: Jan 25, 2007
  10. Jan 26, 2007 #9
    Somebody has to be a rocket scientist - It's a lot easier than most other science, and gives you endless oppertunity to say "It's not rocket science this... Ohh it is, isn't it Ha ha ha..." seconds of fun.

    I agree that if you are drawing power out of a resistive load such as an electric bar heater then inductance and capacitance atrribute to wasted energy, but if your power is being drawn from the inductive component as in a motor where you draw power from the magnetic field then the out of phase energy is not being wasted.
    Big motors are almost all inductive but have energy efficiencies in excess of 90% so I can't believe that all the magnetic energy is being dumped, in fact the resistive energy is all copper losses.
  11. Jan 26, 2007 #10
    There's some seriously smart people on this forum. I followed about 0% of the last few posts! :blushing:
  12. Jan 26, 2007 #11
    The reason the power company wants the power factor to be as close to one as possible is because that is the condition where the current in the transmission lines is the lowest. Any time that the power factor wanders away from 1 and the same amount of work is still being done (power in watts, not voltamperes) then the current in the transmission line will have to go up. When a capacitor is installed across a large electric motor (inductive load) to get the power factor back to 1 we have formed a parallel resonant circuit. A parallel resonant circuit has the highest impedance at resonance and as you go either side of resonance the impedance goes down and the circuit draws more current. However, the current and voltage are not in phase when the load is not resonant and when this happens there is not any actual work done. Volts x Amps does NOT equal watts in this case. It equals Voltamperes, also known as apparent power. The torque on the generator at the generating plant in this condition does not increase. But, the current along the transmission lines (as well as the windings in the generator) encounters resistance in connections and wires which is unavoidable. This creates loss in the form of heat. The best way to prevent this loss is to keep the current as low as possible.
  13. Jan 29, 2007 #12


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    Voltage on a distribution circuit is never constant since it will vary with load and the load is never constant. Line voltage regulators will maintain a usable voltage, not a constant one.
  14. Jan 30, 2007 #13
    I know that but as the maximum permissable deviation in the UK supply grid is +/-6% voltage +/- 1% frequency, it is constant in an engineering sense, if not a scientific sense.

    Very few systems even quite finely calibrated instruments would notice such a fluctuation, unlike some places I've worked in the states where we have had to use our own generators to ensure the instruments didn't shut down.
  15. Jan 30, 2007 #14
    You can't possibly be serious. It is not at all hard to measure that.
  16. Jan 30, 2007 #15
    I meant instruments powered by that mains supply being adversley affected, not instruments measuring that fluctuation.
  17. Jan 31, 2007 #16
    I'm actually being taught about the power factor correction in my university right now. So here: have some notes

  18. Jan 31, 2007 #17
    This is all really basic AC theory. This is usually one of the first things covered in school. Ohms law, DC and resistors with basic knowledge of test equipment is covered first, then it's on to AC theory. If you've gone through basic AC theory and can't understand what's been covered in this thread....:uhh: :confused: :redface: Unless I have a very poor way of explaining it. Possible, but I'm told otherwise from most people.
  19. Jan 31, 2007 #18


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    True, but in terms of revenue metering, assuming a plus or minus 6% variation in voltage as irrelevant to metering accuracy would be tantamount to stealing.

    In Texas, the Public Utility Commision requires plus or minus 2% (or less) error on house meters (most of which in practice run within .5%). Commercial and industrial accuracy requirements are much more stringent.
  20. Feb 1, 2007 #19
    The deviation I quoted was peak and has a 15min maximum duration, if this is exceed the distributer has to pay recompense to affected users and a fine. In reality it is significantly less. I didn't mention the duration as it was in context of wether you needed to measure voltage to determine power.

    The accuracy of meters is a constant error in the meter reading based on a defined input. Which is very different from a deviation.

    If your voltage dropped 6% for 15minutes at some point in a quarter you wouldn't notice any difference, but if your meter ran 2% fast for the whole quarter you definately would.
  21. Feb 1, 2007 #20
    The problem I see with that is that the times that it is MOST likely to have low voltage is when the you are using the most power. Obviously there is no problem when the voltage is low and the meter is running fast when you are using little to no power.
    Does anyone know how wide of a voltage range meters have here in the US? If you have 460 volt 3 phase service then it would require a different meter than 240 volt 3 phase without a voltage winding in the meter. Maybe it already does, but with a high impedance voltage winding in the meter I would think you could use the same meter within a certain range of voltages.
    I'm not saying you are wrong Panda, I'm just saying I disagree with the method.
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