3 Q's on A.C. current

Factual Questions
1

Please enlighten me, any electricity experts here.

  1. How does the A.C. current tester work, the one made as a small screwdriver with some kind of a bulb in the handle, that will glow when the screwdriver tip is inserted into a socket hole and the handle end is touched with a finger?

  2. I live in a place where the supplied A.C. current is 220 volts. To get 110 volts I regularly use one line of the 220 volts grid, and replace the other line with a wire connected to a grounding metal rod driven into the earth outside the house.

As I can understand from my own reading that one line from a 220 volts A.C. current means one half of the sinusoidal wave, hence 110 volts; what I would like to understand is how the earth part supplies the other line to complete a circuit? How does the earth serve as a part of the A.C. generator . . . ?

  1. Can two or more A.C. generators use the same wire for their respective cirtcuits, namely: one generator-circuit lighting a bulb, another serving a toaster, and a third running a computer – of course understanding that the wire is of sufficient gauge.
    Susma Rio Sep
2

  1. The tester contains a glow lamp and a resistor. Your body completes a circuit (earth - AC generator coil - AC transmission line - socket contact - tester contact - resistor - glow lamp - your body - earth). The tester tests not for current but for voltage, of course.

  2. This looks like you are living in the US, with 220/110 symmetrical AC mains to households, right? I am a bit puzzled because from all that I have read about this type of mains there should be a separate neutral wire for you to use. Where do you live?

  3. Of course, that’s what an AC grid is about - a number of generators supply power to the grid, and the electricity consumers draw from what the generators collectively produce. But it is physically not meaningful with several generators to talk of one generator suppling one load, and another one supplying another load.

3

  1. Uses a neon bulb and body capacitance, try to avoid the route to ground.

  2. You might get a 110 voltage reading but there won’t be enough current to run anything, you need the center tap of the transformer.

  3. One generator should be enough for the load you describe.

4

The others are correct; there is a complete circuit, but it’s based on capacitive coupling. The current is usually very low (microamps). One problem with this approach is that it’s difficult to power a bulb directly from this current, so some units boost the signal using a series resistor, amplifier, and battery.

Do not do this!! You neutral should be a copper wire going back to your service panel. This is due to the following:

  1. It’s against code.
  2. The earth has a lot more resistance than a copper wire.
  3. If you have a short there’s a very good chance the breaker will not trip.
  4. There may be a significant voltage between the outlet’s neutral/ground and the service ground when current is flowing.

Again, do not just stick a copper rod in the ground and call it your neutral! You must have a low impedance path between the outlet’s neutral and the neutral/ground bus bar at the service panel. This must be accomplished with copper, not dirt.

I guess I don’t understand… what are you trying to do? Are you using portable AC generators?

5

Some additional info
(2) There are basically two types of AC systems, isolated and grounded. Isolated systems do not have any of their conductors grounded. They have an advantage in that you can touch ground and touch any conductor and not get shocked. They have a disadvantage in that if you try and use them in a very large system, mother nature tends to randomly ground things for you, which ruins your isolation. Hence, isolated systems usually are only used in special circumstances, like in hospital “wet” locations (operating rooms, other areas where bodily fluids might be present).

For grounded systems, you have a lot of choices, but basically you have to pick some conductor in the system and intentionally ground it. Your power is fed from a transformer, which converts the higher voltages used in transmission down to 220/110 volts. This transformer has a center tap, which is grounded. In this system, you know that any grounded conductor is safe, and the other conductors (the so-called “hot” conductors) are not safe to touch. In the U.S. there are 220 volt circuits which are used typically only for appliances like clothes dryers and ovens, then 110 volt circuits which are run all over the house. Typically they are balanced, so roughly half of the circuits will be on one of the “hot” conductors and the other half will be on the other “hot” conductor. Between either hot conductor and nuetral is always 110 volts.

Note that there are other types of power systems where “ground” is not the midpoint of the two conductor voltages. For example there is a thing called a 3 phase delta system, and it’s possible to ground one of the phases. This is used more commonly in transmission and distribution though. By the time it gets out to residential areas you usually only have a single split phase, like what you have.

(3) I don’t think this works the way you think it does. Basically, when you connect all of the wires together, you create a “node.” This node has a total load connected to it, which is the sum of your light bulb, toaster, and computer. The generator(s) feeding this node have no idea what is actually connected. It could be one thing, it could be a hundred thousand things, all the generators know is how much current they are going to draw. Multiple generators hooked up to a single node will share the load, but they may not do it equally, and in fact you can control how much current each generator supplies to balance out the load. But there is no way to know from the generator side how much current each individual load draws, and since everything connects together the currents will all be shared and split.