hi,
i was think'in about this simple issue. remarkably i am stupid enough to not be able to answer it. as we know, ac line have 3 channel, hot-neutral-earth.

afaik, ac line would be just sinusoidal form, that mean both side exchange it's polarity periodically. so, when i think in this context, i can not relate it how can it just hot line can turn the test pen (a special screwdriver with line is hot/neutral by touch the back side and look at the light in it - hot > on, neutral > off) light's on.

doesn't it should be both line can turn it on, if that light was turned on because the sinusoidal form?? i mean its not just the hot one that in one time became positive, but the neutral should be the same. thus, this neutral should be turn that light on too. besides our body can be assumed as the earth, i still don't see the relation.
somebody could help me out of this!?

thanks
Posted on 2004-10-24 21:03:09 by dion
In some countries (such as the Philippines), I believe that the ac line is indeed two-phase, i.e. two 110-volt signals 180deg out of phase to each other. However in other countries, I believe that the ac line has one neutral line WHICH IS ACTUALLY CONNECTED TO GROUND, and another line that is 220-volts. Thus the light test pen will work correctly in the monophase system, but will find no difference between the two lines in, say, here in the Philippines.

BTW I have a 25% trust in what I'm saying, wahahaha.
Posted on 2004-10-25 03:43:39 by AmkG
monophase?? even in monophase i see no cause that it just hot line that turn the light on.
Posted on 2004-10-25 03:52:58 by dion
well, let's say that there are two lines: the phase and null (or zero, don't know the english name) (we don't consider the earth). with the test pen you can only check the phase, a little current flows from the phase through your body to the ground (if the phase is positive), or a current flows from the ground through your body to the phase (when the phase is negative). the testpen won't light when you measure the null line. so if you touch an unknown (2 wires) ac line, you have 50% chance that you'll have pain :). for 3 phase, you have 4 wires: one null and three phases, with a phaseshift of 120? each.
Posted on 2004-10-25 04:20:55 by lifewire
The tester you talk about has a neon lamp (needs very low current to glow) and a large resistor (Megohm range to protect you).

The sytem in your home is monophase. There can be three wire: Line (live), Neutral (null) and Earth (protective earth). Sometimes the protective earth is not present.

Now the voltge is sent between the Line and the Neutral (it's actually the output of a transformer). But the Neutral and the Earth are connected together and connected to earth somewhere close to the point where the electrical wires enter your home. If the protective earth is not used, then only the Neutral is connected to earth.

The fact that the Neutral is connected to earth means that no current can flow from this wire to earth (with a power supply you have two wires, one of which is connected to ground; no current can flow from this wire to ground).
But a current can flow from Line to earth.

So when you touch the tip of the probe to the Line a current flows through the tester, through your body to earth. Even if your feet do not touch the earth the capacitance your body forms with the earth is enough to allow enough current to flow to make the lamp glow. If you touch the Neutral, which is connected to earth no current flows through you to earth.
Line :!: Lamp :idea: You :oops: Earth :o
Posted on 2004-10-25 11:44:18 by VVV
Interesting. So it means that the person holding the test pen is actually acting like a capacitor. That I never knew. :o
Posted on 2004-10-25 12:06:52 by roticv

Now the voltge is sent between the Line and the Neutral

still can't get it right. should that be in 'alternating' direction, as the name ac implied? i mean voltage sent from line to neutral half period and from neutral to line in another half? or maybe i miss something here?

its just... it seems you, VVV, assume that the test pen have diode properties in my point of view.


The fact that the Neutral is connected to earth

this confusing me a bit. actually, how voltage would react to earth? is it cause a current flow from neutral to earth? what the main goal they connect it to earth?
Posted on 2004-10-25 21:14:29 by dion
The voltage is AC, as the name implies, but one of the wires is connected to earth and we call it neutral.

You can play with a transformer, to understand this: connect one lead of the secondary to the ground of your circuit. If you measure with a scope you will see that the voltage on the other lead (with respect to ground) swings both positive and negative.

You can call this wire now your live wire, while the other is your neutral (you call it neutral just because it's connected to ground, if you swap the wires, the other one becomes the neutral, it's just a convention).

If you connect an incandescent lamp (from your flashlight) between the live wire and the ground it will light up, without any diode, since the incandescent lamp works in both directions.
So does the neon lamp in the tester, therefore no diode is needed. The only difference, really, is the much lower current that is needed by the neon lamp.
This low current requirement allows for a large resistor in series, which will limit the current through your body and protect you.

Also, keep in mind that neon lamps generally require at least 80V to glow, so the tester won't work on low AC voltages.

To answer the last question, no the current does not flow from neutral to earth, just between line and neutral. It can flow from line to earth if you connect the load between line and earth, but you should not do it (even in that case it flows from line to neutral, but part of the path is through the earth).

The connection to earth creates a reference point for the system and it prevents high common mode voltages from occurring. As I said, the voltage actually comes from the secondary of a transformer, so it would be isolated from earth. Now if you apply a voltage to any of these wires with respect to earth, both wires will be driven; and you can apply thousands of volts. That would make the system unsafe.

To test this, instead of connecting one wire of the transformer to gound, connect it to the + of a DC power supply and the - of the DC power supply to ground. Now measure with the scope the live wire (with respect to ground, which plays the role of earth) and see what you get: the AC amplitude is the same, but you can vary the DC as much as you want. If you now connect the incandescent light between your live wire (or even your "neutral") and ground, see what happens!
That would be you in the case of the AC power system with the neutral not earthed.
Posted on 2004-10-26 11:24:10 by VVV
The "alternating" in AC refers to the actual electron current flow which alternates direction based on the movement of the armature at the source power plant, but move mostly in one direction due to the applied voltages. Think of the electrons as people running back and forth on an escaltor.
Posted on 2004-10-26 12:57:34 by SpooK
?? looks like i am almost totally lost here :)

i understood your first part, VVV.
but the second one, overwhelming my mind. did my base theory about AC voltage was wrong?

SpooK, i'm hard to understand this :
"but move mostly in one direction due to the applied voltages".

does swinging negatively did not meant as a current flow in reverse direction?

current does not flow from neutral to earth, just between line and neutral. It can flow from line to earth if you connect the load between line and earth


if earth connect to neutral, should that be neutral becomes earth? i mean when you said 'flow from line to earth' should be same like 'flow from line to neutral'?

this bother me, is it a simplified earth's complex properties? as if it just somekind of conductor? from my reasoning, earth is nothing but a follower, so, when neutral connect to earth, the earth follow the neutral. only in this way, i can think it right.

..but you should not do it

i ever do that with voltmeter. it measured about half of line volt.

about high common mode. if both wires driven considered to be unsafe, how one wire driven will be safe?

about the test. 1st one results in ac with offset voltage.
second, the light bright and dimm, but not truly off. is that right?

sorry guys if my too many '?' bored you or drive you mad. please be patient with me :)
Posted on 2004-10-26 23:38:14 by dion
Swinging negative or positive refers to the VOLTAGE with respect to a reference point, which arbitrary, really, but we generally take the ground to be our reference point.


When I said current does not flow between neutral and earth I meant NO CURRENT flows, that is, it's zero all the time. I did not refer to the direction. AC currents also swing negative and positive, but depending on the load their swinging may or may not follow the swings of the AC voltage that produces them: in an inductive circuit, a sinusoidal current "lags" the voltage (changes later), in a capacitive circuit the current "leads", that is it changes before the voltage does.

In the second test, if you connect everything properly, the light should be either bright, or off.

I did not say that a single wire was driven; I said the voltage was developed between the two lines from the secondary of a transformer, but one of them is connected to earth. That way the common mode voltage (the bias, as you call it) with respect to earth is kept at earts's potential. Since you are also at earth's potential no current can flow from the earthed conductor through you to earth.
However, a current can flow through you to earth if you are connected to the earth and touch the Line (live) wire. Yet, the voltage is only your AC line voltage (100-240VAC). While still dangerous, it is limited. Without an earth connection, high voltages from other sources could create a high common mode voltage (possibly much higher than the line voltage) and thus would be much more dangerous.
That is the DC voltage I suggested in the second test. You can increase it as high as the power supply will allow if none of the leads of the transformer is grounded. But if one of them is grounded you create a short and your DC power supply will just go into current limit, but will not be able to deliver any "high" voltage.
Posted on 2004-10-27 11:59:56 by VVV
Polarity does not matter in Neon gas. Once the gas is excited to an ionized state it will cause it to emit photons. AC or DC. "Energy"
Posted on 2004-10-27 14:31:37 by mrgone
Okay, it's this way:

Current flow requires a voltage DIFFERENCE between two points. Absolute voltage is usually meaningless and is a theoretical exercise in most cases. What's needed is relative voltage, i.e. a voltage DIFFERENCE between two things.

In a monophase system, one line is connected to the earth (ground, yes there's an actual metal rod that's buried in the ground and one wire is connected to it). This line (the NEUTRAL line) therefore has a potential equal to the voltage of the earth, which is ASSUMED ARBITRARILY to be zero "0". The other line (the LIVE line) is NOT connected to earth. A voltage source (i.e. your electric power company) creates a VOLTAGE DIFFERENCE between the two lines.

Now, by default, a human body standing on the earth also has the same potential as the earth. By touching the thumb screw (or whatever it is you are supposed to touch in the light pen), you provide an earth or zero potential. If the other end of the light pen is connected to the LIVE wire, you have a VOLTAGE DIFFERENCE between you and the live wire, causing current flow and light. If the other end of the light pen is connected to the NEUTRAL wire, both you and the NEUTRAL are at the same voltage, there is no VOLTAGE DIFFERENCE and therefore, no current and no light.

In some countries (such as here in the Philippines), there are actually two 110-volt lines out of phase to each other (the voltage is referred from the ground, so in our country the voltage difference between the two lines is 220V, but from ground to one line, is 110v).
Posted on 2004-10-28 21:28:23 by AmkG
i guess i think'in too much on this :lol:
anyway, thanks for all your replies :wink:
Posted on 2004-10-28 22:21:51 by dion