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Old 01-24-2009, 12:01 AM   #1
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Lightbulb Neutral voltage

I have a basic voltage characteristics question I'm having trouble getting a clear answer for. In a basic 120V single phase ckt you have the hot and neutral connected to a load such as a light bulb. With the bulb burning I place a clamp meter around the hot wire and get a reading of 1.2A, if I place the meter around the neutral wire I get the same 1.2A value. Now if I place a voltage detector against the hot wire it picks up voltage, but if I place it next to the neutral wire it does not pick up any voltage. So why is there not any voltage going through the neutral wire while the bulb is burning? Just something thats been bugging me, thanks for the help.

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Old 01-24-2009, 01:09 AM   #2
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Originally Posted by lectric_hand6855 View Post
I have a basic voltage characteristics question I'm having trouble getting a clear answer for. In a basic 120V single phase ckt you have the hot and neutral connected to a load such as a light bulb. With the bulb burning I place a clamp meter around the hot wire and get a reading of 1.2A, if I place the meter around the neutral wire I get the same 1.2A value. Now if I place a voltage detector against the hot wire it picks up voltage, but if I place it next to the neutral wire it does not pick up any voltage. So why is there not any voltage going through the neutral wire while the bulb is burning? Just something thats been bugging me, thanks for the help.
Just imagine that your measuring the voltage with a meter to the ground instead of a voltage detector. The hot wire will be 120v to ground and the neutral will read 0v to ground because it is grounded at the panel(there is no potential difference between the the ground in the box and the one at the panel). Now if you measure across the lamp and you will get a 120v voltage drop across it.

As for why your reading 1.2A on the neutral; it's because the circuit is basically a closed loop so for current to flow at one point it has to flow all through the entire circuit in this case.

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Old 01-24-2009, 03:16 AM   #3
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Quote:
Originally Posted by lectric_hand6855 View Post
I have a basic voltage characteristics question I'm having trouble getting a clear answer for. In a basic 120V single phase ckt you have the hot and neutral connected to a load such as a light bulb. With the bulb burning I place a clamp meter around the hot wire and get a reading of 1.2A, if I place the meter around the neutral wire I get the same 1.2A value. Now if I place a voltage detector against the hot wire it picks up voltage, but if I place it next to the neutral wire it does not pick up any voltage. So why is there not any voltage going through the neutral wire while the bulb is burning? Just something thats been bugging me, thanks for the help.
Because the neutral is grounded. Your voltage detector relies on the fact that one circuit conductor is connected to ground. The entire area is one plate of a capacitor. When you put your tester near a hot wire (the other capacitor plate) it will detect the electric field. If you put it near the neutral wire, it will have no reading because the neutral is at the same potential (or very near) as the everything around it.

LZ makes a good analogy with the meter.
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Old 01-24-2009, 05:50 AM   #4
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I did discover I can find the neutral of a circuit at the panel using my circuit finder.
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Old 01-24-2009, 06:22 AM   #5
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Also ----- non contact voltage testers can mislead. When live indicates, it may be that there is no live (HOT) just inductance or capacitance within the circuit that is measured to ground ( capacitive plate as Inphase says) - where the circuit has no ground, as in fault conditions. Also, if the voltage level between neutral and earth is high enough - say 20 volts or so - then a non contact tester may indicate the neutrals as live.

So what have we learned. Use a volt meter for testing voltage levels and don't rely on 'field sniffers',


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Old 01-24-2009, 12:03 PM   #6
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I certainly agree with frank's addition. Tics are good for rough work, but you should never rely on them for any except rough indications.
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Old 01-24-2009, 01:41 PM   #7
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[quote=frank;55018] Use a volt meter for testing voltage levels and don't rely on 'field sniffers',

We often refer to these little devices as "liar sticks"!
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Old 01-24-2009, 09:53 PM   #8
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We often refer to these little devices as "liar sticks"!
"Death Stick"!
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Old 01-25-2009, 06:35 AM   #9
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Well we agree i do not use a tick . batt dead your dead !! think of a voltage sensor as a radio receiver because thats what is inside that voltage sensor its a basic am radio reciever and yes its effected by capacitance the antenna is you tip on that sensor when near a hot wire closer to higher voltage potential its freq inside is changed and sends out a signal to let you know voltage is present . when near a neutral wire its closer to earth ground or ground potential so it doesnt pick up a strong capacitance or field of freq to change its freq. no signal response i maybe out of line please correct me ? best to ya

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