Voltage after throwing breaker

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Old 07-07-08, 09:23 AM
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Voltage after throwing breaker

I have a subpanel with a single double pole 50amp GFCI breaker that fed from a standard double pole 50amp breaker in my main panel. If I throw the GFCI breaker in the subpanel and check its outputs, they correctly read 0VAC. If I meter the inputs of the GFCI breaker in the subpanel they read 240VAC as expected. However, if I throw the parent breaker in the main panel, the voltage drops to 1VAC instead of 0VAC. Is there something wrong with the breaker in the main panel that I should be worred about? I'm assuming I wont' want to do any work on the subpanel unless I completely disconnect all power since I can still read voltage when throwing the main panel breaker.
 
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Old 07-07-08, 09:29 AM
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Lemme take a guess...digital meter? Lots of talk on here about "phantom voltage" with a digital meter, esp when closer to all the still active circuits in the main panel.

No electrician here, just tryin to help out.
 
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Old 07-07-08, 11:04 AM
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Phantom V, or meter reads 1v even when disconnected and it's leads are shorted together.
Check that the meter reads zero amps when the leads are not touching anything. It could be an auto-zero circuitry problem in the meter.
 
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Old 07-07-08, 11:17 AM
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Heck, you can get a potato to register more than one volt. I don't know anybody yet that's been electrocuted by a potato.
 
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Old 07-07-08, 11:50 PM
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Well, the meter isn't your bargain basement variety (Fluke 77). It reads .000VAC when the leads are touched together and .010VAC when they are separated.
 
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Old 07-08-08, 09:07 AM
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The meter is correct in that the voltage is actually there, produced by fields from nearby lines, static charge, EMF, etc. What the meter cannot tell you is that is very, very, very little current behind the voltage.
 
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Old 07-08-08, 09:34 AM
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Voltage alone cannot hurt you. Consider a Van de Graaff generator, which generates millions of volts, but is perfectly safe to touch. That's because the voltage quickly disappears when you apply any load to it. The same thing happens when you stick the two probes of your voltmeter into an apple or a potato (but with much less voltage). The same thing is happening with your breaker. The voltage is really there, but it won't be if you touch it.
 
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Old 07-08-08, 12:31 PM
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It reads .000VAC when the leads are touched together

Autozero is working!

Also, on the current range with no leads connected you should read 0.

Shorting the leads=short circuit=zero volts, by definition.
Open leads=open circuit= zero current, by definition.

Open leads on voltage scale: indeterminate voltage.
Shorted leads on current scale might pick up a nearby AC field so, indeterminate current.
 
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Old 07-08-08, 06:37 PM
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I guess I'm still not sure what's going on. The subpanel has a single breaker and I'm reading 1VAC on its input poles and 0VAC on its output poles (I also read 1VAC at the output poles of the main breaker leading to the subpanel) . If it really was just interference, wouldn't I get the same reading on its output poles?

I would like to work on the subpanel without completely disconnecting power to my house, but I don't want to short anything out.
 
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Old 07-08-08, 07:45 PM
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get yourself a Wiggy or similar selonoid type voltage tester, these devices place a load on the circuit being tested so phantom voltages go away. 1 volt is not going to short out anything in your sub panel, and it will be safe for you to work on.
 
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Old 07-08-08, 07:59 PM
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Originally Posted by mears View Post
The subpanel has a single breaker and I'm reading 1VAC on its input poles and 0VAC on its output poles
I don't get it. What kind of breaker do you have that has "input poles"? Breakers usually have exactly one clip in back where it snaps into the bus (and which you can't even see when the breaker is installed), and one screw on the front where you connect the branch circuit wire.

Double-pole breakers have two of each of the above.

GFCI and AFCI breakers have an extra screw for the neutral wire of the branch circuit.
 
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Old 07-08-08, 09:43 PM
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Originally Posted by John Nelson View Post
I don't get it. What kind of breaker do you have that has "input poles"? Breakers usually have exactly one clip in back where it snaps into the bus (and which you can't even see when the breaker is installed), and one screw on the front where you connect the branch circuit wire.

Double-pole breakers have two of each of the above.

GFCI and AFCI breakers have an extra screw for the neutral wire of the branch circuit.
Yeah, that probably wasn't the clearest choice of words. I measured where the wires from the main breaker connect in the subpanel.
 
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Old 07-10-08, 09:01 AM
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:-\

Originally Posted by John Nelson View Post
Heck, you can get a potato to register more than one volt. I don't know anybody yet that's been electrocuted by a potato.
Actually... I won't say how, but if anybody is interested I'll share the story.
 
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