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Old 04-21-2010, 11:13 AM   #1
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Induced Current


Hi, I have done a number of small electrical projects around our house over the past few months and have sometimes noticed the following. I have one of those no contact live wire detectors and sometimes it still flashes/beeps even after disconnecting a circuit from the fuse box. (not all circuits do this, just some of them). When I use my multi-meter to measure the voltage in these situations, sometimes I do indeed register a small voltage (maybe 5 volts). Is that normal? Is that due to induced current from other circuits?
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Old 04-21-2010, 12:30 PM   #2
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Old 04-21-2010, 01:19 PM   #3
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Yes you are sensing induced current.

When a live conductor is juxtaposed with a dead conductor but not touching, for example in a Romex cable, current leaking between the two can be measured.

The larger the contact area, for example the longer the length over which the conductors are juxtaposed, the larger the current that can leak. Three way switch travelers are a good situation where induced current can be measured.

The closer the conductors are to one another, for example if the insulation is thinner, the larger the current that can leak.

(not really relevant for household wiring) The higher the AC frequency, the larger the current that can leak.

The voltage that the meter shows depends on the "resistance" between the juxtaposed conductors versus the "resistance" of the meter; the larger the latter relative to the former, the larger the voltage reported.
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Old 04-21-2010, 03:40 PM   #4
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Quote:
Originally Posted by mkrawitz View Post
sometimes I do indeed register a small voltage (maybe 5 volts).
And if you measure it with a milliammeter instead of a voltmeter you should get about 1/2 milliamp regardless of the meter range selected and the type of meter used.
But in the unlikely event that it's not a phantom voltage then you pop your meter!

Last edited by Yoyizit; 04-21-2010 at 03:52 PM.
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