david
Senior Member
- Location
- Pennsylvania
Move the NM cable away from the service entrance conductors and test the NM cable again
voltage tester was picking up voltage on a detached wire
- Thread starter michaelkingdom
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Move the NM cable away from the service entrance conductors and test the NM cable again
FionaZuppa
Senior Member
- Location
- AZ
- Occupation
- Part Time Electrician (semi retired, old) - EE retired.
i was not discussing the tester. the coupling i was mentioning was how one of these testers can read "live" downstream of a wire break, etc. its predominantly capacitive coupling across the break, but the voltage developed on the non-connected side of that breal will be extremely small due to the low resistance there, but these non-contact proximity sensors are still sensitive enough to say "live". there is however still a mag field there as the e-field flips at 60Hz.
but yeah, electric field detectors are neat (http://amasci.com/emotor/chargdet.html)
to have fun with the kids, pretend you have a pair of sunglasses that can see through walls, have them inside a bathroom with door closed and you on in adjacent room (even 40ft away if you build it with super gain). have kids comb their hair with a plastic comb, your sensor will show that charge when they comb, you can now "see through walls"
, the kids of course will want to use and try your sunglasses, so i use a cheap $5 pair and tell them it takes lots of practice and concentration to use them to see through walls, so i give them the glasses to keep and practice with.induced voltage from neighboring wires means a magnetic coupling. you would need special circumstances to induce 50v on a neighboring disconnected wire. i suspect your tool is more sensitive than the specs say (why is there a upper limit in the specs?? <-- thats not a sensitivity spec, more of a physical safety spec). does it buzz if its near a 12vac wall-wart?
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- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Induced voltage in neighboring wires which is present only when load current is flowing in the source circuit might be magnetic coupling, but it is much more likely to be capacitive coupling between adjacent wires, caused only by the voltage present on the source wires and not dependent on current flow.
The common usage of "induced voltage" does not imply magnetic induction.
FionaZuppa
Senior Member
- Location
- AZ
- Occupation
- Part Time Electrician (semi retired, old) - EE retired.
induced voltage is most commonly associated with a magnetic field interaction. the notion of induced voltage by way of capacitive coupling is not so common.
a very small voltage may form from e-field, but have you seen any meter measure 50v or more on the wires of NM that has open ends. that could be quite a hazard depending on how much Farad there is holding that voltage. these tiny cheap proximity sensors can detect small e-fields but how can you get 50v on open wire, there is no way for charge to build on a single wire that is not connected to anything (net zero in metals), and to get a diff between wires you would need the ends to be oppositely charged (still net zero though). these things are also typically seen in static charge realm, a tad different with e-field flipping at 60Hz?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
You are certainly not going to get DC on the wire, but if you place an open wire midway between a hot wire and a grounded neutral in open space, a very high impedance tester will measure half the line voltage (i.e. 60V) just from the capacitive voltage divider.
You do not need to have any current flow to raise the voltage of an open wire located in an electric field, and physical current (moving electrons) will flow in hot and neutral corresponding to the series capacitance.
The bookkeeping on closed loop current is balanced by the "displacement current" corresponding to the changing electric field between the wires.
- Location
- Illinois
- Occupation
- retired electrician
While that does test the device itself, it does not tell you that the device won't give you a false negative when testing for an energized state.
If you read all of the disclaimers in the manufacturer's instructions, you would probably never use them for life safety type of testing.
mgookin
Senior Member
- Location
- Fort Myers, FL
- Occupation
- Retired inspector, plans examiner & building official
I think we all agree on that. And of course we'll take that false positive over a false negative any day.
macmikeman
Senior Member
- Location
- Planet macmikeman
Ok, I only read to page three and then skipped ahead to this. Older houses wired using two 14-2 romex cables to do the three way switching , some of them my non contact voltage detectors will ring by touching the tester close to almost any object connected to the inside of the house- door knobs, wooden doors, interior walls, etc. Especially if there is any neutral to equipment grounding buggery going on , which is all to often the case. I have seen and demonstrated the same event on even older dwellings wired with knob and tube. It's not every time, just some of them do it. I wouldn't want to live in one of those places though, too much mystery either's.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Which leaves such testing devices almost of no practical value. They do work for detecting missed connections in an equipment grounding conductor. If you hold them up to metal parts of a light fixture or even near metal yokes of a wall switch - if the object is bonded properly they usually will not "indicate" but if you forgot to bond the object they will "indicate" nearly every time, because that item has a capacitively coupled voltage on it.
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