Ghost or Phantom Voltage Question

cj65

cj65

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Aug 30, 2008
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I bought a repo house, and if I can keep my fingers out of this forum, I actually get some repair work done. One thing that really disturbs me is ghost voltage readings. Tonight I was changing out a light fixture, and before rewiring, I always check for voltage. This is not the last time that I picked up 40 to 70 volts on the multi meter even though the switch was off. I find myself wondering if the switch is really off (I am 99.9% sure) but the darn multi meter shows a 40v reading, or even a 70v? I understand that when you have live wires running through conduit next to open circuit wires (what I am working on), through capacitation, you get the fake readings. I am pretty sure it gives you a little jolt too, unless I am just being jumpy. I would like some opinions from others that know enough about residential wiring, and how to know for sure what you have is ghost voltage. I hear I can buy an older style analog multi meter which will not read the false voltage. Is there any other way to be sure/trouble shoot wiring issues when you come across the Phantom of the fuse box?
 
In California, residential wiring isn't usually done in conduit.

If you're using a digital meter, it may read a voltage when connected to a long open wire. It's energy from radio signals and charge from the ambient environment collected by the long wires; they essentially act like an antenna. Good meters have filters on their input to reduce this effect. Digital meters have very high input impedance (tens of megaohms), so they can tend to read this voltage. But, there's almost no power in this "phantom" voltage, so there's no danger.

To verify, use one of those little test lights instead of your meter.
 
You can also get such voltages with high impedance short circuits. For instance, as a former utility engineer I have seen a cobweb cause such voltage on old wiring in a barn. Current drain is negligible, but high voltages can be created.
 
el dub said:
Or you may have an open neutral. Further checks req. for this.
Click to expand...

That's a good point. Check the bonding of neutral in the box. You may also want to have the ground intergity checked which will probably need to be done by a pro since the equipment is rather expensive.
 
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Hook up a light load to it, such as a 10watt light bulb, if it doesn't even glow then it should be pretty harmless and you can feel relatively safe when hunting down the cause further.
 
Thanks guys. I will continue to investigate with your advice.
 
California wiring regs allow two circuits to have the same neutral. If you shut off only one circuit of such a set you can still get some backflow through the other. I've gotten a couple of buzzes that way. If in doubt, turn off ALL the circuits, then do your work.

I also had one case where the DIY owner had wired at least one of the circuits backward. I plugged my drill into what was supposed to be a grounded outlet (this was before cordless) and started to drill into a grounded piece of metal. Fricking sparks off the drill bit and all of a sudden viola! I was welding and not drilling. So you can never be too careful if you don't know that the wiring was properly done by previous "electrician".
 
knarfeng said:
California wiring regs allow two circuits to have the same neutral.
Click to expand...


Wow! I did not know that. That is shocking... pun intended. I've always thought that California people were crazy; this proves it. There are just so many ways that could go horribly wrong.
 
Thanks Frank! I also need to add some GFCI outlets in the bathrooms. They did that as well. I enjoy it though.
 
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