0

Second-floor outlet indicates voltage with meter but it does not power a lightbulb. We have repeatedly turned off every circuit breaker in the house and none of them has resulted in the voltage going away.

We believe that this is the only outlet remaining on an old 1920s era circuit but that is based on a previous attempt to identify which circuit it was and now we can't get the voltage to stop.

Where do we go next to comfortably get rid of this thing?

Unfortunately I don't have a meter, just a pen indicator. I assume that it is some sort of phantom voltage as mentioned in comments but if it were, it should go away when we turn off whatever close-by circuit is causing it, right? Yet it never goes away.

Based on some comments, I borrowed a meter from a neighbor and it shows not more than 1 volt. I will try shutting down the whole breaker and testing it then. And then I will disconnect (rather than just turn off) the circuit that I think it's on and see if that takes care of it.

1
  • 3
    Could be phantom voltage, picking up voltage from another close power cable. What is the amount of voltage you are reading?
    – crip659
    Nov 23 '21 at 22:10
2

This doesn't answer your question directly, but it will explain why you see what you see with your multimeter.

A multimeter commonly has a 10 Megaohm input. This is very large; at 110VAC the current through the multimeter will be 0.01mA, or 10µA. The power will be 1.2mW.

This is usually seen as a good thing; the instrument should not influence what it measures, and in electronics a 10MOhm input is fine, because you cmmonly have pull-up or pull-down resistors to avoid scenarios where you have stray voltages.

However, we don't have that in house wiring - because we don't care about stray voltages; we care about having power. It's not important if there's 80V and no current available. But it'll throw off any measurements.

The solution is to add a resistor in parallel to the multimeter. This will effectively lower the input resistance of the meter, so it allows you to measure true voltage, shunting away any stray voltages.

As the power grid is built to supply large currents, the resistor can be quite small; passing half an amp is usually no problem in this scenario - but it should at least pull tens of milliamps.

The cheapest resistor available is probably a light bulb. They are built for the voltage, safe to use, and cheap. So simply add a light bulb in parallel to the meter, and measure again. This will show you what's actually present on the wire. Stray voltages will be dissipated in the bulb.

Indicator pens is even worse; they draw no current, but inductively or capacitively couple to whatever circuit they are close to, and amplify that signal by a factor of several thousand. Thus, they can pick up a very weak signal and amplify it. Thus, if it shows no power, it's probably true, but if they show power, it doesn't really tell you anything. Overall, they're next to useless.

4
  • 2
    Good explanation. However, the pens (Non Contact Voltage Testers) are not "next to useless". They are extremely useful as an extra test for safety. As a result, you want to have more change of a False Positive (no harm done) than False Negative (miss a hot circuit and end up zapping yourself when you work on it). Btu that does mean when you are pretty sure that it is a False Positive (all breakers are off, nothing works when you plug it in) that you need an alternate way (as described) to make sure that is phantom voltage and not a real problem. Nov 24 '21 at 0:54
  • 1
    They are next to useless. If they show positive, it's a very good chance it's false.In reality you have to measure in addition. To measure, you should first verify that meter works on a known powered circuit, measure what you're going to work on, and finally verify that meter works on a known powered circuit again.
    – vidarlo
    Nov 24 '21 at 1:01
  • That is the same procedure you use with an NCVT - test live, test what you are working on, test live. Plus an NCVT is a lot safer in many circumstances than a regular meter. Nov 24 '21 at 1:04
  • @vidarlo Or you could just forego the meters and pens, and touch the hot wire to the ground wire - zap! (Of course, without touching either of the bare wires yourself!). Now you have: A) Verified that the circuit was on. B) Verified that your breaker is working. C) Saved yourself a trip to the breaker panel and messing around figuring out which circuit you're dealing with. D) Made the circuit safe to work on. E) When you do go to the panel to turn the breaker back on, you will have determined which circuit it is. :)
    – Glen Yates
    Nov 24 '21 at 19:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.