This is not one of those, “go-out-and-buy-some-test-equipment-and-try-this-at-home-type-blogs.”
Phantom Voltage or “induced voltage” is the result of wire or other metal components appearing to be energized when they in fact are not. How this works is that when you have ungrounded wiring, like Knob & Tube wiring or older ungrounded romex-type wiring in your home and you add metallic pathways (wires and conduit) to these old circuits the metal wires and/or conduit will pick up an induced voltage merely by being in proximity to the hot conductor in the circuit. The ungrounded wire and conduit–and anything attached to it that is not grounded will also appear “energized” (hot) by the simple little Voltage Indicator Tool that is in every home inspector’s tool kit.
Phantom Voltage can make the metal sides of refrigerators, metal light fixtures, metal surface conduit, and metal junction boxes appear energized. As an inspector, it might be my first clue that someone has added newer grounded wiring to the old ungrounded system. With the ground wire not connected to an actual ground source the voltage is induced into the unused ground wire–and just like magic we get Phantom Voltage. It would be important for the inspector to verify that this Phantom Voltage is not “real voltage” with potential because then there could be a serious shock hazard present. Otherwise, Phantom Voltage represents no real danger that I am aware of.
Sometimes homes have grounded-type wires run throughout the home, but the ground wire, for one reason or another, is either not connected to the devices (receptacles, switches, light fixtures etc), or the wire just simply isn’t being used–or even possibly disconnected somewhere.
Homes wired in the early 60’s often have this condition.
It was the early 60’s when we first started manufacturing house wiring that included a ground wire. Because there was no requirement to actually “use” the wire, these houses mimicked older houses with ungrounded wiring.
One can see these ground wires just run back out through the back of the boxes, where they are either twisted together and wire-nutted together, or they are merely cut off. Sometimes they were attached to the metal boxes sometimes they were not. We might see these boxes in unfinished basements or other partially finished areas of the home. Here are a couple of examples.
This becomes a problem for the inspector because it is difficult to check for reversed polarity of two-prong receptacles when the home has been wired this way. Both slots will read hot when approached by a “tick-tester/voltage indicator. In fact the entire area around the receptacle within 6″-8” of the receptacle might read “hot” with a voltage tester–we call this “Phantom Voltage” or “Induced Voltage.”
This Phantom Voltage is high enough to set off a 90-volt “tick-tracer” (voltage indicator). The voltage will actually be the same as whatever the voltage of the circuit is. Because it is an induced voltage, there is no actual amperage present so shocks are not an issue. For the inspector, “CONFUSION” is the issue, because of how all this induced current prevents testing of the device for polarity.
Of course it is critical for the inspector to determine whether this is actually phantom voltage or in fact actual current in the wrong place.
The following pictures demonstrate how the many parts of the receptacle and the surrounding wall test as “hot” with the voltage indicator.
Note that in the next picture, on the left, even the metal screw that holds the cover plate in place reads as “hot.” Simply touching the metal screw with one’s finger is enough to “cancel” the Phantom voltage allowing the inspector to use the tester normally to check for proper polarity of the receptacle.
xxx
Charles Buell, Seattle Home Inspector
If you enjoyed this post, and would like to get notices of new posts to my blog, please subscribe via email in the little box to the right. I promise NO spamming of your email
Despite your warning, I’m going to have to try this for myself.
Reuben as you know—there is really very little danger—as long as it isn’t “actually” hot and you are not grounded if it is 🙂
Good article. Confusion is definitely the issue.
Inspectors can use a 2 prong adapter on their plug in tester to determine reversed wiring. ( AC voltage is not polarized).
I don’t use the pen tester much because they only show EMF. These new-fangled devices have a very high impedance which allows them to read minute secondary induced voltages and therefore are inconclusive and unreliable. You’re actually reading the electromagnetic flux field that is set up around an energized conductor. When you put your finger on the screw you are merely disrupting or collapsing the surrounding magnetic field. I don’t think many electricians use these testers and neither should home inspectors. We should be checking voltage, not EMF. If they really want to get technical they should carry a good old fashioned multimeter which puts a direct load on an energized circuit, but that’s beyond the scope of inspection.
Those metal boxes should be grounded with copper wire attached. They’re metal! If a hot wire touches the metal box it becomes energized and an electrocution hazard. With the box grounded it would simply blow the fuse/breaker. If the house has a ground circuit is should have grounded 3-prong outlets and be grounded back to earth. It’s a defect and safety issue if the ground circuit is not complete no matter what year it was built.
Commercial electricians definitely use the pen indicator or tic tracer. Not for anything precise but one example is before we work on a box that is not supposed to be hot we use our tic tracer to make sure there is no voltage in the box . Just one example.
Ibew member for 8 yrs
I think it is normal to do that but it will not tell you if it is energized with neutral current.
Daniel, I use many different tools—and even the “ticker” has its uses. The ones that are rated at 90 volts (can’t be found at Home Depot and the like) are much less prone to “nuisance” issues. As far as grounding metal boxes go I of course agree that they “should” be grounded. In older systems with ungrounded wiring they just won’t be. If the metal box were to become energized it would only be a shock hazard if somehow someone was able to touch it and something else that was grounded at the same time. Certainly any receptacles near water pipes might meet this criteria. I use the 3-light tester, volt indicator, multi-tester and the Suretest circuit analyzer on most inspections—they all have their advantages and disadvantages and some will find things the others won’t and all can be used to verify what the others find.
@ Daniel, Why not use a tic tester instead of a plug adapter to check polarity of a two prong receptacle? To each his own, I guess.
I find your comment regarding AC current not being polarized interesting. By “not polarized” perhaps you’re confusing DC current charge with AC current charge? The term DC is used to refer to power systems that use only one polarity of voltage or current. DC current charge is continuous, while AC current alternates between positive and negative. A sine wave of DC current is a flat line, while AC is an alternating wave of about 60Hertz in the US. The flow of both currents is however the same through a circuit. Current in a house circuit (AC) flows in one direction, same as in a battery (DC), hence the polarized plugs and receptacles.
As for grounding metal boxes, a good idea if the house wire has three conductors. However grounding does not necessarily mean the breaker will trip when a short occurs. The current will flow to the ground dispersing the current. Someone contacting a grounded circuit that is shorting will most certainly get a shock.
Lastly, what is beyond the scope of the inspection is up to each individual home inspector. You should never do more than you are comfortable with or is within the scope of your knowledge. Standards are the MINIMUM we all must do. In my chapter we like to say, it’s the worst inspection you can do legally.
James,
Mr. Buell is quite correct. DC current flow is polarized whereas AC current is not. There is no such thing as a DC Sine Wave. Alternating current is a sinusoid. A smooth oscillating mathematical function. The behavior of both are entirely different.
I am removing very old two wire romex with cloth over rubber. I have a piece that is still in the wall and both ends have been found and exposed. This romex will read .3-.5 volts AC at either end when totally disconnected.
What would cause this?
Hard for me to say Steve, but I would guess that there is an energized conductor running parallel to this abandoned wire somewhere and it is picking up an induced current from the hot wire. That essentially, is what phantom voltage is.
What would cause AC voltage to to appear on a DC control system?
If it is powered by an AC source that is not grounded it might pick up an induced current—otherwise I have no clue 🙂
Its been a week of trying to figure out why I detect .3 volts ac on a disconnected downstream ground wire while replacing an outlet that I found my answer on your blog explaining phantom voltage. Thank You! It explains my situation perfectly. I’ve been researching shared neutral circuits, floating grounds and 2 circuits in one duplex switch box looking for an answer. All I have is ghost juice!!!! It couldn’t give me a shock but sure gave me a headache!
I have old floor lamp not grounded that reads 24v on AC scale? I think it is phantom voltage but unsure if it is safe or needs to be grounded
You will need to test it for amperage to see if it is energized or induced. Have an electrician check it out.
How can you check for amperage? Maybe placing low ohm resistor to ground?
With a multi-tester. One probe to a known ground and one probe on the metal components
Hi,
would I be able to test a junction box to ensure that the box itself is not “hot” meaning a shock hazard with voltage tester or a different tool?
thx
T.
If you have a junction box that is testing as “energized” with a voltage indicator, you should call an electrician to determine if it is “actually” energized because it may or may not be and sorting it out takes more specialized equipment.
Thank You! I am replacing the last run of some 1940s ungrounded romex and as I tie things in I energize the circuit by connecting to the old (ungrounded) run. I’ve been scratching my head for days trying to figure out why the tester says the ground is hot but the multimeter says its not.
Yes James. Any time that equipment ground wire is not connected to anything—it will make your voltage indicator “think” it is energized.
Thank you, sir. That’s exactly what was happening when I purchased fluke voltage tester from Lowe’s today and started checking my outlets to see if equipment is working right. I was a little disappointed upon finding out it’s picking up “neighbor” voltage on half my outlets and light switches. House is built mid 50’s. I guess back to using an old fashioned method of “neon lamp”.
We have LED light wall scones. The DC drivers are being remotely fed with 120 or 277.
The secondary side when we meter positive to negative we get 33 volts dc both at fixture and remote driver.
When we meter positive and negative to ground we are getting 61 volts ac.
When we get to fixture we are getting 12 – 50 volts ac on fixture itself.
Should we be getting this leakage at fixture?
Should I be reading ac voltage to ground on secondary side of driver?
Sorry, that is nothing I am familiar with.
There are various answers to ungrounded voltage. one of them is static that causes stray voltage like that. I would guess static from your description of rubber wrapped with paper. around that area you probably have a good source of static.
Funny and crazy at the same time. Just had a call on a 1930 home down here in Tampa Fl where the old shingles had been replace with galvanized metal roof and it was reading 60v. First thing in mind was that someone fixing the roof had probably place a nail through a branch circuit. So I rushed and pull the meter out. Went up the stairs and my meter was reading 60v. I could not believe it…