Phantom Voltage


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Old 04-08-14, 03:17 PM
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Phantom Voltage

I was reading a thread and it stated that a cheap analog meter does better with phantom voltage than a cheap digital meter. Does that apply to more expensive digital meters. I borrowed a friends digital meter, a Fluke model 333. Is that meter likely to display phantom voltages? I get 15 or so volts when I think it should be zero.
 
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Old 04-08-14, 04:02 PM
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If you put a load on the circuit, then any phantom voltage will drop to zero.

The typical load recommended for dissipating phantom voltage in a home or small commercial building would be an incandescent lamp of at least 25 watts. It could perhaps be in a lamp fixture plugged into a receptacle that is fed by the hot wire you are testing, or even in a socket with two wires attached to alligator clips that you could connect across the meter probes.

It's phantom voltage if it comes back when you switch off or unplug or unclip the test lamp.
 
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Old 04-08-14, 04:15 PM
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I was reading a thread and it stated that a cheap analog meter does better with phantom voltage than a cheap digital meter. Does that apply to more expensive digital meters.
It can. The price is driven partly by quality but more by the number of functions.

I borrowed a friends digital meter, a Fluke model 333. Is that meter likely to display phantom voltages?
It may. None of has done a bench test comparison of all of the different meters on the market, so it's not possible to comment on a specific model.

I get 15 or so volts when I think it should be zero.
That certainly sounds like phantom voltage or, to use its proper name, induced voltage. It's induced in a conductor that's running with a current-carrying conductor in the same raceway. The reason it doesn't appear when a load is put on the circuit is that the current then has a complete circuit, with a path to ground through the neutral.
 
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Old 04-08-14, 05:02 PM
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Analog meters by their basic nature have a low impedance. Almost like the load of a 25+ watt light bulb so it loads the circuit enough to "drain" phantom voltage. The impedance of digitals are higher.

Edit: Continued in post #7.
 

Last edited by ray2047; 04-08-14 at 11:13 PM.
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Old 04-08-14, 08:32 PM
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Thanks for the responses, all were helpful.

That certainly sounds like phantom voltage or, to use its proper name, induced voltage. It's induced in a conductor that's running with a current-carrying conductor in the same raceway. The reason it doesn't appear when a load is put on the circuit is that the current then has a complete circuit, with a path to ground through the neutral.
When the house was switch to 400 amps two new panels were installed in the garage and approx. 30 cables in a bundle were ran from the old panel box that was in a closet to the new panels. That is what is inducing voltage on the circuits with no load.
 

Last edited by ray2047; 04-08-14 at 09:51 PM. Reason: Remove "Read More" link from quote..
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Old 04-08-14, 09:22 PM
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When the house was switch to 400 amps two new panels were installed in the garage and approx. 30 cables in a bundle were ran from the old panel box that was in a closet to the new panels. That is what is inducing voltage on the circuits with no load.
That'll do it. Especially if the conductors used to extend the circuits weren't derated.

Did they pull #6 wire for those?
 
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Old 04-08-14, 10:01 PM
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To add to my previous post think of the wires as a capacitor. The low impedance meter drains the charge from the capacitor just the way a screwdriver drains a motor capacitor but much more safely. because you can't be sure it is phantom voltage till you try to drain it. If it turns out to be real a screwdriver would be a very bad idea so you can't actually safely use the screwdriver method. So you use a low impedance meter or light bulb as suggested earlier.

http://support.fluke.com/find-sales/...105317_A_w.pdf

Two metal plates (the conductors) separated by the conductor’s insulation and air would appear to fit the definition of a capacitor. We know
from basic electrical theory that ac signals run across a capacitor are coupled while dc volt-ages are blocked. The amount
of ac signal passed through a capacitor is determined by its
capacitance and that capacitance is based on three factors:
1.The surface area of the
plates
2. The distance between the
plates
3. The make up of the insula-
tion separating the plates.

https://www.nema.org/Technical/Docum...d_12_15_11.pdf

ENGINEERING DEPARTMENT
BULLETIN
No. 88
October 1998
Revised February 2003
(Reaffirmed 12/2011)
Phantom Voltages
This Bulletin is intended to address the occurrence of so called “phantom” voltages, a
phenomenon detected during the testing of electrical conductors in thefield.
Due to the high impedance of measuring instruments, a voltage reading may be
detected on open conductors where there is no hard electrical connection to a voltage
source. Conductors that are installed in close proximity to one another, and are
capacitively coupled to each other, can cause this a.c. voltage reading. Such a reading
could be 2 or 3 volts, or it may be as high as the voltage on the adjacent conductors.
This is what is referred to as a “phantom” voltage.

According to Underwriters Laboratories Inc., this can be a harmless reading and can be
caused by the high input impedance of the measuring instrument, which places very
little loading on the circuit under test. The capacitance is increased as the length of the
run is increased. A 50 foot run may produce a pronounced capacitance effect whereas
a one foot sample may not produce any.

Since the “phantom” voltage is a physical phenomenon involving very small values of
capacitance, it cannot energize a load or cause physiological damage to a
person.

Care must be taken to be sure that the voltage reading is a phantom voltage, which is
caused by improper use of high impedance multimeters, and not as a result of a cable
defect or improper installation, which may result in a shock hazard.

In order to help minimize the likelihood of reaching a wrong conclusion from this
phenomenon, NEMA recommends the use of a Listed low impedance multimeter in
place of a high impedance multimeter or other high impedance measuring device for
testing on open conductors where there is no hard electrical connection. Without a low
impedance measuring device, a high voltage reading is an inconclusive indication of
possible faults in the cable.
Thanks to PCBoss for help on this post.
 

Last edited by ray2047; 04-08-14 at 11:14 PM.
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Old 04-09-14, 07:02 AM
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Thanks Ray2047 and PCBoss, the capacitor thing is something I hadn't thought of.

That'll do it. Especially if the conductors used to extend the circuits weren't derated.

Did they pull #6 wire for those?

Not sure what "derated" means. My house is all electric including heat. I have 11 240v circuits (2 with 6-3 with ground (60 amp breakers) and 9 with 10-3 with ground) and 25 120v circuits all 12-2 with ground (20 amp breakers). 8 of the 240v circuits (including the 2 #6 cables) and 20 of the 120v circuits run from the old panel to the new panels in a bundle and enter the new panels at the top via 2-1/2" plastic conduit. On a few of the 120v circuits, I have found the circuit extended using 14-2 with ground and I have switched to a 15 amp breaker for those circuits.
 

Last edited by ray2047; 04-09-14 at 07:07 AM. Reason: Remove "Read More" link from quote..
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Old 04-09-14, 09:45 AM
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Derating refers to a reduction in ampacity. For example, a #12 conductor in a bundle where it cannot radiate the heat will have an adjusted ampacity of 70 percent or less depending on the number of conductors. To compensate for this you need to install larger conductors to preserve the full ampacity.
 
 

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