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The electrical fields around us

By Chuck Newcombe

June 2012

The Fluke 2AC VoltAlert™ non-contact wand

The Fluke 2AC VoltAlert™ non-contact wand detects alternating electric fields near a live conductor.

Anyone who has been involved with electrical testing has come across the simple voltage testers (non-voltmeter) used to detect the presence of voltage. They range from a light bulb with test leads used in low voltage automotive applications, to neon testers that glow when the probes are connected across a live 120 or 240 volt circuit.

Today, of course, voltage testers include non-contact wands, such as the Fluke 2AC VoltAlert™ Voltage Detector, which detect alternating electric fields near a live conductor. These testers have no exposed metal, the internals being completely encased in a plastic shell.

So, how do these various testers work?

Let's revisit the neon tester for a moment. This unit cannot indicate the presence of a voltage below the 70 to 80 volts required to ionize the neon gas in the bulb, making it glow orange. It works for both ac and dc. Both electrodes glow if ac is present and only one if the voltage is dc. You can even figure out the polarity of the connection in the latter case.

If the probes in these devices were directly connected to the leads of the neon lamp, then too much current would flow when connected to 120 V or more, and the lamp would fail. So, manufacturers place a high resistance in series with the lamp to limit the current to a value that allows ionization but does not exceed the wattage rating of the lamp. That resistor is usually one megohm or greater in value.

Because the resistor is in series, a trick (which I don't recommend because of the unknown configuration of various neon testers, and the conductivity of the technician's shoes) has long been used by troubleshooters to determine the hot versus the neutral lead at a receptacle by placing one of the probes in a receptacle slot while holding the other probe in the hand. The idea is to take advantage of the capacitance of the human body with respect to ground as a reference. When the free probe is placed in the live slot, the tester glows, although not as brightly as one that is directly connected.

Today, much safer non-contact testers are available to sense nearby electric fields. They rely on the capacitance between the probe electronics and the technician's hand and body, which gives the unit a ground reference.

Why does the human body represent a ground reference?

There is a physical constant sometimes referred to as "the permittivity of free space" that is used to describe the force of an electric field between two conductors in a vacuum. Any isolated conductive body can exhibit the properties of a capacitor plate, and the earth, or metal conductive bodies, may represent the second plate, making a capacitor. As a result, the human body can represent one of those plates.

How can we measure the body's capacitance?

To measure current, and thereby estimate the effective capacitance of the body, repeat the test using a digital multimeter (DMM)

To measure current, and thereby estimate the effective capacitance of the body, repeat the test using a digital multimeter (DMM).

The human body has a capacitance to ground that varies, depending on its nearness to ground. It is lower when the body is farther from ground.

As it turns out, the dimly glowing neon lamp in the test mentioned above is an indication that a small current is flowing from the hot lead of a receptacle through the technician's body acting as a capacitor plate.

To measure that current, and thereby estimate the effective capacitance of the body, we can repeat the test using a digital multimeter (DMM). To be absolutely safe, we'll use a GFCI receptacle as our voltage source.

The procedure is as follows:

  • Verify that the voltage input resistance of the dmm is greater than 1 megohm.
  • Set the dmm function switch to ac volts and measure the voltage between line and neutral at a receptacle; in my case I read about 120 volts.
  • Remove the black (common) test lead from the receptacle and from the meter.
  • Connect the red dmm (input) lead to the neutral slot and note the voltage.
  • Now, connect the red lead to the hot slot of the receptacle.

When I did this, with the meter perched on top of a four-foot (1.2-meter) fiberglass ladder, I read 1.14 volts. I'll show the calculations for the effective capacitance to ground of the meter under these conditions in a table below.

Continuing the test:

  • Reconnect the black lead to the common terminal.
  • Take the probe on the black lead in your hand while standing on a wood floor and note the reading.
  • My reading went up to 19.36 volts in this test.
  • Finally, step up on the ladder, further insulating your body from the floor, and note how the reading changes. Mine went down slightly, to 16.23 volts

What does it all mean?

The process to make use of this data is to first calculate the current through the meter input by using the above voltages, the input resistance of the dmm, and Ohm's Law. Then do the calculations to estimate the capacitance to ground for the various tests. Here is a table with the results.

Line Voltage = 120 V Line Freq = 60 Hz  
dmm input R = 10 Mohm      
  Current through dmm V across C Xc  
Measured V (Amps) (120-Meas) (using Ohm's law) C
1.14 1.14E-07 118.86 1.04E+09 2.54E-12
19.36 1.94E-06 100.64 5.20E+07 5.10E-11
16.23 1.62E-06 103.77 6.39E+07 4.15E-11
At home or at work, use the Fluke 2AC non-contact voltage detector to safely determine whether the conductor of ac voltage is live.

At home or at work, use the Fluke 2AC non-contact voltage detector to safely determine whether the conductor of ac voltage is live.

In summary, the power factor of the meter alone was about 2.5pf, my body capacitance to ground ranged from 40pf to 50pf, depending on my distance from ground (or floor, in my case.) These are very rough estimates.

In summary, the new Fluke 2 AC non-contact voltage detector is a much safer way to determine whether an conductor of ac voltage is live, or if the metal of a lamp base is ungrounded. I recommend it over the experimental process I used for this column.