Ghost and Stray Voltages

In a Post Elsewhere; that now points to this blog entry; I said I was doing the Annual or Biannual Floor Fan Inspections at my home and had an Unexpected Finding:

While unplugged:

I inspected the Cord, Plug, and Cord at the Strain Relief and the Strain Relief itself. I removed the Grills, removed the Fan Blade, cleaned the inside of the Fan Casing, used a commercial can of Dust Blowing agent to remove the Dust from various points inside the fan casing, as well as in the Motor Windings (without removing the motor, just through the openings for air circulation).

The Fan uses an Ungrounded Cord, and with Metal Case Fans, it’s possible that a strand of wire in one of the windings is contacting the Motor Casing, or Laminated Core, and the Motor is connected to frame of the Fan. Due to the length of the wire used to make the windings, it can produce a reduced voltage on the metal parts of the fan. To check for that, I used a Fluke Model 12 Meter, and in reference to ground, it showed 48 Volts on one fan on set to High, lesser voltages on the other settings. It showed about 30 volts on the other fan.

Tester Fluke 12

I needed to see if the voltage was really there, or if it was a Stray Voltage that, technically, really isn’t there. I have seen appliances with Low Voltages imparted to their metal shells, such as a Refrigerator or Dishwasher. A relative by marriage was shocked while touching the Dishwasher and grabbing the faucet. The Shell to the Dishwasher was Electrically Hot, and the Grounded Plug was inserted correctly into a grounded outlet that itself WAS NOT Grounded.

When working as an Industrial Electrician, finding wires that showed 60 Volts or less, when 0 Volts was expected, was not uncommon. These were voltages Coupled from other wires in a Cable, and in the old days (just as I started) the Electrical Testers Used had Low Input Impedance and WOULD NOT show stray voltage.

Tray Cable

Above is at least a 20 Conductor Cable. Assuming you can read schematics, are not color blind (I worked occasionally with a Color Deficient Electrician, he had to frequently ask what color wires were), and the cable wan’t replaced since it was designed, you have an expectation of which wires are Electrically Energized and which aren’t. Nevertheless, before handling wires they need to be tested to be sure they were de-energized. If some wires were energized, it’s possible for some conductors to induce a voltage in other conductors. Following is an Explanation why this happens:

New Multimeter Detects Ghost Voltages

Ghost Voltage Can Look Real

Ghost voltages can be caused when energized circuits and non energized wiring are located in close proximity to each other, such as in the same conduit or raceway. This condition forms a capacitor and allows capacitive coupling between the energized wiring and the adjacent unused wiring.

When you place your multimeter leads between the open circuit and the neutral conductor, you effectively complete the circuit through the input of the multimeter. The capacitance between the connected, hot conductor and the floating conductor forms a voltage divider in conjunction with the multimeter input impedance. The multimeter then measures and displays the resulting voltage value.

Most digital multimeters today have an input impedance that’s high enough to show this ghost voltage, giving a false impression of a live conductor. The meter is actually measuring voltage coupled into the disconnected conductor. But at times, these voltages can be 80-85 % of what the “hard” voltage should be. If not recognized as a ghost voltage, additional time, effort and money may be lost troubleshooting circuit problems.

The Old Electrical Testers were Solenoid Types. An Electric Solenoid would be activated by the incoming voltage and it moves the plunger and the pointer attached to it, to the voltage that corresponds with the input. It takes a specific amount of current to drive the Solenoid, and actual voltages will be sufficient to drive the coil. Phantom or Stray Voltages on the other hand, do not have sufficient current to drive the solenoid, so they don’t product movement of the solenoid indicator, but may illuminate the Lights on Solenoid Voltage Testers that show whether the source is AC or DC.

Tester Ideal

Hunting Down the Ghosts

In the past, tracking down such a voltage indication could send the technician off on a time wasting wild goose chase, looking for a problem connection that was never there. He could ferret out the ghost voltage with a low-impedance analog meter or a solenoid tester, or ‘wiggy’ – but that would require packing or fetching an extra tool.

To be sure, I used my Tester, similar to the above, and it didn’t show any voltage that would register, but I believe it illuminated the AC Indicating Lights. But it occurred to me, the Fluke 12 Has a Low Input Impedance Mode.

VCHEK AND HOW TO USE IT

VCheck is a subset of the continuity/ohms function. In VCheck, the meter is designed to automatically display an ac or dc voltage when the meter detects a voltage greater in magnitude than about 4.5 V and the meter is not in the manual range mode. THIS
WILL NOT HARM THE METER. VCheck is always enabled (and k is displayed) when
the meter is in the continuity/ohms function unless the meter is in one of the following:
  1. The manual range mode (i.e., Z is displayed)
  2. The MIN MAX mode (i.e., j is displayed)
  3. The capacitance function (i.e., µF is displayed)

Repetitive transients on a dc bus will cause i to select ac volts, even though a hazardous dc voltage may be present. To avoid a misleading display and possible electric shock, manually select the proper volts function for measurements on these circuits.

In VCheck, the meter has a low input impedance (~2 kᾨ). When a voltage is displayed, LoZ is also displayed to remind you of this, and the beeper momentarily sounds a VCheck  Alert™. To disable the i Alert in the ohms function, press and hold down [g] while turning the meter on.

Use VCheck only on power supplies and other power sources that have a low output
impedance. Do not use VCheck to measure voltage in electronic circuitry unless a 2 k
load will not damage the circuit.

So I simply put the meter in VCheck Mode, made sure the Meter was in the AC Mode, checked between the Fan Casing and Ground, and voila, I measured 3 Volts. Not a concern, I don’t believe it represents the windings contacting the metal laminated core or Fan Motor Casing, and both fans has very slight readings, 3 volts and less, so I think they are ok.

I learned writing this, I thought the ghost voltages were Inductive, they are according to the literature I found Capacitive. The article I linked to above is a meter that detects Ghost Voltages, but Fluke sells a Stray Voltage Adapter for use with many Meters:

Fluke SV225 Stray Voltage Adapter 

 

Tester Stray Voltage Eliminator

“Key features

  • On energized wires, the meter will indicate the real voltage
  • On non-energized circuits the meter will read close to zero (even if there are stray voltages)
  • It can be used with all modern meters with standard input spacing
  • Rated CAT III 1000V, CAT IV 600 V” 

So, what started out as an annual, or biannual, Fan Cleaning, revealed an unexpected phenomenon, that is likely exacerbated by not using a grounded plug and cord set (as manufactured) and connecting the ground to the Fan and/or Motor Casing. But the 48 Volts was significant enough for me to want to verify that no voltage was actually present. Which I did. I hope this explains Stray Voltages, I’ve blogged it before, I saw the Stray Voltage Eliminator in my Media Files on the Blog, but don’t remember deleting the original blog or its topic.

 

Author: Dr-Artaud

A Doctor that is not a Doctor, but named after a character in the movie "No Such Thing", as is the Avatar.

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