Non-Contact Voltage Detector -- less than 100% effective

[drcampbell]

During an outage, I powered a trouble lamp from the 120-volt convenience receptacle in an electric vehicle.

I tried to demonstrate a non-contact voltage detector. It didn't light up, despite the trouble lamp being powered and lighted.
Probably because the 120-volt receptacle in the EV is floating -- not ground referenced.

Just thought I'd share my discovery: A NCVD might not indicate the presence of power.

 

[don_resqcapt19]

I put this together a few years ago with information from the instructions...everyone of them says that lack of voltage indication on the tester, does not indicate lack of voltage on the equipment or conductor being tested.

This is from Fluke.

When using the Tester, if tip does not glow, voltage could still be present. The Tester indicates active voltage in the presence of electrostatic fields of sufficient strength generated from the source (MAINS) voltage. If the field strength is low, the Tester may not provide indication of live voltages. Lack of an indication occurs if the Tester is unable to sense the presence of voltage which may be influenced by several factors including, but not limited to:
●Shielded wire/cables
●Thickness and type of insulation
●Distance from the voltage source
●Fully-isolated users that prevent an effective ground
●Receptacles in recessed sockets/ differences in socket design
●Condition of the Tester and Batteries



This is from Klein.

WARNINGS:
It is important that users of this tester read, understand, and follow all warnings, cautions, safety information, and instructions in this manual before operating or servicing this tester. Failure to follow instructions could result in death or serious injury.
Risk of electric shock and burn. Contact with live circuits could result in death or serious injury.
Use caution with voltages above 30V AC as a shock hazard may exist.
• A steady red glow and continuous beep indicates voltage present.
• If no indication, voltage could still be present.
•In bright light conditions, the LED visual indicators will be less visible.
• Do not use if green LED is not illuminated.



This is from Ideal.

IMPORTANT SAFETY INFORMATION
Read and understand all of the instructions and safety information in these operating instructions before
using this tester.
WARNINGS
To avoid possible electric shock, personal injury or death follow these guidelines:
• Do not use if green light is not illuminating on tester.
• Do not use if tester appears damaged or is not operating properly.
• Do not use tester for more than the rated voltage on the tester.
• Do not use without the tester fully assembled.
• This tester will not detect the presence of DC voltage.
• This tester will not detect voltages at a distance, such as through walls, metal conduit, or shielded wire.
• Voltage detection maybe affected by outlet design, insulation thickness/type.
• Do not assume that no voltage indication means the circuit is de-energized.
• Voltages below 40VAC will not be detected by the 61-025 but are still shock hazards that can result in injury or death.
• Use the 3 Point Safety Method. (1) Verify tester operation by measuring a known voltage. (2) Apply tester to circuit under test. (3) Return to
the known live voltage again to ensure proper operation.
• Ensure all fingers are behind finger guard when testing for voltage.
• If this tester is used in a manner not specified by IDEAL, protection provided by the product may be impaired.
• Comply with local and national safety requirements, including the use of appropriate personal protective equipment.

 

[LarryFine]

Right. Non-contact testers depend on the earthing of the system, not unlike GFCIs.

 

[jaggedben]

I don't understand how the earthing of the system alone makes a difference. Does it also depend on the earthing of the user?

 

[LarryFine]

I don't understand how the earthing of the system alone makes a difference. Does it also depend on the earthing of the user?

Usually, yes.

 

[jaggedben]

Usually, yes.

So trusting one of these while standing on a fiberglass ladder ... probably a bad idea.

 

[ptonsparky]

So trusting one of these while standing on a fiberglass ladder ... probably a bad idea.

Probably.

 

[drcampbell]

. . . When using the Tester, if tip does not glow, voltage could still be present. The Tester indicates active voltage in the presence of electrostatic fields of sufficient strength generated from the source (MAINS) voltage. If the field strength is low, the Tester may not provide indication of live voltages. . . .

I'm sure I read that once upon a time, but that was many years ago and it seems to have fallen out of my brain. A reminder doesn't hurt.

So trusting one of these while standing on a fiberglass ladder ... probably a bad idea.

If it lights up, you can trust it.

 

[jim dungar]

I'm sure I read that once upon a time, but that was many years ago and it seems to have fallen out of my brain. A reminder doesn't hurt.


If it lights up, you can trust it.

Yep. This is primarily a training issue.


These devices are great for detecting the presence of voltage, the problem is they are all but useless for detecting absence of voltage.

 

[Speedskater]

Voltage is always measured between two points.
With the NCVD the probe tip senses one point and your body's capacitance to Planet Earth senses the other point.
Typically your body's capacitance to Earth is about 100 to 200 picofarads. But a tall fiberglass ladder might reduce that number.
The voltage supply that you are measuring also needs to be referenced to Earth.

 

[jaggedben]

...

If it lights up, you can trust it.

Actually I have found they deliver false positives too.

 

[electrofelon]

I have found there are two things that improve the reliability of these things greatly:

1. Only use a fluke, throw any others in the garbage immediately.

2. Keep/make sure the batteries are good.

 

[mayanees]

They are acceptable however, per 70E, 120.5(7) even if permanently mounted.
(7) Use an adequately rated portable test instrument to test each phase conductor or circuit part to test for the absence of voltage. Test each phase conductor or circuit part both phase-to-phase and phase-to-ground. Before and after each test, determine that the test instrument is operating satisfactorily through verification on any known voltage source.
Exception No. 1 to 7:
An adequately rated permanently mounted absence of voltage tester shall be permitted to be used to test for the absence of voltage of the conductors or circuit parts at the work location, provided it meets all of the following requirements: (1) It is permanently mounted and installed in accordance with the manufacturer’s instructions and tests the conductors and circuit parts at the point of work; (2) It is listed and labeled for the purpose of testing for the absence of voltage; (3) It tests each phase conductor or circuit part both phase-to-phase and phase-to-ground; (4) The test device is verified as operating satisfactorily on any known voltage source before and after testing for the absence of voltage.

 

[VirutalElectrician]

So trusting one of these while standing on a fiberglass ladder ... probably a bad idea.

Or rubber soled footwear?

 

[TwoBlocked]

So trusting one of these while standing on a fiberglass ladder ... probably a bad idea.

You'll probably be holding onto a conduit or something with the other hand.

 

[jaggedben]

You'll probably be holding onto a conduit or something with the other hand.

Not if it's romex.

 

[TwoBlocked]

Not if it's romex.

Hmm, can't say if the capacitance in the Romex would be low enough or not to still get it to work. Only times I've had a false negative (thinking there was no voltage when there was) is when I was convinced there was no voltage and just kinda waved it around like a magic wand. Once was a 120 heater circuit for a 480 motor being replaced. Clipped the wires and tripped the breaker in a panelboard. False positives, though, many times due to inductive, stray voltage. Used to carry one of those neon light screwdriver testers for those situations.

 

[synchro]

The circuit path used to sense the presence of AC voltage includes 1. capacitance from the metal probe inside of the tester to the conductors being tested, 2. capacitance from the tester to the hand that is holding it, and 3. capacitance from the body to its environment including grounded metal objects as discussed in some posts above. The capacitance in item 3. is also important in electrostatic discharge that can damage semiconductors (discussed at web link below).

Since all of these capacitances can vary, I prefer non-contact testers where the sensitivity can be varied with a knob or other means.
If you suspect a false positive reading showing that a conductor is "hot", then you can turn down the sensitivity to the point where you have to get very close to known hot conductors in order to get it to light up / buzz. Then if the conductor you want to test still sets it off, you'll have more confidence that it is in fact a "hot" conductor.
If the tester is not responding when you probe some point, but you want to get more confidence that there's no significant voltage present, then you can crank up the sensitivity to the point where it just starts indicating (if that's possible). Then you can see how far away from known "hot" conductors you have to get before it will stop indicating there's a hot conductor. If you have to get much further away than you were with the conductor that you want to test, then that gives more confidence that it really is a "dead" conductor.

Of course, I certainly wouldn't rely on a non-contact tester alone without metering it.

https://incompliancemag.com/article/human-body-model-and-electrostatic-discharge-esd-tests/

 

[acin]

Voltage is always measured between two points.
With the NCVD the probe tip senses one point and your body's capacitance to Planet Earth senses the other point.
Typically your body's capacitance to Earth is about 100 to 200 picofarads. But a tall fiberglass ladder might reduce that number.
The voltage supply that you are measuring also needs to be referenced to Earth.

Thanks for that info. I was told it was showing heat

 

[GoldDigger]

I don't understand how the earthing of the system alone makes a difference. Does it also depend on the earthing of the user?

Look at one simple case: an isolated 120V winding on a transformer, used to power a single light bulb. With no ground reference, there will still be a voltage difference between the two terminals at the light socket, whether the light is screwed in or not. Most likely the absolute voltage of the two terminals, set by a capacitive voltage divider to ground, will be 60V at each terminal, but with opposite polarity.
If the holder of the NCVD is solidly grounded or has a relatively high capacitance to earth ground the tester will see the electrtic field associated with at most 60V. Depending on the sensitivity of the NCVD it may or may not light.
But if the capacitance to earth in the circuit under test is low enough, the current through the tester capacitance in series with the user capacitance may be enough that the NCVD will see even less than 60V on each terminal when it moved close to that terminal. Even more likely that it will not indicate the presence of voltage.