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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Steven Cohen on November 14, 2018, 10:28:28 am

Title: Possible phantom readings on non-contact voltage testers
Post by: Steven Cohen on November 14, 2018, 10:28:28 am
Hello,

I have a quick question concerning possible phantom readings on non-contact voltage testers. I have a Fluke 2AC VoltAlert that I used to test for power on a residential GE load center. When I put the voltage tested near the main ground wire that goes to the ground stake, the tester lit up. The main breaker of the panel was off. Just to verify power was off, I used my Fluke 87 multi meter and tested across hot and neutral, hot to ground, and hot to hot and found no voltage. This testing of the hot lines and neutrals was done post of the main breaker, which was off.

My question is, why did the Fluke non-contact voltage tester light up near the main ground. The tester is rated to pick up voltages between 90 volts and 1000 volts.

Thank you in advance,
Steve
Title: Re: Possible phantom readings on non-contact voltage testers
Post by: John Roberts {JR} on November 14, 2018, 10:41:30 am
Those testers sense for voltage difference from electrostatic fields. Perhaps the ground wire is indeed ground, but you and the room have a measurable voltage field, and that difference is triggering the NCVT.

Another possibility is that the ground stake is not making a good low resistance electrical bond to local earth.

Some NCVT are more sensitive than 90V. I have a cheap one that is almost too sensitive to use.

JR
Title: Re: Possible phantom readings on non-contact voltage testers
Post by: Steven Cohen on November 14, 2018, 11:00:58 am
Thank you John for the response.

I forgot to mention, this load center is outside. I recently had a Federal Pacific load center and sub panel changed out due to the Federal Pacific panel being unsafe.

 When the new load center was installed, the electrician installed two new 8' ground rods due to recent electrical code changes.

Steve

   
Those testers sense for voltage difference from electrostatic fields. Perhaps the ground wire is indeed ground, but you and the room have a measurable voltage field, and that difference is triggering the NCVT.

Another possibility is that the ground stake is not making a good low resistance electrical bond to local earth.

Some NCVT are more sensitive than 90V. I have a cheap one that is almost too sensitive to use.

JR
Title: Re: Possible phantom readings on non-contact voltage testers
Post by: John Roberts {JR} on November 14, 2018, 11:36:05 am
It is unlikely the reading is spurious, but likely <90V.

One trick I use with a VOM in AC Volts scale (Volts only, same test using current scale could be dangerous), is to grasp one probe lead with my one hand and then probe around with the other test lead. If you measure voltage on your earth ground this way, it suggests that your room (and you) are energized with some residual voltage.

JR
Title: Re: Possible phantom readings on non-contact voltage testers
Post by: Mike Sokol on November 14, 2018, 08:04:43 pm
It is unlikely the reading is spurious, but likely <90V.

One trick I use with a VOM in AC Volts scale (Volts only, same test using current scale could be dangerous), is to grasp one probe lead with my one hand and then probe around with the other test lead. If you measure voltage on your earth ground this way, it suggests that your room (and you) are energized with some residual voltage.

JR

Exactly.... If there's other high-voltage wiring in the area, your own body can become the voltage source, and placing the NCVT next to an actual ground can cause it to trigger.
Title: Re: Possible phantom readings on non-contact voltage testers
Post by: Steven Cohen on November 15, 2018, 10:48:28 am
Thanks Mike and JR for the responses.

I was very close to the meter and the main breaker that was hot on the line side, so that's is where the stray high voltage must have been coming from.

Mike, in the past I have read some of your articles on non-contact voltage testers. I ASSumed that if the if the non-contact voltage tester lit up there was voltage with enough current to cause injury. Thanks again for the responses,


Steve


 
Exactly.... If there's other high-voltage wiring in the area, your own body can become the voltage source, and placing the NCVT next to an actual ground can cause it to trigger.
Title: Re: Possible phantom readings on non-contact voltage testers
Post by: Kevin Conlon on November 15, 2018, 08:15:21 pm
Thank you John for the response.

I forgot to mention, this load center is outside. I recently had a Federal Pacific load center and sub panel changed out due to the Federal Pacific panel being unsafe.

 When the new load center was installed, the electrician installed two new 8' ground rods due to recent electrical code changes.

Steve

 
When did 2 grounds happen? I ask because i plan on adding a new one since we have had 2 lightning strikes and i understand they can do bad things to grounds.
Title: Re: Possible phantom readings on non-contact voltage testers
Post by: Stephen Swaffer on November 15, 2018, 10:50:36 pm
It has been at least 15-20 years in the NEC-I took my exams in around 1998 or so and the code was written that way then.

Actually, code requires a grounding electrode-if the resistance to ground is greater than 25 ohms it requires a second one-but once you have two impedance to ground is not considered.  The time,effort and equipment required to measure impedance to ground usually makes it easier and more cost effective to drive a second ground rod.

Properly install 2 ground rods and there is no question and most inspectors are happy.  Though all electrodes present are required to be connected-so metal water lines, structural steel on building, etc all need to be bonded.

In Iowa, new construction that has concrete footings is required to use a UFER (concrete encased electrode) ground.

Keep in mind that code is a minimum requirement-not necessarily the best practice.  I considered bidding on a project-several cabins in a state park.  The engineer required a ground ring made of 1/0 copper connecting 8 or 10 ground rods.  So, if there are extenuating circumstances you might want more.
Title: Re: Possible phantom readings on non-contact voltage testers
Post by: Jonathan Johnson on November 16, 2018, 12:15:10 pm
Keep in mind that code is a minimum requirement-not necessarily the best practice.  I considered bidding on a project-several cabins in a state park.  The engineer required a ground ring made of 1/0 copper connecting 8 or 10 ground rods.  So, if there are extenuating circumstances you might want more.

Such extenuating circumstances could include extremely dry or poorly conductive soils. A properly installed "Ufer" ground -- bonding to the steel rebar in a concrete footing or pad -- can serve as an effective grounding electrode, especially in dry soils. But might have to do a fall-of-potential test to satisfy the inspector/engineer.

For what it's worth, many non-contact voltage testers will light up if you rub the tip on your shirt. I have yet to be shocked by my shirt. Other people might be shocked by my shirts, but that's a topic for a different discussion.
Title: Re: Possible phantom readings on non-contact voltage testers
Post by: Lyle Williams on November 16, 2018, 01:43:22 pm
NCVT are handy, but they provide misleading results >10% of the time in my experience.

Not something to trust you life to.  You would be dead in less than a week.
Title: Re: Possible phantom readings on non-contact voltage testers
Post by: John Roberts {JR} on November 16, 2018, 02:27:39 pm
NCVT are handy, but they provide misleading results >10% of the time in my experience.

Not something to trust you life to.  You would be dead in less than a week.
Trust but verify.. I would caution about taking warnings too lightly. Not random, but not a precision measure.

JR
Title: Re: Possible phantom readings on non-contact voltage testers
Post by: Jonathan Johnson on November 16, 2018, 03:14:56 pm
Trust but verify.. I would caution about taking warnings too lightly. Not random, but not a precision measure.

JR

Yes. It can warn you of the presence of electricity, but it cannot prove the absence.

Nobody hears the dog that doesn't bark.